SoulFetish 5/14/2018 2:11 AM
Characteristics of Guitar Amplifier Output Transformers, and criteria for design
I'm trying to better understand magnetic circuits and have been doing some informal reading. I've found discussions and threads in this forum to be insightful; particularly in inspiring me to step back and get some better fundamental knowledge on the subject. Quite frankly, I had some uniformed assumptions that were just wrong. I'm sure I still have some (and there's obviously much more to learn) but I'm starting to piece it together and I'm getting those light bulb/"Oh Shit" moments when it makes total sense.
I've been mostly reading about Mains type power transformers, and have found the articles on Rod Elliot's site to be really great. They are a really good primer in laying some groundwork for understanding some of the other info I've found in some other sites.
But, I've been thinking about Output Transformers lately, and would like to have a no bullshit conversation about what we are really looking for in OTs for guitar amps.
Most of the "output transformer shootouts" that people have put out there aren't that informative at all, I find. Everyone I've seen pits one manufacturer against another and one is left to choose which manufacturer's iron sounds better.
I would rather understand the different material characteristics and construction techniques and set up the pepsi challenge under those conditions to see if I preferred one method/lamination grade/interleave over another.
Really, I'd like to try my hand at maybe designing one from the ground up. I think that will help me understand how different materials react under different conditions.
And I think you guys are just the sort of folks to talk to about it. We need, like, a bat symbol send out for R.G. with threads like this.

[IMG]https://img00.deviantart.net/4de1/i/2018/134/d/2/rg_batsymbol_by_soulfetish-dc8kqzt.jpg[/IMG]


So, where do I begin? I imagine I start with deciding what the power and bandwidth requirements will be?
 
Malcolm Irving 5/14/2018 2:50 AM
This is a good source (but I expect you may already have found it):

http://jensen-transformers.com/wp-co...rs-Chapter.pdf

and this one:

https://ieee.li/pdf/introduction_to_...chapter_12.pdf
 
SoulFetish 5/14/2018 3:43 AM
Quote Originally Posted by Malcolm Irving View Post
This is a good source (but I expect you may already have found it):

http://jensen-transformers.com/wp-co...rs-Chapter.pdf

and this one:

https://ieee.li/pdf/introduction_to_...chapter_12.pdf
Thanks for those links, Irv. I also tracked down a pdf of Lee’s “ electronic transformers and circuits “.
 
jmaf 5/14/2018 5:26 AM
There's a text by RG somewhere where he analyzes old and new trafos and finds subtle differences in old iron and as usual for RG in the process he yields several knowledge pearls. I can't find it now, it's not the top results at Google. You should definitely give his other texts a look as well.

Sergio Hammernik of Mercy Magnetics talks about rust in transformers in a column for a mag, that PDF can also be found online, which is a second resource that comes to mind about old iron actually being good.

Just my 2 cents about one of the aspects that isn't discussed very much. Like some wines, good transformers age well, so that's a rich field for research. What's in old iron?
 
Leo_Gnardo 5/14/2018 6:57 AM
Quote Originally Posted by jmaf View Post
What's in old iron?
Impurities, those magic impurities. Like what's in old copper wire, for pickups and transformers. Our local pickup builder-repairman is constantly scanning for the oldest rolls of wire he can find "because that's necessary to get that old-fashioned sound." Modern metal is too pure, too good.

Good advice on the OT's, why reinvent the wheel. Besides being very time consuming, and wondering exactly what it is you're hearing (takes double blind, um, make that double deaf tests), bound to be expensive.

AND may I add it's good to see you back jmaf, where have you been? It's been a while.
 
jmaf 5/14/2018 7:31 AM
Quote Originally Posted by Leo_Gnardo View Post
AND may I add it's good to see you back jmaf, where have you been? It's been a while.
Thank you, Leo. Great to chat with you again as well.

Been on and off several projects, times have been a bit rough here in Brazil. But as they say here, I'm still above the ground and below the heavens.

Certainly wanna be more in touch with the great discussions and folks on this forum.

Cheers!
 
Gregg 5/14/2018 10:14 AM
The short answer to your topic is: there's no specific criteria for designing a guitar OT. Few simple facts from the past decades illustrate that very well:
1/ Many different types of iron were used
2/ Many different types of winding configurations were used
3/ Most of the time the basic criteria for the OT (especially for the big manufacturers) was the price - the cheaper, the better.

That's why you can see and OT made from 0.5mm PT grade laminations, one primary/one secondary, without any interleaving. On the other side you can see (not very often) HiFi grade OTs made from M6 iron, many pri/sec, heavily interleaved.

My sincere advice to you is don't bother designing a guitar OT from zero. If you're into a specific type of sound better find out what specific OT was used in that amp. Also if you think that designers of a specific amp spent endless hours designing that specific OT to achieve that specific sound you'll be as far from the truth as you can get. However if you would like to learn how an OT is generally designed and learn new stuff that's entirely different story. There's room for many experiments in this field.
 
R.G. 5/14/2018 10:57 AM
Don't beat yourself up if you don't get transformers clearly in your mind quickly. It can take years for the information from the texts and tinkering to soak in. It did for me anyway.

Here are some quick and dirty maxims to think of.

> There is a big inductor on the primary side. This inductor is the model for the loading of the core that your incoming signal must pay to get through the transformer. It is the fundamental low frequency limit on the transformer. The low frequency rolloff point at half power happens at the frequency where the primary inductor's impedance is equal to the reflected load from the secondary.

Example: If you have an OT with a speaker load, and you've get the winding ratios right, then the speaker load impedance (e.g., 8 ohms, 4 ohms, etc.) is reflected into the primary side by the transformer. For a pair of 6L6's, the plate to plate loading is often set at 4400 ohms. So if you want a half-power bass point of 40Hz so that your guitar's 82Hz low string is a full octave above the rolloff, you instantly know that the primary inductance must be no smaller than
Zl = 4400 @ 40hz = 2*pi*40hz*L
or L = 4400/(2*pi*40) = 17.5 henries.
That's a minimum, and for hifi work they like the half-power point to be lower than a tenth of the actual signal response. In this case, for 1/10 of 82Hz, the inductance needed gets to be 85.4H.

Things get out of hand rapidly, as making ~100H inductors that can handle tens to hundreds of watts requires BIG cores and lots of copper windings.

That sets the low end. Doing this stuff well means picking big enough iron and stacking the Es and Is in a way that maximizes the magnetic properties of the stacked iron and minimizes the contribution of air gaps.

The above is for continuous AC transformers or push-pull. Single ended designs get yet more complex because the magnetic field in the iron has to be able to put out a half cycle of the maximum maximum output energy in a lowest-frequency half cycle. There's only one output device to pull power on its active half cycle, so the energy for the "relax" half cycle of the output device can't come from anywhere but the magnetic field in the core. This gets out of hand in the iron design, as a practical matter, because you must include an air gap, and designing the air gap is tricky and may well need multiple passes.

> For the high end, the fundamental limits are set by a combination of the leakage inductance and interwinding capacitance, as well as funny stuff like interwinding balance and cross-winding coupling. These are all properties of the winding method, mostly.

Leakage inductance is caused by the magneto-motive force ( or MMF; that's the current in the primary windings times the turns) forcing a magnetic field to form that energizes the core. Iron cores are not complete "conductors" of magnetic fields, and free space is not a perfect "insulator" of magnetic fields, so some of the MMF sent into the primary coils leaks out into space and does not get inside some of the secondary windings.

To combat this, special winding schemes are cooked up to intermix primary and secondary windings and layers interspersed with one another so that leakage MMF has a hard time getting out into free space without coupling with the intermixed secondary wires. Ideally, you'd wind the primary and secondary wires side by side, so that every primary wire was parallel with every secondary wire. That's not possible in a world where we want 20-30 times as many primary turns as we want secondary turns, unfortunately.

The next best thing is to wind a layer of primary, then a layer of secondary, then another primary, another secondary, etc. That is GREAT for reducing leakage, but is a real PITA to actually wind, and makes capacitive coupling worse. One of my first tasks as an engineerling was to derive the equations for leakage inductance from the physical geometry of coils of X width and Z winding height. Then I had to go wind the trans former in the lab and explain why I got it wrong. It was very instructive, and in only another year or so I found where I'd gone wrong with that.

As a practical matter (there's that word again) commercial ventures can't make layer per layer interleaves. They subdivide the primary and secondary into sections, and wind, for instance, three or four layers of primary, then a couple of secondary, then some more primary, then some more secondary until the right number of turns is on. This can be shown (I finally got the equations right) to reduce leakage by the square of the number of alternations between primary and secondary. Generally, splitting primary and secondary into two sections each or two/three will reduce leakage by a factor of four over just primary over secondary.

> Interwiring capacitance fights you at every step. If there is a voltage difference between two conductors and insulation between them, capacitive current flows through the space between the conductors. This is true for even two side by side turns of copper wire in a single turn of wire. The intra-and inter-winding capacitance is a distributed thing that depends on where each wire is actually put in the coil. Random winding makes this unpredictable (if smaller - no parallel wires) and different for each article. Layer winding maximizes it. Sigh.

> The general figure of merit in an OT was defined as the "Goodness Factor". No, I didn't make that up, that's what it was called in the OT biz back in the 50s and 60s. This was the ratio of the primary inductance to the leakage inductance. An OT with a primary inductance of 100H and leakage of 10mH (representative values, these) gives a GF of 100/0.01 = 10,000. This is the range you want to shoot for if you're designing hifi OTs.
Write up your last will and testament before trying to get to that with an SE design.

> Other complications happen if you're doing push-pull as anything other than Class A. When half a winding turns off, as it does once each half cycle for Class AB or B, that winding's coupling and magnetic effects (and inductance!) vanishes as it quits participating in the magnetic goings-ons. Not being very careful to make each half-primary have beautiful, complex interlayering/interleaving with the entire secondary causes little glitches to happen in the output right at the spots where current in the primary halves turn on and off. This imposes a fundamental limit to how low crossover distortion can go in a Class AB push-pull output stage. It is the fundamental reason for the "unity coupled" McIntosh OTs and the big hoopla in the hifi market it caused. Guitar amp designers mostly ignore this as an issue, as the silly guitarists LIKE distortion. Let'em live with it. And it's too expensive to fix, anyway, so the makers said.

Hope that helps rather than confusing issues. Yell with questions.
 
