Bob M. 5/14/2018 12:52 PM
Updated Champ amp heater circuit
Ok, guys, here's a real low-tech question:

I'm updating my workhorse and very reliable, around the house SF 70's Fender Champ amp. I"m gonna put in a twisted pair heater circuit w/ 2x 100 ohm ground reference resistors. Do I want to attach these resistors to the bias supply (approx 28 Vdc)? Sometimes, I've just grounded the ground reference resistors and other times, I've 'elevated' them via the bias supply. I've never really made a study of what yields the quietest amp but I've have had very good luck, noise-wise (or lack thereof) with just grounding these resistors in the past.

Any and all on-point comments are welcome.

Bob M.
 
Chuck H 5/14/2018 1:03 PM
If you REALLY want to know, try it both ways. It's one wire. If there doesn't seem to be any advantage to elevating the false CT at the cathode resistor just ground it. The last two I did were elevated, but only because I didn't try it both ways so I can't say what difference it may have made.
 
Justin Thomas 5/14/2018 1:08 PM
And my '79 was dead quiet with just the standard heater wiring as it left the factory...
So, they all can work. But if it ain't broke...

Jusrin
 
Chuck H 5/14/2018 1:43 PM
Ok... Let's take it next level. Since the twisted pair is only effectively reducing hum in the single preamp tube (the tremolo tube isn't in the signal path and the power tube has one filament) you could consider a DC circuit. The winding would probably handle it (though I haven't researched it for certain) since that PT was also used in the Princeton Reverb which draws A LOT more filament current.

EDIT: Rethinking this... Short of DC filaments, a long as the cathodes of the first preamp tube are fully bypassed (the first triode is, the second virtually), as it IS in the stock circuit then the stock circuit should work fine. Now, if you've done something like a partial bypass of the first triode then you may benefit from a different filament winding.
 
bob p 5/14/2018 2:02 PM
> Any and all on-point comments are welcome.

oops.

Does anyone know where that "virtual center tap" term originated? It's totally wrong, but it seems to have caught on so that people use it anyway. I'm not sure why. Maybe it's just because it looks like a center tap if you draw it that way, but electrically that's not what's going on. No offense intended, Bob and Chuck. I know that you know that it's not really a center tap and that the purpose of the resistors is just to provide a DC offset to the heater supply -- I'm just wondering where that "virtual center tap" terminology came from, because even though it's wrong it sure seems to have become sticky.
 
mozz 5/14/2018 2:04 PM
Even if you can't hear any difference, put your true rms meter on the speaker leads(or dummy load), you will measure millivolts on the AC scale. Rewire with twised pair, remeasure, elevate, remeasure again.
 
bob p 5/14/2018 2:07 PM
Quote Originally Posted by Justin Thomas View Post
And my '79 was dead quiet with just the standard heater wiring as it left the factory...
So, they all can work. But if it ain't broke...

Jusrin
My SFVC is dead quiet too. If it got noisy all of a sudden, I'd be thinking about a bad cap or a bad tube.
 
Chuck H 5/14/2018 2:32 PM
Jusrin () and Bob,

Do your VC's have the first triode fully bypassed like the stock circuit? Because I REALLY think that's the key to quiet operation with the stock filament wiring.
 
Justin Thomas 5/14/2018 3:15 PM
Yeah, mine does. Cuz I'm not clown shit crazy.

Jusrin
 
bob p 5/14/2018 6:30 PM
absolutely stock.
 
SoulFetish 5/14/2018 7:04 PM
Quote Originally Posted by bob p View Post
> Any and all on-point comments are welcome.

oops.

Does anyone know where that "virtual center tap" term originated? It's totally wrong, but it seems to have caught on so that people use it anyway. I'm not sure why. Maybe it's just because it looks like a center tap if you draw it that way, but electrically that's not what's going on. No offense intended, Bob and Chuck. I know that you know that it's not really a center tap and that the purpose of the resistors is just to provide a DC offset to the heater supply -- I'm just wondering where that "virtual center tap" terminology came from, because even though it's wrong it sure seems to have become sticky.
Bob, it's a good thing your here to enforce nomenclature compliance. If it weren't for you, I'd still be calling it "dampening".
 