SoulFetish 5/14/2018 4:47 PM
Quote Originally Posted by Gregg View Post
My sincere advice to you is don't bother designing a guitar OT from zero. If you're into a specific type of sound better find out what specific OT was used in that amp. Also if you think that designers of a specific amp spent endless hours designing that specific OT to achieve that specific sound you'll be as far from the truth as you can get.
I appreciate the advice. Look, I've though about leaving the whole thing alone and walking away from the topic all together. There's a part of me that has a sneaking suspicion that when the dust settles, and we near the ass end of this conversation, I'll already be 3 orders deep and waiting for more supplies so I can wind my own OT.
...ugh. (Don't let me go down that rabbit hole, guys)

However if you would like to learn how an OT is generally designed and learn new stuff that's entirely different story. There's room for many experiments in this field.
Yeah, more this ^^ I want to be a little more purposeful in my approach to this area of the design. I generally think learning empowers creativity, and the creative part of this is one things I enjoy most about music electronics.
(even though I'm not breaking any new ground here)
 
Chuck H 5/14/2018 5:31 PM
Maybe a good place to start from an amateur perspective would be to reverse engineer the OT from an amp you like the sound of (as much as possible without destroying one I suppose) and then apply your research to HOW those physical properties affect the end result and WHY those results sound good. In doing this you're bound to discover some construction aspects you'd like to hear tweaked a little
 
SoulFetish 5/14/2018 7:54 PM
Quote Originally Posted by R.G. View Post
Don't beat yourself up if you don't get transformers clearly in your mind quickly. It can take years for the information from the texts and tinkering to soak in. It did for me anyway.
Yeah, I have a lot of questions that kind of stretch out over all the different factors at play. For instance, primary inductance is an important factor in audio transformers, where it is of no real design consideration in power transformers? The inductance needs to increase with loading impedance on the driving signal. This seems to indicate a relationship between inductance and magnetizing current? Also, making practical sense of BH curves in the context of voltage/current/frequency conditions. For instance, is there risk of saturation at low signal levels in high permeability cores? Anyway, I have many more, but at this point I'm trusting the learning process. I know it will come together. So let's start off where you did.

Here's what I'm designing for:
2XEL84s with a p-p load impedance of 10k2 and a peak sine wave output of around 17.5W. But square wave output at full drive, i'm guessing +25W maybe? Let's account for alternate tuning and the rare bass player who may want to plug in and set the -3dB at 20Hz.
That means I require a primary inductance of around 81.5H rated for min 25W. (Ideally)
Now What? (lets make some big iron)
 
Chuck H 5/14/2018 9:37 PM
Hammond 1609

The 1608 at 8k has a very good rep for 18W type builds and it's what I have in my personal (prototype) amp. I solicited Heyboer for the production model and the amps are apples to apples. The Hammond sounds better. Maybe try one and see what you think, then reverse engineer it if you want to play with that. The transformer is rated for 10W, but trust me when I tell you that it will handle anything 2Xel84's will throw at it under any operating conditions. I have that from a tech at Hammond and I've experienced it for myself.
 
J M Fahey 5/14/2018 11:57 PM
Quote Originally Posted by Gregg View Post
Also if you think that designers of a specific amp spent endless hours designing that specific OT to achieve that specific sound you'll be as far from the truth as you can get.
True.
Amp designers work on the *electronic* side of the problem , and just *order* transformers from an Industry supplier, leaving him to work the details.
 
Malcolm Irving 5/15/2018 4:09 AM
Quote Originally Posted by SoulFetish View Post
… . For instance, primary inductance is an important factor in audio transformers, where it is of no real design consideration in power transformers? ...
Primary inductance is in parallel with the load reflected from the secondary, so for an audio transformer the tubes supplying the OT have to provide current into that parallel inductance as well as into the proper load. At low audio frequencies, the reactance (2.pi.f.L) of the primary inductance is low and the extra current causes the primary voltage to drop (due to voltage drop across the output impedance of the tubes - if you like).

For a PT, the internal impedance of the mains supply is very low (say 1 ohm or so). The mains is almost a perfect voltage source. In this case some reactive current is 'wasted' into the primary inductance, but the primary voltage holds up and the secondary (load) side is happy. There still needs to be a reasonable primary inductance (and hence reactance at 50 or 60 Hz) otherwise the primary would draw very high current and overheat.
 
Malcolm Irving 5/15/2018 4:20 AM
Quote Originally Posted by SoulFetish View Post
... The inductance needs to increase with loading impedance on the driving signal. This seems to indicate a relationship between inductance and magnetizing current? …

Yes. We can think of whatever is driving the primary as a Thevenin Equivalent, i.e. voltage source in series with an internal impedance.

Then the magnetizing current is Vs / (Zth + Zmag)

where Vs = source voltage, Zth is source internal impedance, and Zmag = magnetizing reactance ( = 2.pi.f.L ).
 
Malcolm Irving 5/15/2018 4:36 AM
Quote Originally Posted by SoulFetish View Post
... Also, making practical sense of BH curves in the context of voltage/current/frequency conditions. …

In the B - H curve, H is proportional to magnetising current and 'rate of change of ' B is proportional to primary voltage.
For a sine wave, the 'rate of change' is just another sine wave (but advanced by 90 degrees, i.e. a quarter of the wave period).

In a PT, for example, the primary and secondary voltages are close to sine waves, but the B-H curve means the current into the primary is distorted. The primary inductance (also called the magnetizing inductance) is a non-linear inductance (due to B-H).

The B-H curve shows the non-linearity and hysteresis between primary voltage and magnetizing current.

The frequency of the signal is how many times 'we travel around' the B-H loop per second.

I hope some of this is helpful and I apologise if I am 'teaching my grandmother to suck eggs'.
 
Helmholtz 5/15/2018 8:05 AM
That means I require a primary inductance of around 81.5H rated for min 25W. (Ideally)
Lp values this high are quite realistic in guitar amplifier OTs. But only around rated output power and at low frequenies (max. ac flux). I measured Lp of around 40 different OTs at 650Vpp and 50Hz and found Lp to lie between 60H and over 300H. But when measuring with an LCR meter @ 1kHz, the values were lower by a factor of 5 to over 10. (The reason for the different values is that the LCR meter measures at very low flux amplitudes where the ac ĩ is close to the initial permeability. The ac ĩ rises strongly with flux amplitude up to the onset of saturation.)

The results seem to indicate that the major design concern was to avoid excessive power loss from high magnetizing current at max. output, while the bass response at low output power may suffer. Of course NFB can take care of this.
 
R.G. 5/15/2018 9:18 AM
In transformer design, everything depends on everything else, so getting an optimal transformer is an exercise in optimizing many things at once. Accordingly, with so many ways to make things better in some sense, there are many ways to look at the transformer and its drive and loading. I mention this not least because Malcolm has presented some views that are correct. Here are some other views that strike me from your questions.

Quote Originally Posted by SoulFetish View Post
For instance, primary inductance is an important factor in audio transformers, where it is of no real design consideration in power transformers?
That particular question is answered in my mind as a question of objectives. Primary inductance is still a big deal in power transformers, but it's hidden behind other factors that are quoted. For PTs, the "bass response" is more subtle. You know the frequency response needed going into the design, so that's not the issue. The issue is how much of the incoming AC current leaks through that primary inductance, and correspondingly primary inductance is manipulated to get a "goodness factor" measured in dollars.

A power transformer will be fed a single frequency at a relatively fixed size for 100% of its life. A better primary inductance "bass response" means that the amount of electricity that unavoidably leaks across the primary winding as a result of the primary inductance impeding it. The primary inductance spec is tied up in the "magnetizing current" or "efficiency" specifications. Big primary inductances make magnetizing current smaller, and to the PT customer, that makes the dollars spent on electricity over the lifetime of the trannie lower. It also makes the transformer run cooler (in concert with several other factors), so primary inductance still matters, it's just hidden as a specification.

It would still be good if the primary inductance was huge, but huge costs money in terms of iron, copper, and labor.

Interestingly (to me anyway) is that primary inductance being too small in power transformers is what got line frequency wall warts made illegal. The unavoidable primary current leakage got noticed by the protectors of the planet in California. Wall warts that stay plugged in all the time "spend" this electricity all the time, and the multiplication exercise of X zillions of wall warts at Y milliwatts each, times Z hours plugged in came up to "Oh my &deity. we have to stop that!" and they legislated that no wall wart could have a no load current bigger than "too small to be a line frequency transformer", and also got the EPA to issue the same regs for the country. This had the effect of making only switching power supplies be legal. And so the world was saved from primary inductance.

PTs are all about power transfer (duuuh...) so their specs get wrapped up in volts, amps, phase angle and power loss terms. Fidelity is of little concern, so the funniness of BH curve bending and minor BH loops gets ignored, where it can't be for output trannies. Primary inductance and even leakage inductance gets hidden in the power specs.
The inductance needs to increase with loading impedance on the driving signal. This seems to indicate a relationship between inductance and magnetizing current?
Absolutely correct. Primary inductance is always across the driving signal and must be energized before any signal gets to the secondary. It's a price that has to be paid. To minimize magnetizing current, you maximize inductance.

More subtly, you minimize the area of the BH loop in the core. Transformer designers work with a chart of flux density (B) versus magnetomotive force (H). The more closely the graph of B versus H approaches a straight (and vertical!) line, the better the material is. The slope of the BH graph at any point is proportional to the primary inductance at that flux density, so higher slope is better. Real materials do not retrace the same path on the BH curve going up and coming down, making each cycle of signal be an odd, squashed-S loop. The area inside the loop is representative of the energy wasted per cycle in iron losses.

And put another way, the inductance varies at different levels of flux density. Yes, that means that an OT has distortion all on its own, due to the nonlinearity of the iron's magnetization. The more the iron "eats" of your signal (i.e. the lower the magnetizing inductance), the more the iron distorts your carefully prepared audio.
Also, making practical sense of BH curves in the context of voltage/current/frequency conditions. For instance, is there risk of saturation at low signal levels in high permeability cores?
I already got into that a bit. A BH curve is just a plot of how much magnetic flux density (magnetic field intensity) exists per unit of MMF. Flux density used to be measured in (maginary) field lines per square foot/inch/yard/etc. Now it's measured in Teslas, after the guy who had the nerve to read a newspaper inside a lighting generation machine. Whatever the units, it's still magnetic field intensity. MMF is measured in Oersteds, after a guy I don't have a good anecdote for at the moment. It's units are abstract as well, but it's proportional to ampere-turns, and so designers universally think of H as ampere turns times some funny constant to make the units come out right.

There is no concept of frequency in the BH chart. Iron has no frequency, and BH charts are ways of making statements about the magnetic material. Frequency gets into the whole picture because you have to drive the iron to a certain amount of ampere-turns to get a certain flux density, and how fast you can get there depends on how hard you can drive the primary inductance (slope of the BH line). It's always V = L *di/dt, so how fast you can get to a certain I depends on how much voltage you can impress on the total number of turns around the iron. All this is external to the material itself, and BH curves are about the material.

And yes, there is some issue with saturation of high permeability materials with low signals, but as with all things magnetic, it happens a funny way. Ferromagnetism happens BECAUSE unpaired electon spin directions in the atoms of iron, nickel, and cobalt can be oriented by a magnetic field. This orientability is what lets an M field "flow" through ferromagnetic materials more easily than they flow through free space. When all the spins are in random directions inside the material as they are in virgin materials, making a few of them line up to follow a magnetic field is easy, so you get a lot of alignment for little energy input. That means, lots of field density for little MMF, so the BH slope is high and the inductance is high. As you get more and more of them aligned, it gets harder to get the NEXT atom's spin aligned because the easy ones to align are already in line. So you get incrementally less alignment and field strength per unit of MMF trying to align them., And the inductance is resultingly less. At some point, all of them are lined up, and with no more easy alignments to be made, the material is saturated, and any additional MMF can create B only at the rate it does it in free space. That's saturation.