Chuck H 5/14/2018 9:49 PM
Quote Originally Posted by bob p View Post
> Any and all on-point comments are welcome.

oops.

Does anyone know where that "virtual center tap" term originated? It's totally wrong, but it seems to have caught on so that people use it anyway. I'm not sure why. Maybe it's just because it looks like a center tap if you draw it that way, but electrically that's not what's going on.
Well... Both an actual CT and the 100R balance resistors act as a 0V reference with very little current aspect. What would YOU call it? Not being snarky, it just seems like a fair question. I didn't make up "virtual center tap" but I'll use the term for lack of a better one. With 0V reference grid bias resistors we call them "grid load". Maybe "filament load"? Except it's not really a load WRT the filament circuit impedance where a grid load IS a load.
 
the fatch 5/15/2018 1:30 AM
Quote Originally Posted by bob p View Post
> Any and all on-point comments are welcome.

oops.

Does anyone know where that "virtual center tap" term originated? It's totally wrong, but it seems to have caught on so that people use it anyway. I'm not sure why. Maybe it's just because it looks like a center tap if you draw it that way, but electrically that's not what's going on. No offense intended, Bob and Chuck. I know that you know that it's not really a center tap and that the purpose of the resistors is just to provide a DC offset to the heater supply -- I'm just wondering where that "virtual center tap" terminology came from, because even though it's wrong it sure seems to have become sticky.
An actual center tap creates out of phase currents at 3.15V with respect to ground (or to an elevation voltage), so that with tightly twisted wires the electrostatic field due to one wire is cancelled by the field due to the other wire. The two 'virtual center tap' resistors perform the same function. Perhaps 'artificial center tap' would be a more accurate description.
 
Tqi 5/15/2018 1:55 AM
Quote Originally Posted by the fatch View Post
Perhaps 'artificial center tap' would be a more accurate description.
The Valve Wizard

This person agrees with you.
 
Justin Thomas 5/15/2018 7:52 AM
"Fake Center Tap" is easier to say and takes up less interweb space...

Justin
 
bob p 5/15/2018 12:28 PM
Quote Originally Posted by Justin Thomas View Post
"Fake Center Tap" is easier to say and takes up less interweb space...

Justin
As does saying that SF Fender amps are "ultralinear".
 
bob p 5/15/2018 1:32 PM
I guess it's time for an old curmudgeon rant. Feel free to skip this post if you like.

Quote Originally Posted by Tqi View Post
This person agrees with you.
I like Merlin's books. I think they're well written and they contain a wealth of useful information. But at the same time, I understand that their target audience is the homebrew type of amp builder who may not have formal electronics training and definitely does not have a Golden Era electronics background. They are written for the modern hobbyist who wants to learn how to build amps, they are not intended to be engineering reference material. As an example: On that page you linked, he uses the term "humdinger" right after he uses the term "artificial centre tap."

Humdinger. That's a scientific term if I've ever heard one.

That's just a tongue-in-cheek jab on my part, no disrespect intended, but the reference illustrates my point. His books commonly use imprecise slang conventions to get points across in a way that makes people understand with a minimal amount of writing. That's good. But slang can have it's downsides.

The "artificial centre tap" is an example. When imprecise slang becomes conventional there is a risk of a downward step in the level of comprehension. We know that the "artificial centre tap" isn't remotely close to being a real center tap, but we use the term anyway. We like to use terms like "fake center tap" to abbreviate the amount of writing that has to be done to express our ideas about heater elevation. In doing this there is a price that we have to pay, in the form of obfuscating the difference and perhaps diminishing some readers' understanding of what a center tap really is and what it actually does. There are people out there who read these books and read these threads who actually think that the resistors create out of phase waveforms. One problem that comes along with the belief that the resistors actually create a virtual center tap is that sooner or later somebody is going to come along and try to use them to wire up a full wave rectifier. Of course we know that the current flow through the windings just won't be the same as it would be with a real center tap.

My purpose isn't really to cause a debate on whether or not the terminology is correct; we all know that it isn't. What I'd really like to know is where the "virtual center tap" term came from originally. I know that Merlin uses it in his books, but I don't think he created the term, did he? And just because he uses the term "artificial centre tap" doesn't necessarily mean that he thinks it's technically equivalent to a real center tap, he's just using a slang convention.

To me this is a lot like saying that Fender amps are UL. They aren't. But someone began incorrectly applying that term and because most people who looked at the schematic didn't have the expertise to realize that the term was wrongly applied, the term has now become sticky. The result is that most people wrongly believe that SF Fender amps are something they are not.

Chuck, I don't know what the best name is for them. I don't think virtual center tap is a particularly good name, though it is in common use. Me? I just think of them as providing DC offset, so I call them DC offset resistors.
 
Justin Thomas 5/15/2018 1:42 PM
Quote Originally Posted by bob p View Post
As does saying that SF Fender amps are "ultralinear".
"Ultralinear..." "Distributed Load..." But what <IS> the correct term?
Same number of syllables to me!

Jusrin***

***Maybe I'll use Jusrin as my goofy alter-ego... dang, Chuck, I'm gonna owe you a beer or seven if I ever get out west, as much of an "inspiration" you've been to me.
 
Justin Thomas 5/15/2018 1:47 PM
What do you call something that does the same job with the same end result as something else, even if done in a different way?
I don't know, I'm just asking. By my logic, a full wave bridge rectifier should be called a full wave fake rectifier... I just posit that anything called "artificial" should just be called "fake." And it's easier than "counterfeit," too...

Justin
 
bob p 5/15/2018 1:49 PM
I believe the term you are looking for is "Fugazi."
 
g1 5/15/2018 1:56 PM
Pseudo might raise fewer objections than fake.
 
bob p 5/15/2018 3:00 PM
ongoing curmudgeon rant:

Is pseudo really any better than fake? I think that both terms are inappropriate because they continue to use the "center tap" analogy, and perpetuate the premise which is founded on ignorance. While those resistors do provide a DC offset, they don't actually do anything in the way of emulating the functionality of a center tap. In that respect any kind of a reference to a center tap is technically incorrect.

This is a case where there is no real equivalence in function between the "virtual center tap" and a real center tap -- the only reason that anyone is using the term is because the picture just looks superficially similar to someone who either isn't paying attention or just doesn't understand the difference. I'm thinking that the origin of the term had to originate in the paint-by-numbers school of amp building... you know, that school where they don't teach you to read a schematic and you build an amp by copying someone else's build photos or maybe by using a layout diagram. In that school the picture looks close enough to a center tap, so the same words get applied to both circuits.

In the past I've made hair-splitting comments about things like "ultralinear." At least with UL the people who mis-apply the term are in the right ballpark when it comes to understanding the circuit's operation and the distinction amounts technically to hair splitting between operating modes. In the case of the virtual center tap, the misnomer isn't even close to accurately representing the operation of the circuit. It's totally wrong.

I know that ranting about this is a bit of a Qixotic quest, as the term isn't going to go away just because I ranted -- and I'm not calling for us to stop using the term. But it's persistence makes it hard for me to distinguish between the people who use the term because they don't know any better and those people who use the term just to appease the people who don't know any better. IMO if we use the term we all end up looking ignorant irregardless* of which group of people we belong to.

I guess my point is that this being the Theory section of the MEF, accuracy in nomenclature is more important than it is at a place like The Amp Garage.



* I use the emphatic form of regardless/irrespective in honor of Jusrin.
 
Chuck H 5/15/2018 3:25 PM
Ok, how about 'center tap substitute balance resistors'?

Say it ten times fast!
 
Bloomfield 5/15/2018 3:45 PM
I think it's just a fixed humdinger; nothing fake there..
 
bob p 5/15/2018 4:36 PM
Quote Originally Posted by Chuck H View Post
Ok, how about 'center tap substitute balance resistors'?

Say it ten times fast!
I guess we could just call them dummy resistors, but that would be ambiguous too.
 