Some special recipes for iron, nickel, cobalt, aluminum, copper, and other metals and elements can make it easier for alignment to happen than in a pure sample of the big three (iron, nickel and cobalt), and so there exist special alloys with higher incremental inductance as reflected by steeper slopes on their BH curves. This being the real universe, there is no free lunch and you have to pay for this with lower saturation flux density, higher losses in terms of bigger area inside the BH loop, or both. So yes, there are materials with insanely high per-unit inductability ( I just made that word up) but these often saturate at low flux densities, so you can have high inductance, but low saturation. Still depends on how many turns and how big a lump you're driving with signal, though.
2XEL84s with a p-p load impedance of 10k2 and a peak sine wave output of around 17.5W. But square wave output at full drive, i'm guessing +25W maybe? Let's account for alternate tuning and the rare bass player who may want to plug in and set the -3dB at 20Hz.
That means I require a primary inductance of around 81.5H rated for min 25W. (Ideally)
Now What? (lets make some big iron)
And this is where designing gets easier. You're designing for your own edification, so you can spend as much on iron and copper, and winding labor and insulating materials as you'd like. If your job is designing transformers, you get pay raises and promotions in return for making the next design return higher profit, not for making it pleasing.
 
Jazz P Bass 5/15/2018 9:53 AM
At a lecture, Oersted passed a compass over a current carrying wire.
The compass needle was deflected.
(What the heck was that!)

When the current in the wire was reversed, the compass needle also swung the other way.
(Holy, Moley!)

So the man is credited with the 'discovery' that a current carrying wire has a magnetic component,

In actual fact, to this day, no one knows exactly what he was trying to prove at that lecture.
 
Malcolm Irving 5/15/2018 10:45 AM
Quote Originally Posted by Jazz P Bass View Post
At a lecture, Oersted passed a compass over a current carrying wire.
The compass needle was deflected. …
Oersted having shown that electric current creates magnetism, Faraday had the idea 'well maybe magnetism should create electric current'. He put a large coil around a big permanent magnet, and measured current in the coil using a galvanometer. There was no current. But then he noticed that there was a current when he removed the coil from the magnet or replaced it on the magnet. Faradays law: induced voltage is proportional to 'rate of change of flux linkage'.
 
Mick Bailey 5/15/2018 11:07 AM
Design is one thing. Build is another - at least here in the UK. I was discussing transformer design with an amp builder who had done the design legwork and then went from factory to factory to get the things built. The best deal he got was to meet the minimum order of 10,000 stampings for each lamination type. The amps that used the transformers had a short run so he's left with lots of stock.

Even when I've approached companies to build short runs of replica transformers they've directed me to current standardized sizes using off-the peg components, as anything custom is way too expensive for the small numbers involved.
 
SoulFetish 5/15/2018 12:22 PM
Quote Originally Posted by Malcolm Irving View Post
Primary inductance is in parallel with the load reflected from the secondary, so for an audio transformer the tubes supplying the OT have to provide current into that parallel inductance as well as into the proper load. At low audio frequencies, the reactance (2.pi.f.L) of the primary inductance is low and the extra current causes the primary voltage to drop (due to voltage drop across the output impedance of the tubes - if you like).
.
See, I knew I came to the right place. This makes complete sense. Thank you, this is a great exclamation!
 
Gregg 5/15/2018 1:45 PM
Even when I've approached companies to build short runs of replica transformers they've directed me to current standardized sizes using off-the peg components, as anything custom is way too expensive for the small numbers involved.
If you need a small run of something at maybe half UK prices (without quality loss) you should check some central and/or eastern EU countries where you can find small businesses that will accept small orders without any problems. Knowing a local also helps a lot.
 
SoulFetish 5/15/2018 6:55 PM
Quote Originally Posted by R.G. View Post
Now it's measured in Teslas, after the guy who had the nerve to read a newspaper inside a lighting generation machine.
R.G. c’mon man, I obviously know who Tesla is! I don’t remember the lightning thing though, I wasn’t at that show.
https://youtube.com/watch?v=9vwHuCC6nP8
 
ric 5/15/2018 7:41 PM
Quote Originally Posted by SoulFetish View Post
R.G. c’mon man, I obviously know who Tesla is! I don’t remember the lightning thing though, I wasn’t at that show.
https://youtube.com/watch?v=9vwHuCC6nP8
Well yeah, he's the guy building those cars and flying rockets.

What I don't get is which is the right OT ...Hammond 1609 or 1608?

From simple recipes to building your own induction cooktop and genetically engineering the ingredients, it's all here.
 
Mick Bailey 5/16/2018 2:01 AM
Quote Originally Posted by Gregg View Post
If you need a small run of something at maybe half UK prices (without quality loss) you should check some central and/or eastern EU countries where you can find small businesses that will accept small orders without any problems. Knowing a local also helps a lot.
I needed some gearbox parts for an old Harley and sent the worn ones to a machine shop in Poland to get them replicated. Heard nothing back for over 6 months and then a greasy parcel arrived. I was knocked out by the quality, finish and fit. Proper materials, properly hardened.
 
SoulFetish 5/18/2018 6:23 AM
So.... What's my next move? Choosing a core lamination grade and begin the process of sizing it out?
From what I've been reading, it seems that there are more than a few designers/builders of transformers who prefer to use non grain oriented steels for guitar OTs. (I understand that you'll find M6-M50 type lamination, with a variety of winding styles historically).
For instance, I know that Heyboer uses M50 for many OTs, and a few designers prefered M19. Any thoughts either way on this?
 
Gregg 5/18/2018 7:13 AM
Since you're asking about iron grade here's your first idea for experiments: wind a bobbin and use the same one with several types of iron to see (hear) the difference. Use a miked cab to capture the sound otherwise it will be all subjective due to ear fatigue as well. Also make sure the amp has the same settings at all times.
Finding laminations retail in quantities for single transformers is not easy though. Most of the time the manufacturer will sell them as 20-22kg stack from specific grade.
Over the years I've used M330 (0.5mm) and M6 (M165-35S according to DIN, EN 10107 - this is how it's known in Europe) grade iron and the results were quite good however I wasn't trying to replicate any specific transformer sound.
 
J M Fahey 5/18/2018 4:31 PM
Donīt overthink it

As Enzo says, these are guitar amps, not NASA Mars Mission Landers.

You must *really* strethc design limits and carefully balance conflicting requirements in Hi Fi transformers, and thatīs fine, not audiophoolery by any means, but on *Guitar* amps?
Why try to:
* lower distortion from 0.5% to 0.2% when tubes by temselves have 5 to 7% distortion ... when *clean* , and are usually clipped anyway?
Reach 50kHz so 20kHz response is still flat when Guitar speakers drop at 24dB/oct above 3500Hz?
And a speaker reaching 4500Hz (Italian Jensen) is deemed "unbearably nails-on-blackboard harsh/icepicky?"
* guitar lowest frequency is >80Hz and in many respected amps (Marshall) itīs attenuated below 160Hz?
Mind numbing below 700Hz or so in VOX amps.
* transformer self resonant peaks which in Hi Fi amps must be tamed and when present held >>25 kHz become not that important when considering speaker bandwidth.
* hard to achieve low phase shift to allow higher NFB without breaking into oscillation is not important in amps which either have relatively low NFB factor to begin with (Fender) , attenuate/kill higher frequency NFB (which is where problems appear), think Tweed and Marshall Presence control, or plain have NO NFB at all (VOX and many others).

An OT must be very horrible to damage Guitar sound ... and in many cases restricted bandwidth may be a bonus.

I wind my own, interleave a little within reason, from 1/2 Pri - Sec - 1/2 Pri to **at most** 4 primary sections interleaved with 3 Secondary ones, and that for Hi Fi amps, consider Marshall does 1/2 Pri in one continuous wind, 0-4 ohm winding , 1/4 Pri , 4-8-16 ohm winding (which has exact same number of turns as 4-8) , 1/4 Pri so itīs 5 sections in total.

Champ transformers are a joke ... on paper .... are wound for 200Hz minimum frequency ... not that bad considering the tiny light speakers usually fitted to them also fall like a brick below 200Hz, so ...
 
SoulFetish 5/18/2018 7:26 PM
I appreciate this, Juan. I know you know your way around iron, and this is practical advice.
I'm actually not interested in achieving the kind of fidelity required by hifi designs. But, in this particular case, I do want to design for (relatively) wide bandwidth, avoid saturating the core, and operate at full drive output without breaking a sweat.
As far as some of the classic amps you mentioned go, IMO they often were able to overcome some of their deficiencies and sound good, I don't necessarily think they sounded good because of them. (Although, I'm really surprised at the 700Hz figure for voxes. I wouldn't have guessed that.)
At the other end of the spectrum, I currently have a transformer which was made using M6/-1dB@20Hz to -1dB@20kHz/probably heavily interleaved. I'm not sure if I like this type of OT for a guitar amp... I'm not sure if I don't either. I would like something to compare it to. Something in between this and not close to a champ.
 
Gregg 5/19/2018 2:15 AM
But, in this particular case, I do want to design for (relatively) wide bandwidth, avoid saturating the core, and operate at full drive output without breaking a sweat.
Most of the time guitar OT's can go down to 50 Hz at full power without problems and saturating the core is not so easy. Concerning the top end 15kHz is more than enough and is not difficult to achieve. You can use these as a start. In calculations shoot for -3dB. For guitar OT-1dB is overkill.


Couple of examples from real guitar OT's (and what you'll actually get from calculations at -3dB):

1/ 50W - iron sizes EI96x32-40mm,
primary 1600-1800 turns/ 0.25-0.3mm
secondary 60-68 turns/0.8-1.2mm
2/ 100W - iron sizes EI96x45-60, EI108x40-50mm, EI115x35-45mm
primary 1000-1200 turns/ 0.35-0.45mm
secondary 52-57 turns/1.2-1.5mm
The wire thickness for the secondary is for a single 4 Ohm winding which means that if you have two secondaries in parallel you must divide that by square root of 2 etc. All wire thicknesses are without insulation/lacquer.

After you get your numbers it's time to choose the winding configuration and there are many. Then you have to see what is the actual wire thickness (including the insulation/lacquer) and do some calculations how many turns will fit in one layer on your transformer bobbin so they can spread equally in all layers. Some corrections may be necessary but if you get to that stage we'll help.
 
Malcolm Irving 5/19/2018 2:23 AM
In a band context, with a Bass player, I think it sounds better when the low frequencies of the guitars are attenuated. Especially when there are two guitars, their low frequencies just muddy the sound and 'get in the way' of the clarity of the real bass line. (Keith Richards was famous for 'inventing' the five-string guitar by throwing away his low E string. )

For a solo jazz player, in the style of Joe Pass, a bit of hi-fi low frequency could be good, though.
 
J M Fahey 5/19/2018 7:16 AM
Quote Originally Posted by SoulFetish View Post
(Although, I'm really surprised at the 700Hz figure for voxes. I wouldn't have guessed that.)
I was also amazed when I calculated it

See for yourself: original VOX AC30 from the 60's

[IMG]http://www.voxamps.com/uploads/SupportPage_Downloads/ac301960.jpg[/IMG]

Check Bright Channel volume control and its coupling cap, at low volume you have C1 500pF and VR2 500k .
Use this handy online calculator:RC pad corner frequency upper and lower cutoff frequency calculation filter calculate time constant tau RC voltage power calculator capacitance resistance - sengpielaudio Sengpiel Berlin

Using those old series nominal values you get some 639Hz cutoff (at mild 6dB/octave, so you *still* have some Bass and Low Mids, just quite attenuated) , with modern normalized values it would be 470pF and 470k = 720 Hz. In practice same thing.

And that is with volume set low; when set to "10" capacitor also sees R9 and R7 to ground (depending on Normal Channel Volume setting, but which can be assumed is set to 0 if unused) so cutoff frequency starts at some 1200/1400 Hz

I BET Chris Jennings did not fire up his slide rule to calculate a cutoff frequency, but most probably had somebody playing his prototype LOUD and tried different capacitors until he found one which cleaned the distorted sound a lot .... tried and true design technique

FWIW revered Trainwreck amps are *basically* "a Fender amp with an extra tube for gain/sustain and a VOX type strong Bass cutoff for clarity"

[IMG]http://music-electronics-forum.com/attachments/1185d1195008139-express-input.jpg[/IMG]

Notice 0.002 coupling cap and 180k grid resistor at the third triode, which is also a cold cathode clipper, cutting below 440Hz.
The rest of the circuit is again a basic Fender with tweaked (Marshallish) Tone Control values.

Like at Mc Donalds, where they can offer a couple dozen "different burgers" using basic 6 or 7 "components".
Actually less, because they *always* need to use the bun and at least 1 patty
 
Helmholtz 5/19/2018 7:40 AM
Most of the time guitar OT's can go down to 50 Hz at full power without problems and saturating the core is not so easy.
True, but an OT being down 3dB@50hz at full power may be down 3dB@500Hz at low power.
Saturation headroom for a given core increases with the number of primary turns.
 
Gregg 5/19/2018 12:49 PM
True, but an OT being down 3dB@50hz at full power may be down 3dB@500Hz at low power.
I didn't quite get that?
 
Helmholtz 5/19/2018 1:28 PM
Quote Originally Posted by Gregg View Post
I didn't quite get that?
What is the specific question? (Maybe you missed my post #17)
 
J M Fahey 5/19/2018 5:39 PM
I think heīs baffled by the claim that at low power the OT cuts off Bass below 500Hz:
True, but an OT being down 3dB@50hz at full power may be down 3dB@500Hz at low power.
Saturation headroom for a given core increases with the number of primary turns.
something we do not experience when actually playing ... or even measuring amp frequency response on the bench.

A real World amp is as flat at 1W as at, say, 50% to 80% of full power (simply to put saturation out of the question).
yet I donīt doubt your experimental results, just think that a signal generator with its tiny weak output, most are designed to drive >600 ohms and, say, 1V RMS which means some 1.5 mA current capability definitely can not meet any magnetizing current requirements .... but any real World amplifier can pre-magnetize (consider it some kind of magnetic bias) output iron simply with power tube current imbalance ... let alone if itīs a Class A output.

Just guessing but find it a plausible explanation for the fact that actual Guitar amps do not show that huge Bass loss you mention.
 
nickb 5/20/2018 2:37 AM
Quote Originally Posted by Helmholtz View Post
Lp values this high are quite realistic in guitar amplifier OTs. But only around rated output power and at low frequenies (max. ac flux). I measured Lp of around 40 different OTs at 650Vpp and 50Hz and found Lp to lie between 60H and over 300H. But when measuring with an LCR meter @ 1kHz, the values were lower by a factor of 5 to over 10. (The reason for the different values is that the LCR meter measures at very low flux amplitudes where the ac ĩ is close to the initial permeability. The ac ĩ rises strongly with flux amplitude up to the onset of saturation.)

The results seem to indicate that the major design concern was to avoid excessive power loss from high magnetizing current at max. output, while the bass response at low output power may suffer. Of course NFB can take care of this.
I think the variation of permeability is something that is easily overlooked. Well, by me at least...

I did find a nice plot of permeability vs flux density here fig 27, which incidentally is a great introductory text on transformers (start here). Do take note that the x axis is a log scale so you only see the biggest lowering of permeability at very low flux densities. I doubt it has any audible effect in practice but does serve to illustrate that the simple transformer models need to be applied carefully.

[ATTACH=CONFIG]48899[/ATTACH]
 
jmaf 5/20/2018 6:35 AM
Quote Originally Posted by Gregg View Post
I didn't quite get that?
Iron cores are slow. As you increase frequency the core will be unable to react in due time and eventually power will get lost more and more easily.

3 dB loss means losing "half the power". So what he said is as you raise the frequency the point where you lose half your power to core saturation is lower and lower.
 
Helmholtz 5/20/2018 7:49 AM
I think heīs baffled by the claim that at low power the OT cuts off Bass below 500Hz:
True, but an OT being down 3dB@50hz at full power may be down 3dB@500Hz at low power.
Saturation headroom for a given core increases with the number of primary turns.
something we do not experience when actually playing ... or even measuring amp frequency response on the bench.
I admit that the example I gave is extreme and rather pessimistic. It was meant to sensitize to not only measure frequency response (and primary L) at rated power in cases where strong bass response matters. Of course the OT's bass response only directly shows without NFB. The magnetizing L of the OT decreases at lower currents and as the -3dB point depends on the ratio of reflected load to Lmag (Raa/Lmag), it will shift to higher frequencies at lower power. I think an increase of the OT's corner frequency by a factor of 3 at reasonably low levels is not an unrealistic assumption. NFB can compensate.

Imbalance means DC offset/bias and this further decreases primary/magnetizing inductance and results in a higher corner frequency.
 
R.G. 5/20/2018 8:40 AM
Just like the current gain of a transistor cannot be expressed as one number, neither can the primary inductance of a transformer or the permeatility of a loop of iron. The BH curve for a magnetic material is just that - a curve - and the "inductivity" of the material is the slope of that line at any given point to small signals. To large signals, the signal gets distorted by this constantly-changing loading.

The size of the primary inductance does indeed change with the size of the signal applied, and its frequency (that's a side effect I didn't get into in the preceding polemics ) and with any "operating condition" such as a smaller signal riding on a much lower frequency.
 
Mike Sulzer 5/21/2018 11:40 AM
Quote Originally Posted by R.G. View Post
The BH curve for a magnetic material is just that - a curve - and the "inductivity" of the material is the slope of that line at any given point to small signals.
It is not that simple. The observed effect is larger inductance at larger flux levels (higher transformer power). This requires an analysis that takes into account the movement around the hysteresis loop. I do not think that it is so easy to get a good intuitive understanding of why the inductance is higher at higher levels.
 
Malcolm Irving 5/21/2018 12:19 PM
I have on old textbook: 'Transformers for Electronic Circuits' by Nathan R. Grossner, 2nd Edition, 1983.

On page 8 it shows a family of B-H curves. For small values of B and H, the 'principal axis' (if I can call it that) of the B-H loop is rotated clockwise.

Also, on page 181 he states that: 'Initial permeability … at the instep of the curve is physically associated with domain growth and reversible boundary displacement'.
 
SoulFetish 5/21/2018 8:58 PM
Quote Originally Posted by Malcolm Irving View Post
on page 181 he states that: 'Initial permeability … at the instep of the curve is physically associated with domain growth and reversible boundary displacement'.
- goes without saying I'm sure.

But there's still one thing that confuses me in this whole conversation. What the hell does "'teaching my grandmother to suck eggs" mean?? Sounds very British, and with the last name Churchill, I feel like I should know this!
 
SoulFetish 5/21/2018 9:00 PM
Quote Originally Posted by nickb View Post
I think the variation of permeability is something that is easily overlooked. Well, by me at least...

I did find a nice plot of permeability vs flux density here fig 27, which incidentally is a great introductory text on transformers (start here). Do take note that the x axis is a log scale so you only see the biggest lowering of permeability at very low flux densities. I doubt it has any audible effect in practice but does serve to illustrate that the simple transformer models need to be applied carefully.

[ATTACH=CONFIG]48899[/ATTACH]
Nice work, Nick! I just bookmarked those and am looking forward to the read.
 
Gregg 5/22/2018 1:19 AM
Don't get scared from the theory talk. It may be interesting (to some) but most of it has no practical use whatsoever. When you start calculating an actual OT you'll see what I mean.
 
Malcolm Irving 5/22/2018 2:17 AM
Quote Originally Posted by SoulFetish View Post
… What the hell does "'teaching my grandmother to suck eggs" mean?? …
It means 'trying to teach something to someone who probably already knows more about it than you do'.

It is harder to say why anybody would want to suck an egg though. On the internet somewhere I found that in Europe and Russia, in an earlier century, kids were taught to put a pin-hole at each end of an egg and suck out (and consume) the contents as a 'health tonic'. Not recommended these days due to fears of Salmonella etc.

Probably there are old-fashioned practices in guitar amp design which fall into a similar category.
 
J M Fahey 5/22/2018 3:28 AM
Quote Originally Posted by Malcolm Irving View Post
It means 'trying to teach something to someone who probably already knows more about it than you do'.

It is harder to say why anybody would want to suck an egg though. On the internet somewhere I found that in Europe and Russia, in an earlier century, kids were taught to put a pin-hole at each end of an egg and suck out (and consume) the contents as a 'health tonic'. Not recommended these days due to fears of Salmonella etc.

Probably there are old-fashioned practices in guitar amp design which fall into a similar category.
Well, MY Grandmother used to do that when she was young, tired, and needing a little Energy boost without stopping her farm work. We are talking 1928 or so.

I am quite certain that Salmonella was not such a big threat way back then, since chickens were free to roam in the open, all around the farm house, eating insects, "natural" seeds and vegetables and whatever food scraps were thrown at them.

The real "infection factories" are modern concentration camp type chicken farms: 5000 or 10000 chickens growing inside a huge "barn", with lights on 24/7 and as I was told, subject to noise so they never sleep, running all over and trampling each other, and of course over chicken poop covered floor.
Thatīs why they are pumped with tons of antibiotics, they are in such crowded contact that "one sick bird means **all* sick in a matter of hours" and so losing the whole batch.

Eggs , specially raw ones, were considered a quick and gfood source of Protein.

I didnīt personally see my Grandma suck raw eggs from the shell although itīs Famiy tradition and one of my old Aunts mentioned "she kept the nail Grandma used to punch eggs in her youth" , go figure.

What I DO clearly remember is visiting "Milk Bars" which where very popular when I was a kid (late 50īs) , and seeing mid to old aged guys ordering "Candéal", which was made on the counter by beating inside a glass a raw egg (or was it just the yolk?), a "measure" (the standard sized ration, probably a fluid ounce or so) of Brandy and a teaspoonful of sugar.
Might also have a drop of vanilla or a pinch of cinnamon.
Distantly related to Eggnog but without even a drop of milk and always with alcohol.

Besides the Health angle, I guess it was probably a loophole to allow "officially non drinking" people to get a couple shots now and then.
 