Chuck H 5/15/2018 6:17 PM
well, with all due recognition of the semantics and respect for my fellow posters... I think I'll just keep calling it a false/fake/artificial center tap in the interest of communicating modern parlance. But I promise to wince when I do. If that helps.
 
g1 5/15/2018 7:46 PM
One of the functions of the center tap is to reduce hum. If the resistors do that, how is that so absolutely not the same function? Do both methods not result in a ground reference that would otherwise be non-existant?
I'm certainly ready to be schooled on this, and hope some around here are willing to forgive my ignorance.
 
The Dude 5/15/2018 7:58 PM
Yep. I'm with you g1. The two terms "center tap" and "virtual center tap" are differentiated by the word "virtual". Of course it's different than an actual center tap, as it's description indicates. I don't see a problem with that- just hair splitting- potato, pototo- coke, pop, soda, etc. Virtual reality is not actual reality. If you want to start dissecting the English language as it relates to terminology, you're gonna be busy a while.
 
Leo_Gnardo 5/15/2018 8:52 PM
Quote Originally Posted by Chuck H View Post
I think I'll just keep calling it a false/fake/artificial center tap in the interest of communicating modern parlance. But I promise to wince when I do. If that helps.
You left out ersatz.

However it's not bogus. Because it works.
 
the fatch 5/16/2018 7:33 AM
Quote Originally Posted by bob p View Post
ongoing curmudgeon rant:

Is pseudo really any better than fake? I think that both terms are inappropriate because they continue to use the "center tap" analogy, and perpetuate the premise which is founded on ignorance. While those resistors do provide a DC offset, they don't actually do anything in the way of emulating the functionality of a center tap. In that respect any kind of a reference to a center tap is technically incorrect.

This is a case where there is no real equivalence in function between the "virtual center tap" and a real center tap -- the only reason that anyone is using the term is because the picture just looks superficially similar to someone who either isn't paying attention or just doesn't understand the difference. I'm thinking that the origin of the term had to originate in the paint-by-numbers school of amp building... you know, that school where they don't teach you to read a schematic and you build an amp by copying someone else's build photos or maybe by using a layout diagram. In that school the picture looks close enough to a center tap, so the same words get applied to both circuits.

In the past I've made hair-splitting comments about things like "ultralinear." At least with UL the people who mis-apply the term are in the right ballpark when it comes to understanding the circuit's operation and the distinction amounts technically to hair splitting between operating modes. In the case of the virtual center tap, the misnomer isn't even close to accurately representing the operation of the circuit. It's totally wrong.

I know that ranting about this is a bit of a Qixotic quest, as the term isn't going to go away just because I ranted -- and I'm not calling for us to stop using the term. But it's persistence makes it hard for me to distinguish between the people who use the term because they don't know any better and those people who use the term just to appease the people who don't know any better. IMO if we use the term we all end up looking ignorant irregardless* of which group of people we belong to.

I guess my point is that this being the Theory section of the MEF, accuracy in nomenclature is more important than it is at a place like The Amp Garage.



* I use the emphatic form of regardless/irrespective in honor of Jusrin.
The center tap to ground on a 6.3V AC heater winding results in currents of opposite polarity at 3.15V AC with respect to ground on the heater wires, as do the two resistors of the 'virtual center tap' connected to ground. This seems a lot like 'emulating the functionality of a center tap' to me.
 
pdf64 5/16/2018 8:00 AM
The usual action of the heater winding CT is to force the heater circuit to become 'balanced', with respect to 0V / circuit common.
As (in normal operation) there's no current flow via the CT, a suitable pair of resistors can do the exact same thing when there's no CT, nothing artificial / false / fake about it.
Hence I suggest they may be referred to as 'heater circuit balancing resistors' or just 'heater balancing resistors'
 