SoulFetish 5/24/2018 12:50 AM
should we designing for a some DC content due to stage imbalance (as is common in guitar output stages)? From some of my reading, if even a small 3% of DC offset is present it can result in a large error, increasing the magnetizing current to 135%. Umm,... what kind of things can we do about that?

see article below:

[ATTACH]48939[/ATTACH]
 
Gregg 5/24/2018 1:15 AM
No. With articles like this you're about to set foot onto the very well oiled high slope HiFi surface
 
eschertron 5/24/2018 5:26 AM
From my quick skim through the article, the 3% appears to refer to % of full load current. This would require a 20..25% imbalance at idle. At which point there are other, more audible, side effects to worry about. I'm thinking heater hum.
 
Helmholtz 5/24/2018 6:46 AM
Quote Originally Posted by SoulFetish View Post
should we designing for a some DC content due to stage imbalance (as is common in guitar output stages)? From some of my reading, if even a small 3% of DC offset is present it can result in a large error, increasing the magnetizing current to 135%. Umm,... what kind of things can we do about that?

see article below:

[ATTACH]48939[/ATTACH]
Thanks for the interesting link. One of the most important aspects of transformer design is to avoid core saturation under worst case conditions. And these have to include a reasonable amount of DC offset in the peak value of H. For a given transformer design saturation depends on the peak magnetizing current Imag. Increasing the number of primary turns decreases Imag and thus increases saturation headroom.
Saturation does not depend on the load current/power.
 
Gregg 5/24/2018 9:05 AM
In practice OT saturation in a PP guitar amp under it's usual operating conditions is a non issue and very unlikely to happen (despite current imbalances) unless it's a poorly designed cheap iron crappy OT.
 
Helmholtz 5/24/2018 9:15 AM
Quote Originally Posted by Gregg View Post
In practice OT saturation in a PP guitar amp under it's usual operating conditions is a non issue and very unlikely to happen (despite current imbalances) unless it's a poorly designed cheap iron crappy OT.
Yes, but I thought this thread was about ab initio DIY design of OTs. Saturation easily occurs when Nprim is chosen too low.
 
R.G. 5/24/2018 9:19 AM
Quote Originally Posted by Mike Sulzer View Post
It is not that simple. The observed effect is larger inductance at larger flux levels (higher transformer power). This requires an analysis that takes into account the movement around the hysteresis loop. I do not think that it is so easy to get a good intuitive understanding of why the inductance is higher at higher levels.
Of course it's not that simple. However, when you're trying to introduce complex, slippery subjects to someone new to the concepts, you start with the simplest views first, then add on modifications as they get the grosser concepts under control. I didn't get into the quantum mechanical view of how magnetism arises, either.

Quote Originally Posted by Malcolm Irving View Post
I have on old textbook: 'Transformers for Electronic Circuits' by Nathan R. Grossner, 2nd Edition, 1983.
Hang on to that book. It's one of the few textbooks I still refer to. Solid gold.

Quote Originally Posted by Gregg View Post
Don't get scared from the theory talk. It may be interesting (to some) but most of it has no practical use whatsoever. When you start calculating an actual OT you'll see what I mean.
And we have another winner. Practical considerations of how much iron and copper you can afford and get into the available space will be much larger than highly detailed theory. I do find that theoretical considerations about the iron are much more necessary in the design of a SE output trannie, where you're actually relying on the core to store energy sufficient for one half cycle of the biggest outputs.

Quote Originally Posted by SoulFetish View Post
should we designing for a some DC content due to stage imbalance (as is common in guitar output stages)? From some of my reading, if even a small 3% of DC offset is present it can result in a large error, increasing the magnetizing current to 135%. Umm,... what kind of things can we do about that?
see article below:
As a practical matter, don't sweat designing for DC offset in a push-pull design. The way DC offsets are always handled is to introduce some air gap into the iron core. In non-toroidal cores, it is impossible to NOT introduce slight air gaps. There's a great deal of effort expended in stacking E-I laminations to reduce these air gaps as much as possible.

Most "toroidal" transformers are made by winding a coil of transformer iron into the toroidal core. There is still some effect of air gap because the iron is not really continuous, but is in thin layers with mostly air (or varnish) in the gap. It's much less air gap than E-I laminations. The article you found looked at offset effect in punched rings of iron; this is done so the testing can actually have a single loop of real iron to test the iron completely without air gaps. It is and is intended to be completely free of air gaps and the real material --can--- be tested. This is most definitely not a real-world condition.

As a practical matter, no transformer outside some lab setups is a true toroid. This is because (1) it's so very difficult to get a true, solid-iron toroid to wind and (2) solid iron toroids are impractical because they are solid. You want the core iron to be divided into very thin laminations so that eddy currents in the iron itself are thwarted by being forced to run in circles in a thin piece of iron. Eddy current losses act like a nonlinear resistance in parallel with the primary. The magnetic field in the iron itself causes current to flow in loops in the iron, which is conductive itself. Lamination breaks up the electrical path in the iron layers and makes the path for eddy currents be thin and higher resistance, cutting the losses.

As a side note, reducing eddy current losses is one reason that transformer iron is made with silicon in the alloy. The silicon makes the iron be much higher in electrical resistivity, further reducing the losses to eddy currents in the iron itself.

Eddy current losses and lamination is another of those cross-purposes things so common in transformers. Assuming you could get your core made from a solid lump of iron in the right shape and still thread your copper into the right places, you'd have horrible losses to eddy currents from current loops in the iron. Splitting the iron into laminations allows you to (1) wind the transformer coils then stack the iron around it, saving a whole lot of labor and (2) dramatically reduce eddy current losses. So the more laminations, approaching iron foil, the better, right?

Wrong. The thinner the laminations, the harder they are to stack and the more fragile they are, and also the more air gaps you introduce between laminations. The transformer industry homed in on a narrow range of lamination thicknesses as the relative optimums for most transformers. You can get super thin special material laminations, but the price, difficulty in stacking, and other ugly issues get out of hand rapidly. That's one reason you don't see iron foil cores, or pure-nickel OTs.

DC offset, air gaps, eddy current losses, the list of competing things to optimize just goes on and on.

Quote Originally Posted by Helmholtz View Post
Thanks for the interesting link. One of the most important aspects of transformer design is to avoid core saturation under worst case conditions. And these have to include a reasonable amount of DC offset in the peak value of H. For a given transformer design saturation depends on the peak magnetizing current Imag. Increasing the number of primary turns decreases Imag and thus increases saturation headroom.
Saturation does not depend on the load current/power.
That brings up another of those sideways steps. We all (should have) learned that V = L * di/dt. Or, put another way, di = V*dt/L. This is a way of saying that the magnetic field change in the iron depends on the voltage applied and the time it's applied. That assumes a constant value for L, which we know is not strictly true, but let's start simple.

The current in windings of an inductor is equal to the volt-time integral divided by a constant (L in this case, which isn't completely constant...) and this current is the current that drives the ampere-turns in the B-H curve. It has almost no relation to the current which merely goes THROUGH the magnetic field and out to the secondaries. As a sidelight, this is also the reason that transformers are frequency-sensitive - the core has to be wound to NOT go into saturation on a full half-cycle of applied primary voltage from zero crossing to the next zero crossing. Looked at another way, the core plus windings give you a certain volt-time "endurance" before you hit saturation. If you double the frequency, you halve the time, and you can then apply twice the voltage and still stay out of saturation.
 
Malcolm Irving 5/24/2018 9:33 AM
Quote Originally Posted by R.G. View Post
.... We all (should have) learned that V = L * di/dt. Or, put another way, di = V*dt/L. This is a way of saying that the magnetic field change in the iron depends on the voltage applied and the time it's applied. That assumes a constant value for L, which we know is not strictly true, but let's start simple.
...
Yes. I seem to remember from a theory class, in the dim and distant past, that the full formula is:

V = L di/dt + i dL/dt

But if inductance L is constant, the second term vanishes, of course.

In iron-cored coils, where reluctance is varying, the inductance of the coil would also vary, making the situation complicated.
 
Helmholtz 5/24/2018 12:07 PM
Quote Originally Posted by Malcolm Irving View Post
Yes. I seem to remember from a theory class, in the dim and distant past, that the full formula is:

V = L di/dt + i dL/dt

But if inductance L is constant, the second term vanishes, of course.

In iron-cored coils, where reluctance is varying, the inductance of the coil would also vary, making the situation complicated.
The OTs' primary inductance is not constant at all. As my measurements (above) and manufacturers' information show, L rises with increasing current typically by around a factor of more than 4 (even though there is some unavoidable airgap) up to a maximum from where it decreases steeply.

In my transformer and choke designs, avoiding saturation always was a major concern.
 
Gregg 5/24/2018 1:16 PM
In my transformer and choke designs, avoiding saturation always was a major concern.
Chokes and SE OTs are a different story. PP OTs however are very unlikely to saturate. Over the years I was able to saturate guitar PP OTs only with 30-40Hz signals which they were not designed to handle anyway. In rare cases I've seen some of them go down to 35Hz at -3dB.
Contrary to the popular belief even a toroidal OT won't saturate with DC imbalance. I've tried and seen that myself with one tube running at 30mA the other at 40mA and I'm talking full power not just couple of Watts.
 
Helmholtz 5/24/2018 1:23 PM
Quote Originally Posted by Gregg View Post
Chokes and SE OTs are a different story. PP OTs however are very unlikely to saturate. Over the years I was able to saturate guitar PP OTs only with 30-40Hz signals which they were not designed to handle anyway. In rare cases I've seen some of them to go down to 35Hz at -3dB.
Contrary to the popular belief even a toriodal OT won't saturate with DC imbalance. I've tried and seen that myself with one tube running at 30mA the other at 40mA.
Short and simple: For a given core, saturation determines the minimum number of primary turns for max. peak current to be expected.
 
R.G. 5/24/2018 2:46 PM
Quote Originally Posted by Gregg View Post
Contrary to the popular belief even a toroidal OT won't saturate with DC imbalance. I've tried and seen that myself with one tube running at 30mA the other at 40mA and I'm talking full power not just couple of Watts.
Whether they will saturate with DC imbalance or not greatly depends on the size of the imbalance and whether it's a current limited imbalance or a voltage imbalance that can ramp up over time. Practical toroids necessarily have some air gap no matter how you try to reduce it. It's just much smaller than you can get with EI stacks.

And I did see a write up of a fellow whose high-dollar hifi setup started humming. A lot. But only after dark. The power amps had toroidal PTs. A great deal of fire drill activity ensued, and only ended when he remembered putting those light-dimmer life-extending pellets in his outdoor garage lights. Got the pellets in the same electrical direction for both of them, and this introduced a diode's worth of voltage offset in his AC ///voltage///. There's a lot of current available there, at least the total current offset made by the garage lights and the resistance of the toroid primary windings, so it walked the toroids up to saturation. Removing the diode pellets fixed it.

Toroids are one of those be-careful-what-you-wish-for things.

Quote Originally Posted by Helmholtz View Post
Short and simple: For a given core, saturation determines the minimum number of primary turns for max. peak current to be expected.
Not exactly. Core saturation determines the number of turns and core area needed to withstand a certain amount of volt-time integral. That's not directly related to the peak output current to be expected. Primary magnetizing current is not the same as reflected secondary current. The two are only very indirectly linked. It's not in general possible to saturate a transformer core from the secondary. There is an exception to this for half wave rectification in small transformers with high resistance primary windings, and I have never seen this exception demonstrated.

You pick wire sizes for the secondary and primary so that the full load current (or max peak rms average ) can be managed without spending all your power heating transformer wires. Given that wire cross section, you proceed to pick a transformer with enough core area and window area to get your wires inside the window and still get enough turns in to get a primary inductance that does not let you get into saturation (very far... saturation is a soggy, soft slippery slope with most transformer iron), then you go back and see if you need to increment wire size because you've wound so many turns, and then so see if you have to add more stack to reduce turns by adding core area or jump to the next core size.

If an AC-line transformer is running a no-load magnetizing current more than a few percent of its full load current, it's defective or of low quality in materials and/or design. If your employee turns in such a design, you educate him/her or fire her/him. Given that no-load current is and must be so small, the current to magnetize the core is inconsequential in dealing with the peak loads.

Quote Originally Posted by Helmholtz View Post
The OTs' primary inductance is not constant at all. As my measurements (above) and manufacturers' information show, L rises with increasing current typically by around a factor of 10 (even though there is some unavoidable airgap) up to a maximum from where it decreases steeply.
That is correct. Core inductance is very, very much not a constant. The nature of the iron and the BH curve it traverses, including minor loops and offsets make sure of that. A conservative OT designer gets familiar with his iron and picks some minimum value to be used, then designs as though the core will always show that minimum inductance, all the while knowing he may and almost certainly will get more at some operating conditions.

Quote Originally Posted by Malcolm Irving View Post
Yes. I seem to remember from a theory class, in the dim and distant past, that the full formula is:
V = L di/dt + i dL/dt
But if inductance L is constant, the second term vanishes, of course.
In iron-cored coils, where reluctance is varying, the inductance of the coil would also vary, making the situation complicated.
That is correct. Most transformer practice was codified long before we had machine ways to handle systems of partial differential equations. The transformer designers of that time went off and made up charts and graphs of measure quantities of every transformer they built and use that as a way to iterate in on a design. The history book of transformer designs made and measured was a highly treasured intellectual asset of every transformer design shop. A great deal of transformer practice depends on knowing some kind of bounds on what you might see, then going off and designing with small variants and extrapolations of what you did see.
 
Helmholtz 5/24/2018 3:01 PM
Quote Originally Posted by Helmholtz View Post
Short and simple: For a given core, saturation determines the minimum number of primary turns for max. peak current to be expected.
Not exactly. Core saturation determines the number of turns and core area needed to withstand a certain amount of volt-time integral. That's not directly related to the peak output current to be expected. Primary magnetizing current is not the same as reflected secondary current. The two are only very indirectly linked. It's not in general possible to saturate a transformer core from the secondary. There is an exception to this for half wave rectification in small transformers with high resistance primary windings, and I have never seen this exception demonstrated.

As said, for a given core. Of course I meant max. peak value of magnetizing (not total) current. In a well coupled OT the (symmetrical) load current has no influence on saturation as primary and secondary load fluxes cancel. Magnetizing current is measured without load (open secondaries).
 
nickb 5/24/2018 3:26 PM
Quote Originally Posted by R.G. View Post
Most transformer practice was codified long before we had machine ways to handle systems of partial differential equations. The transformer designers of that time went off and made up charts and graphs of measure quantities of every transformer they built and use that as a way to iterate in on a design. The history book of transformer designs made and measured was a highly treasured intellectual asset of every transformer design shop. A great deal of transformer practice depends on knowing some kind of bounds on what you might see, then going off and designing with small variants and extrapolations of what you did see.
Don't underestimate the importance of this point. This is why I write the specs and hand them over to the transformer manufacturer to do the actual design. Not one failure so far equates to much happiness.
 
R.G. 5/24/2018 3:44 PM
Quote Originally Posted by Helmholtz View Post
As said, for a given core. Of course I meant max. peak value of magnetizing (not total) current. In a well coupled OT the (symmetrical) load current has no influence on saturation as primary and secondary load fluxes cancel. Magnetizing current is measured without load (open secondaries).
I think I misunderstood your comment, for which I apologize. I think I mentioned earlier that there are many ways to view a transformer's operation. I tend to view it from the standpoint of the volt-time integral for most core operations.

Yes, for a given core (which sets the magnetic path length and for a given stack of laminations sets the core area and also residual air gap) there is a maximum ampere-turn product you can allow. That ampere-turn MMF results in a flux density, which you want to keep out of the soft knee of saturation. So you keep adding turns until the designed criteria of maximum AC input voltage and minimum half-cycle time do not let a a magnetizing current flow that causes enough ampere turns to get the flux too deeply into saturation.

From the volt-time point of view, the core allows a current through that is the time integral of V(t)dt/L, and in a real world, non-simplified view includes the fact that L is actually L(t), or more accurately, L(i) where i is also time varying. I just view the magnetizing current in the primary as a result of volt-time. Raising the number of primary turns increases L by the square of the increase ratio whatever the "L" is as a time function, and that in turn slows down the magnetizing current that gets ramped up to.
 
Malcolm Irving 5/24/2018 4:07 PM
One experiment to find out more about the magnetising current in guitar amplifier output transformers would be to run tests with the OT secondary open-circuit! Horror of horrors - that is exactly what you shouldn't do to a tube guitar amp! But if we can guarantee to keep the input signal sinusoidal and only at low frequencies, I think it would be OK. The magnetising inductance would present a low impedance load (albeit inductive) to the tubes.

We could then measure the current going into the OT primary and see a pure magnetising current waveform (not obscured by load current reflected from the secondary).

Any volunteers?
 
Helmholtz 5/24/2018 4:26 PM
Quote Originally Posted by Malcolm Irving View Post
One experiment to find out more about the magnetising current in guitar amplifier output transformers would be to run tests with the OT secondary open-circuit! Horror of horrors - that is exactly what you shouldn't do to a tube guitar amp! But if we can guarantee to keep the input signal sinusoidal and only at low frequencies, I think it would be OK. The magnetising inductance would present a low impedance load (albeit inductive) to the tubes.

We could then measure the current going into the OT primary and see a pure magnetising current waveform (not obscured by load current reflected from the secondary).

Any volunteers?
No problem, if you disconnect the feedback loop and measure at low frequencies, where the peak current and the voltage-time-integral is maximal. Alternatively you could measure at line frequency with a step-up transformer and a variac. But the standard method is to measure the voltage-time-integral, which determines primary net flux and does not depend on the load.
 
Malcolm Irving 5/24/2018 4:31 PM
Quote Originally Posted by Helmholtz View Post
... Alternatively you could measure at line frequency with a step-up transformer and a variac. … .
Well yes, but what I was interested to find out more about is how much the output tubes can drive the OT into saturation. (I should have said that.)
 
Helmholtz 5/24/2018 5:06 PM
Quote Originally Posted by Malcolm Irving View Post
Well yes, but what I was interested to find out more about is how much the output tubes can drive the OT into saturation. (I should have said that.)
Just measure the max. V/f-value the amp can produce at the primary with a (speaker) load and adjust the variac accordingly. I mention speaker load because the speaker impedance rises considerably (up to a factor of maybe 10) at the bass resonance, allowing primary voltage to rise.
But if you have a scope with an integrating function, it is simpler to measure the max. primary voltage-time-integral over a half-wave.
 
SoulFetish 5/24/2018 8:18 PM
Quote Originally Posted by nickb View Post
Don't underestimate the importance of this point. This is why I write the specs and hand them over to the transformer manufacturer to do the actual design. Not one failure so far equates to much happiness.
Could you elaborate on this point. I'm realizing that the best way to learn and understand the concepts of transformer design and operation intuitively is to actually build one(or more). But, I would like a professional, working transformer by the end of June. So I may have one wound for me while I read over the texts and materials and hopefully continue this conversation.
 
nickb 5/26/2018 5:45 PM
Quote Originally Posted by SoulFetish View Post
Could you elaborate on this point. I'm realizing that the best way to learn and understand the concepts of transformer design and operation intuitively is to actually build one(or more). But, I would like a professional, working transformer by the end of June. So I may have one wound for me while I read over the texts and materials and hopefully continue this conversation.
There's not much to say really (sorry to disappoint). I've always been able to find suitable OPTs off the shelf so it's really PTs that are of interest which I don't think is your focus. For these I specify the nominal primary and secondary voltages, the max VA and max primary voltage, the frequency (50/60 Hz), then each secondary voltage, regulation and current. Also mechanical stuff: general construction i.e drop thru', maximum dimensions etc of course. All one or two off replacements so cost is not is not the most important factor.

Well there is an exception which is a funny little story. Dude came to me in a panic with his dead (very expensive) boutique amp as he has a big recording session in a couple of days and he MUST have this amp. Naturally no schematics. Turns out the OPT has a short. The manufacturer and winder are both out of stock. The design used a couple of bottles running a single ended class A and each one drives a separate winding on the OPT. The only thing I can find that will physically fit is a push pull type. So I explain it all to the customer and he's happy for me to modify it to class AB push-pull with about twice the output power and then change it all back once crisis is over and I can get the original part. I did the work. He did the session and never came back to get it reinstated. He was happy with how it was. I'm saying no more. Draw your own conclusions.


PS: Here is a pretty good practical guide and a great insight into the process [ATTACH]48957[/ATTACH]
 
J M Fahey 5/27/2018 6:04 AM
Quote Originally Posted by SoulFetish View Post
Could you elaborate on this point. I'm realizing that the best way to learn and understand the concepts of transformer design and operation intuitively is to actually build one(or more).
Well, not really

That will give you some practical skills, but nothing on Theory or new design knowkledge.

You would need to build *many* , varying winding turns per volt, interleaving, core material, core stacking, gap if introduced on purpose, and then measure and compare them all, to see what affects what and how much.

That is not happening for any standard Tube amp builder, not even for Commercial "wind by the book" or "clone standard stuff" winders, I bet only a few Pro winders, think MM, Hammond, Schumacher, Drake, Heyboer, Dagnall, etc. will do that.

Way too high for us, mere Mortals.
 
J M Fahey 5/27/2018 6:20 AM
Quote Originally Posted by nickb View Post
There's not much to say really (sorry to disappoint). I've always been able to find suitable OPTs off the shelf so it's really PTs that are of interest which I don't think is your focus. For these I specify the nominal primary and secondary voltages, the max VA and max primary voltage, the frequency (50/60 Hz), then each secondary voltage, regulation and current. Also mechanical stuff: general construction i.e drop thru', maximum dimensions etc or course. All one or two off replacements so cost is not is not the most important factor.