Chuck H 5/16/2018 8:10 AM
Quote Originally Posted by pdf64 View Post
The usual action of the heater winding CT is to force the heater circuit to become 'balanced', with respect to 0V / circuit common.
As (in normal operation) there's no current flow via the CT, a suitable pair of resistors can do the exact same thing when there's no CT, nothing artificial / false / fake about it.
Hence I suggest they may be referred to as 'heater circuit balancing resistors' or just 'heater balancing resistors'
Ok. Good call. And I think Bob would be better with that I don't think anyone would become confused WRT a filament winding if any of us were to write "balance resistors" or "balancing resistors" (since we actually HAVE done this) instead of "virtual center tap". I may try to commit to this.
 
eschertron 5/16/2018 8:15 AM
Quote Originally Posted by pdf64 View Post
The usual action of the heater winding CT is to force the heater circuit to become 'balanced', with respect to 0V / circuit common.
As (in normal operation) there's no current flow via the CT, a suitable pair of resistors can do the exact same thing when there's no CT, nothing artificial / false / fake about it.
Hence I suggest they may be referred to as 'heater circuit balancing resistors' or just 'heater balancing resistors'
I agree too. However, we will need to begin calling the heater "center tap" a more functionally-correct name: the heater "balance tap" in order to keep the nomenclature proper.
 
bob p 5/16/2018 1:11 PM
Quote Originally Posted by pdf64 View Post
The usual action of the heater winding CT is to force the heater circuit to become 'balanced', with respect to 0V / circuit common.
As (in normal operation) there's no current flow via the CT, a suitable pair of resistors can do the exact same thing when there's no CT, nothing artificial / false / fake about it.
Hence I suggest they may be referred to as 'heater circuit balancing resistors' or just 'heater balancing resistors'
Just to be clear, I'm not intent on forcing terminology on anyone, and I'm not trying to force a change in what the resistors are called. I started off asking where the "virtual center tap" term originated and then I complained that that terminology wasn't a particularly good description because although we observe similar behavior in one circumstance, those similarities can't be extrapolated to all center-tapped secondary behavior.

Anyone can use any term they like that gets their idea across. (Chuck, I'd prefer if you didn't even pause to think about it.) I'm just glad that some people think enough of the this topic to humor me by suggesting other names.

I do like a lot of the ideas that have been suggested. Being a curmudgeon, I just prefer terms that don't inject confusion by oversimplifying and potentially implying that a pair of resistors can transform a non-center-tapped secondary into the equivalent of a center-tapped secondary. Of course we know better.

What to call the resistors? Any term that gets the point across is a term that works. Me? As far as slang goes, I'm biased toward descriptions that describe the desired result (DC voltage offset) or effect (hum reduction), rather than descriptions that are based on the superficial appearance of a schematic rendering that doesn't accurately represent an equivalent circuit. Because the resistors' effects on hum is their intended purpose, I guess it makes sense to refer to them that way, with a name that describes their function. In that respect I like the suggestion for "hum balancing" or even "humdinger" though I personally like "DC offset." Those hum-based terms seem to describe their audible effect, which seems to be what some people are after. And there's the added benefit that "hum balancing" and "humdinger" don't have very many syllables, for those who care about that sort of thing.

All of the terms that have suggested so far sound a lot better to me than "virtual center tap". For some reason that term has always bugged me (as if you haven't noticed).
 
Bob M. 5/16/2018 1:47 PM
Well, I had no idea what a semantic can of worms I was opening.

My comments were about my Fender Champ amp, not the VibroChamp although they are 'kissin' cousins' and pretty much the same thing, minus the tremolo. My Champ is somewhat stock but over the years I've developed some things that improve these little workhorses and I decided to dive in and do that to this one while at the same time trying out a new preamp design that I like alot. The amp wasn't broken but I just had a free day and it was raining hard outside, a perfect shop day. I've got about six of these Champs and VCs lying about: I've got a Rem-Champ (Remaking of a Champ - Glass Audio), a Swamp Champ (really a one channel Bandmaster in a VC box), a super stock VC (a tricked out, blueprinted but still pretty stock VC - great amp) and a couple of beaters as well.

To set the record straight, I don't I've ever said (or used) virtual center tap. I've normally said ground reference resistors. I think (but I'm not sure) that I picked it up from Kevin O'Conner and my rationale being if it was good enough for him, it's good enough for me. I think everyone knows what I mean.