Well there is an exception which is a funny little story. Dude came to me in a panic with his dead (very expensive) boutique amp as he has a big recording session in a couple of days and he MUST have this amp. Naturally no schematics. Turns out the OPT has a short. The manufacturer and winder are both out of stock. The design used a couple of bottles running a single ended class A and each one drives a separate winding on the OPT. The only thing I can find that will physically fit is a push pull type. So I explain it all to the customer and he's happy for me to modify it to class AB push-pull with about twice the output power and then change it all back once crisis is over ans I can get the original part. I did the work. He did the session and never came back to get it reinstated. He was happy with how it was. I'm saying no more. Draw your own conclusions.
Happens all the time, meaning the Booteek babble is usually that, babble, no real improvement (in at all) over "things properly done the established way".

He must have found that regular Push Pull is better (certainly louder) than his clunky "side by side single ended" arrangement.
Definitely louder , and strains transformer core way less.
FWIW I know two guys who carried the single ended concept (which I admit has an interesting flavour of its own) a little beyond its comfortable area (which is, say, a single 6V6/EL84 for very good 5W); one is a Brazilian friend who makes a single KT88 "20W" amp. Must actually be 14/15W at most.
Everybody "loves" the idea .... but their "wallet votes" say the contrary, he hardly sold any, beyond the original order.
290*| AcedoAudio Amplificadores Valvulados
It sounds good, but maybe the fact that it uses a humongous OT (old style Fender OT), has only 14/15W , not enough to play along a (Brazilian or Argentine) Drummer, and is quite expensive for what it is, has conspired against sales.
More important, I fail to hear a sonic advantage over a similar, conventional PP 15W amp, which sounds as good and is way less expensive.

And now an Argentine colleague is offering a similar , "15/18W" one with *three* 6V6 in parallel.
Oh well, I wish him luck ... heīll need it.
 
Helmholtz 5/27/2018 6:23 AM
...and don't underestimate the safety aspects like creepage and clearance distances, hi-pot testing and the likes.
 
J M Fahey 5/27/2018 6:35 AM
Quote Originally Posted by Helmholtz View Post
...and don't underestimate the safety aspects like creepage and clearance distances, hi-pot testing and the likes.
I very much doubt booteek makers worry about that
 
R.G. 5/27/2018 9:42 AM
Juan's right, all counts.

Winding transformers is a different skill from designing transformers. The design process involves picking core material type, core lamination size and stack, copper wire sizes, division of windings into sections and layers, computing the build height and width interatively, picking layer materials, and choosing insulation classes for wire, former, and layer insulation.

Once you get the design done, you can start procuring materials and finally winding.

The materials are not prohibitively expensive, but they are hard to find in small quantities. So while you're messing with your design, it would be good to be finding suppliers. It's hard to buy just enough of the materials. iron core laminations, magnet wires, interlayer insulation sheet, core former tubes, insulating tape, and so on. Minimum quantities will be a constant misery on all of these.

You're probably going to need to make or buy some kind of winding machine. Both hands will be busy guiding the wire where it goes on each layer, taping things in place, and so on. It is entirely possible to build something like this, but it's a PITA. I had access to a prototyping transformer winder at my work way back when, along with a selection of core sizes, magnet wire, insulating sheet and tapes. And it was still a lot of work winding a transformer.

I've done some transformer winding for myself, in my garage. Juan's done more of this than I have, I think, so he can provide a good bit of advice on the practicalities. My take on it is that is that you have to really LIKE the process of winding the things and doing all that work, buying and stockpiling all the supplies and so on, to do the several iterations of each transformer design that have historically been necessary. That is, you would need to be into transformer design and building as a hobby nor business.

I must admit - I'm not. I have some understanding of and experience with design and winding, but I don't love it enough to make a hobby of it.
 
SoulFetish 5/27/2018 12:46 PM
Quote Originally Posted by J M Fahey View Post
Well, not really

That will give you some practical skills, but nothing on Theory or new design knowkledge.

You would need to build *many* , varying winding turns per volt, interleaving, core material, core stacking, gap if introduced on purpose, and then measure and compare them all, to see what affects what and how much.
Yeah, but that just sounds like a lot of work

 
J M Fahey 5/27/2018 1:30 PM
Yes, thatīs the point.
Thatīs why I said winding a few will definitely improve your ... um .... "mechanical" skills.

As of designing, measuring, testing and comparing different designs, obviously itīs also possible, but it takes a lot more time and dedication.
And $$$$$.
 
Tom Phillips 5/27/2018 4:31 PM
I concur with Juan. You may wish to consider winding a transformer for practice and to find out first hand the skills needed and the hassles involved. I suggest the approach of re-winding an existing transformer so that you don’t have to procure all the parts. You could do a PT for the first project and you might even take the opportunity to make a special configuration that is not available off-the-shelf but which you need for a project. Donor transformers can be defective ones that you have removed during an amp repair or an old transformer that doesn’t currently have voltage ratings useful to you.

I wound a couple of PTs when I built my first amps and I just recently found the QST magazine article that I used as a reference. Attached is a copy of that article. It is very basic as it was written for the novice. The article along with a copy of the Radio Armature’s Handbook and the RCA receiving tube manual were my source of initial technical information. After a lot of scrounging on a high schooler’s budget and many interesting learning experience mistakes I ended up with a working 6G14A Showman head.

[ATTACH]48962[/ATTACH]

I also find that it is a good learning experience to take things apart. It's interesting to see how they were built even if you don't plan on repairing the thing.
 
nosaj 5/27/2018 5:15 PM
Tom you Rock. Thanks for pulling out these old articles for us.

nosaj
 
R.G. 5/27/2018 6:45 PM
That's the kind of stuff that hooked me on electronics (and transformers) back in the 1960s. Great article, Tom.

The bones of a rewind-your-driver-transformer article is up at geofex:
Thomas Vox Driver Transformer
It outlines some of the skills to rewind a small mains donor transformer into a replacement for the otherwise-unavailable driver transformers for the Thomas Vox amps. It includes some tips that make the process simpler, like dummy spacer winds at the ends of layers to preserve layer margins and edge stability, and some tips on impregnation. Of course, winding on a coil form with ends makes this much easier than winding self-supporting layers in a stack.

I had an article up on de/re-winding a semi-toroidal trannie to give many secondaries for isolated pedal supplies. I took that down because of the great lurking demon of everything electrical today: litigation. I became seriously worried about the possibility of being sued by the parent of some junior genius that could not be bothered to follow the safety warnings. That is one big difference between the USA today and the USA of fifty years ago.
 
Mike Sulzer 5/28/2018 5:53 AM
Quote Originally Posted by R.G. View Post
Of course it's not that simple. However, when you're trying to introduce complex, slippery subjects to someone new to the concepts, you start with the simplest views first, then add on modifications as they get the grosser concepts under control. I didn't get into the quantum mechanical view of how magnetism arises, either.
Yes, there is a truth there. Someone did the physics, and showed that the B-H curve, something that can be measured for a material without understanding the QM, is what you need to know to design a transformer. But no one said to would be easy, and it is not. Most people just pick out an output transformer for the job, and that is a good thing. If you figure out how much primary inductance you need and then pick out a transformer with that in mind you could be in big trouble. In a push pull design, at low signal, the inductance to support the magnetizing current is the low signal inductance, several times smaller than the high signal inductance, as Helmholtz most recently pointed out. The specs usually show high level inductance, but might not say that. Use the wrong inductance and you lose bass at low power.

A related issue is the myth that you have to have an air gap in a transformer for SE use, or it would be gigantic, yes GIGANTIC!. An air gap does two things: it postpones saturation to a higher current, but at the cost of lowering the effective permeability. These effects tend to cancel out, meaning you just have to wind more wire on to get the missing inductance back, BUT there is something very different about how an SE versus a PP transformer is used. The difference is where they operate on the B-H curve with no signal. Unlike the PP transformer, the SE transformer has a big dc current through it and thus is at a different location on the curve. Does that immediately tell you why you can lower the effective permeability with a gap? Not in any easy way, because this stuff is hard to understand. I doubt that most of the people who make transformers could explain exactly how an air gap in an SE transformer works, but they know that it does, and they know how to take advantage of that.
 
Malcolm Irving 5/28/2018 6:27 AM
If an air-cored output transformer was built it really would have to be huge, and would need to be kept a long way from the rest of the electronics (and anything else which is susceptible to magnetic fields). However, it would be perfectly linear – no hysteresis – no saturation – no funny business at low excitation levels!

I wonder if any hi-fi enthusiasts have done it?

An air-gapped core is a short air-core magnetically in series with the ferromagnetic core. A linear component in series with a non-linear component must improve the overall linearity.

EDIT: A push-pull air-cored output transformer would have the added advantage that you could check the DC balance of your output tubes using a compass.
 
Chuck H 5/28/2018 7:41 AM
Quote Originally Posted by Malcolm Irving View Post
A push-pull air-cored output transformer would have the added advantage that you could check the DC balance of your output tubes using a compass.
It would have to be a pretty fast compass! You could use it as a fan to improve cooling
 
R.G. 5/28/2018 8:55 AM
Interesting concepts.

One effect of air gapping that is fairly well buried in transformer design is energy storage. An SE transformer must store energy to release on the half-cycle where the output device is turning off. It has to store enough to complete a half cycle of the lowest frequency you want to come out undistorted. It being an output transformer, it has to store enough speaker energy to fill that half cycle, so this is a pretty stiff requirement. Looked at in this way, SE OTs are different in nature from PP in that they are energy store-and-release devices, not flow-through devices.

It turns out that energy storage in a transformer with an air gap is lopsided in favor of the energy being stored in the gap, not the iron. This is much like in a capacitor, where the energy is stored in the insulation, not in the plates. The iron is a "conductor" to conduct the m-field into the air gap. Much less energy is stored in the iron than the gap as you increase field strength by forcing current into the coils.

I have not done the math, but I suspect a non-air-gapped SE transformer would be much larger for the same output than an air gapped one because the air gap stores the permeability times more energy at the same flux density B. The iron helps you constrain the field intensity into a defined volume of space.

A purely air gapped SE transformer would be fighting another problem - leakage. One screaming advantage of combining iron and air (vacuum works, too ) is that the iron channels the field outside the air gap back to the air gap with mu times less leakage. A purely air core coil would have much worse leakage for the same cross sectional area times magnetic path length, so the losses to leakage at the upper end of the frequency range would be a lot worse. This is one of those things that it's hard to add more wire to fix. The bigger the stack of the coil, the worse leakage gets. Couple that with lower primary inductance and the need to put even more coils of wire to get inductance back up and it gets much, much harder to design to wide bandwidth audio.
 
Malcolm Irving 5/28/2018 9:04 AM
Quote Originally Posted by R.G. View Post
...
An SE transformer must store energy to release on the half-cycle where the output device is turning off. It has to store enough to complete a half cycle of the lowest frequency you want to come out undistorted. It being an output transformer, it has to store enough speaker energy to fill that half cycle, so this is a pretty stiff requirement. Looked at in this way, SE OTs are different in nature from PP in that they are energy store-and-release devices, not flow-through devices. ...

Yes, I've observed that effect in the waveforms of a SE guitar amp at low frequency. I think it gives another type of distortion, which people sometimes incorrectly attribute to saturation.
 
J M Fahey 5/28/2018 9:58 AM
Not in any easy way, because this stuff is hard to understand. I doubt that most of the people who make transformers could explain exactly how an air gap in an SE transformer works, but they know that it does, and they know how to take advantage of that.
Not really
The core of the answer is that yes, you lose inductance because of the gap, but core does not saturate and transformer is heavier than expected for the power handled because now you need more copper and iron.

If ypu do NOT use a gap, core saturates, period, so it BOTH loses inductance/Bass (tons of it) and to boot, saturated iron is terrible to handle Audio.

"Saturated" by definition implies "not change", while Audio is "all about change".

If you donīt use a gap (you still have one anyway, because of mechanical imperfections in punched lamination) , you need to use 5X or more than if you had used a proper gap to begin with.
I fail to see the savings/advantage.
 
Malcolm Irving 5/28/2018 10:25 AM
Iron will saturate when its flux density gets high enough. The increased overall reluctance due to the air-gap means that more ampere-turns are needed to get up to that flux density.
 
Mike Sulzer 5/28/2018 10:27 AM
Quote Originally Posted by R.G. View Post
Interesting concepts.

One effect of air gapping that is fairly well buried in transformer design is energy storage. An SE transformer must store energy to release on the half-cycle where the output device is turning off. It has to store enough to complete a half cycle of the lowest frequency you want to come out undistorted. It being an output transformer, it has to store enough speaker energy to fill that half cycle, so this is a pretty stiff requirement. Looked at in this way, SE OTs are different in nature from PP in that they are energy store-and-release devices, not flow-through devices.

It turns out that energy storage in a transformer with an air gap is lopsided in favor of the energy being stored in the gap, not the iron. This is much like in a capacitor, where the energy is stored in the insulation, not in the plates. The iron is a "conductor" to conduct the m-field into the air gap. Much less energy is stored in the iron than the gap as you increase field strength by forcing current into the coils.

I have not done the math, but I suspect a non-air-gapped SE transformer would be much larger for the same output than an air gapped one because the air gap stores the permeability times more energy at the same flux density B. The iron helps you constrain the field intensity into a defined volume of space.

A purely air gapped SE transformer would be fighting another problem - leakage. One screaming advantage of combining iron and air (vacuum works, too ) is that the iron channels the field outside the air gap back to the air gap with mu times less leakage. A purely air core coil would have much worse leakage for the same cross sectional area times magnetic path length, so the losses to leakage at the upper end of the frequency range would be a lot worse. This is one of those things that it's hard to add more wire to fix. The bigger the stack of the coil, the worse leakage gets. Couple that with lower primary inductance and the need to put even more coils of wire to get inductance back up and it gets much, much harder to design to wide bandwidth audio.
All output transformers store and release energy because of the magnetization current. At the lowest frequency of operation, the magnetization and signal currents in the primary are about the same. So this energy is of the same order as the energy needed for a half cycle. The SE is more demanding, of course. It has to store the energy from the dc current as well. But that dc current is nearly the same as the peak ac current, and so I think we have just a few times, not many times.
 
Mike Sulzer 5/28/2018 10:36 AM
Quote Originally Posted by Malcolm Irving View Post
Iron will saturate when its flux density gets high enough. The increased overall reluctance due to the air-gap means that more ampere-turns are needed to get up to that flux density.
Yes, and the increased reluctance means that the inductance is down, and the magnetization current is up, and so you get those ampere turns, if you can provide them. Transformer design must consider all the effects at once, and it is certainly true that an SE transformer is most efficiently done with an air gap. I think you have to go through the whole design to understand why.
 
Roberto 5/28/2018 1:42 PM
Quote Originally Posted by SoulFetish View Post
What the hell does "'teaching my grandmother to suck eggs" mean?
In northern Italy we say "teaching the cat to climb": basically try to teach something to someone who knows it way better than you. Until early decades of the last century a warm-fresh (sorry for the oxymoron) egg was the "redbull" of the era: just two holes on its symmetrical axis and suck the inner part.
 
SoulFetish 6/7/2018 9:13 PM
I've been bouncing between authors and whatever industry data on EI lamination standards im coming across. I'm piecing the design together as I'm figuring this out.
I'll say this though, "Audio Transformer Design Manual" by Robert G Wolpert (2004) is fantastic. Not only is it incredibly practical and informative, it's been serving as an outline, kind of guiding me through the learning process.
I've settled on M19/29ga lamination
*M19 0.014" (29 gauge); ASTM A677. 1.55 W/lb • @Magnetic Field 1.50 T • Frequency 60.0 Hz – 3.42 W/kg • @Magnetic Field 1.50 T • Frequency 60.0 Hz)
Nominal Silicon % 2.5-3.8
Approx. Permeability μ = 7,500
Max recommended operating flux density 12 to 13 kilogauss (but I've seen figures as high as 14 kilogauss)

One question I have is, when looking at the loadline and specifying the primary voltage and current, should I use RMS or peak numbers?
 
cerrem 6/7/2018 11:40 PM
Quote Originally Posted by J M Fahey View Post
I was also amazed when I calculated it

See for yourself: original VOX AC30 from the 60's

[IMG]http://www.voxamps.com/uploads/SupportPage_Downloads/ac301960.jpg[/IMG]

Check Bright Channel volume control and its coupling cap, at low volume you have C1 500pF and VR2 500k .
Use this handy online calculator:RC pad corner frequency upper and lower cutoff frequency calculation filter calculate time constant tau RC voltage power calculator capacitance resistance - sengpielaudio Sengpiel Berlin

Using those old series nominal values you get some 639Hz cutoff (at mild 6dB/octave, so you *still* have some Bass and Low Mids, just quite attenuated) , with modern normalized values it would be 470pF and 470k = 720 Hz. In practice same thing.

And that is with volume set low; when set to "10" capacitor also sees R9 and R7 to ground (depending on Normal Channel Volume setting, but which can be assumed is set to 0 if unused) so cutoff frequency starts at some 1200/1400 Hz

I BET Chris Jennings did not fire up his slide rule to calculate a cutoff frequency, but most probably had somebody playing his prototype LOUD and tried different capacitors until he found one which cleaned the distorted sound a lot .... tried and true design technique

FWIW revered Trainwreck amps are *basically* "a Fender amp with an extra tube for gain/sustain and a VOX type strong Bass cutoff for clarity"

[IMG]http://music-electronics-forum.com/attachments/1185d1195008139-express-input.jpg[/IMG]

Notice 0.002 coupling cap and 180k grid resistor at the third triode, which is also a cold cathode clipper, cutting below 440Hz.
The rest of the circuit is again a basic Fender with tweaked (Marshallish) Tone Control values.

Like at Mc Donalds, where they can offer a couple dozen "different burgers" using basic 6 or 7 "components".
Actually less, because they *always* need to use the bun and at least 1 patty
Wow... You have a copy of a schematic I had written on many years ago.... That is my hand writing for the OT secondary colors ..... I may still have the original copy with my pen writing in BLUE ink... I must have done it when I was Kenny Fisher's shop back in the 80's ....
 
J M Fahey 6/8/2018 6:39 AM
Quote Originally Posted by cerrem View Post
Wow... You have a copy of a schematic I had written on many years ago.... That is my hand writing for the OT secondary colors ..... I may still have the original copy with my pen writing in BLUE ink... I must have done it when I was Kenny Fisher's shop back in the 80's ....
Small World indeed
 
R.G. 6/8/2018 9:51 AM
Another step sideways.

My all time favorite power amplifier for guitar was one I constructed myself using the power and output transformers from a Magnavox hifi (not stereo! console from the late 50s or early 60s that I found in a vintage/antique furniture shop. It ran a single 6AQ5/EL84 single ended. I bet it did all of 4W fully cranked.

But "fully cranked" was so very, very sweet a sound. It had almost all of the adjectives I've heard describing smooth overdrive, touch sensitivity, creamy distortion and so on. It just wasn't all that loud.

Couple that with one of the 100/500/1000W Class D amps and you get a breadbox sized giant killer. The available power makes it a real challenge to build speakers that will take the output. There are smaller output Class D's of course, if you insist on lower power. The speaker/amp interaction can easily enough be handled by putting something like a real speaker you like as a load on the actual low powered amplifier.

I'm guessing that these kinds of amps will become ever more popular with working musicians in the near future.
 
J M Fahey 6/8/2018 11:04 AM
Cool.
But remember you still need to drive a suitable speaker as a load for the first tube amp and tap *that* to drive the big one, Class D or even AB.

The complex impedance, phase shifts, etc. offered by the speaker interacting with the tube amp are an important part of the signature sound; if you use a resistor there, youīll find it lacking "something".

Or at least use a very good reactive load, such as Aikenīs.
 
nickb 6/8/2018 1:26 PM
Quote Originally Posted by SoulFetish View Post
One question I have is, when looking at the loadline and specifying the primary voltage and current, should I use RMS or peak numbers?
I think that what you are concerned about is the heating effect (of the copper in the transformer) so RMS is appropriate. Now you get to make another design choice. Should you use current at max output or something, perhaps more realistically, go for something lower since you're probably not going flat out 100% of the time?
 
Mike Sulzer 6/8/2018 2:26 PM
Quote Originally Posted by nickb View Post
I think that what you are concerned about is the heating effect so RMS is appropriate. Now you get to make another design choice. Should you use current at max output or something, perhaps more realistically, go for something lower since you're probably not going flat out 100% of the time?
If the purpose of a load line analysis is to maximize the RMS power, then a part of doing that involves paying attention to what happens to peak voltage and current for both positive and negative deviations from the quiescent point.
 
SoulFetish 6/10/2018 5:31 PM
Quote Originally Posted by Mike Sulzer View Post
If the purpose of a load line analysis is to maximize the RMS power, then a part of doing that involves paying attention to what happens to peak voltage and current for both positive and negative deviations from the quiescent point.
Well, that's an interesting point. I suppose, the primary reason I asked the question was for the purpose of properly sizing diameter of copper winding in the primary for handling the heat, as well as appropriately choosing an appropriate core stack area. Here is some load line data for my operating conditions:
(courtesy of https://bmamps.com/ivds.html)

[ATTACH=CONFIG]49226[/ATTACH]

[ATTACH=CONFIG]49227[/ATTACH]
Power out watts ≈ 17.1
Max Power Dissipation watts(squre wave, per tube) ≈ 11.47
Max Dissipation watts (sine wave, per tube) ≈ 10.53

The circuit is designed to push the amp into output stage into clipping early, and will likely be operated with some level of overdrive during operation. So, I would like to design it to handle 25-30W. According to the M19 data posted above, for 30W power, I calculated a minimum core stack area (mm2) of about 1469.7 (which I probably would just give to a manufacturer to fit with available EI stock, as I have flexibility in mounting space). Does this seem correct?
I'm just wondering how I should size the primary copper diameter (knowing that I need to account for wire diameter and insulation for a given winding window).
 
Chuck H 6/10/2018 8:52 PM
Quote Originally Posted by J M Fahey View Post
Or at least use a very good reactive load, such as Aikenīs.
I used that Aiken design (or rather, an 8R impedance adjusted version) with a large-ish 50R rheostat for my own attenuator. So it's still resistive well below the true impedance spikes of a speaker, but it's really, really good. I've never felt the need for a "bright" switch or other tweaks at any level. So +1 on the Aiken load.