You know, the music (and amp) biz is littered with expressions we've all coined over the years to describe our gear, some really descriptive, some not so and very few are actually scientific: hardtail, sag, dive-bombing, speaker farting come to mind immediately. A guy came over the other day with his Strat and was talking about about his affinity for the #2 and #4 positions on his 5-way switch calling them the out of phase sounds. I said, you know, they are really make-before-break sounds, not out of phase sounds. He said, I know but everyone calls them that. And I said, you're right. Leo called it vibrato (the Vibro-this and a Vibra-that) but we all know that its tremolo and we know what he meant.

Loved Mozz's comment about measuring AC millivolts at the speaker or dummy load.

If you had 20 Fender (tube) amps, ten of which had a heater winding center tap and ten which used 2x 100 ohm resistors (I'll refrain from calling these resistor by any name as to not offend any hard-line semanticists) and then you measured the heater voltage at both sides of the heater winding, I think you'd find that generally, amps using the resistors gave closer (to each other) winding readings and also made for generally quieter amps. But not in all cases: The amp I just finished with, a 1965 BF Pro Reverb had very close to identical voltages on each side of the heater winding and used the stock center tap and the amp was dead-quiet (I followed Justin's sage advice: If it ain't broke...). The great side benefit of using the 2x resistors instead of the stock center tap is when the dreaded plate-to-heater short rears its ugly head, with the resistors in place you might only have to replace these resistors (provided they are sized for this important side function) rather than a tube socket or a transformer, both of which would be a bigger deal. That's the kind of forethought I like.

Steel guitarists have a similar saying to Justin's "if you build it, it will hum". They say once you put your bar on the strings, you're out of tune.

Thanks for the lively give and take,

Bob M.
 
Malcolm Irving 5/17/2018 2:34 AM
One other use of the term 'virtual' in electronics, which is widely accepted, is 'virtual ground' in an operational amplifier (op-amp) circuit. In that case the 'virtual ground' has very close to the same voltage as the signal ground, even though they are not directly connected.

If we connect two equal resistors in series across a (centre-tapped) transformer winding, the voltage at the junction of the two resistors will be very close to the voltage at the centre-tap.

So the term 'virtual centre-tap' seems OK to me.

EDIT: Of course, we only use a 'virtual centre-tap' when the winding has no centre-tap, but in that case the voltage at the 'virtual centre-tap' is the same as the voltage would be at a real centre-tap (if there was one). I think I'll stop there before I get even more confusing.
 
pdf64 5/17/2018 12:45 PM
The problem with the terminology focusing on the centre tap, real or virtual, is that folks then get thinking it's the centre tap that's the important thing here, which seems to be putting 'the cart before the horse'. As I see it, what's important is that the heater circuit gets balanced and referenced.
Here's an example of confusion that may have been caused by the 'centre tap' terminology Is a PT secondary center tap a must? - The Amp Garage

Quote Originally Posted by Bob M. View Post
...The great side benefit of using the 2x resistors instead of the stock center tap is when the dreaded plate-to-heater short rears its ugly head, with the resistors in place you might only have to replace these resistors (provided they are sized for this important side function) rather than a tube socket or a transformer, both of which would be a bigger deal...
I think that is a downside of the resistor balancing method, rather than a benefit. Electrically robust resistors might best be used to mitigate the risk of it happening, eg >3 watt vitreous wirewound.
The issue being that if the heater circuit loses its 0V reference and a tube short pulls it up towards HT, then the heater-cathode insulation of every tube in that circuit may be damaged.
To avoid the risk of transformer etc damage, the correct value and type of fusing must be used of course, but that's a fundamental (if often ignored) aspect of amp ownership anyway.
 
bob p 5/17/2018 6:27 PM
I was only kidding with my previous Amp Garage comment. I had no idea there was such a thread out there.
 
g1 5/17/2018 6:32 PM
bob p, I know you are being very precise with your terminology, so your preference for "DC voltage offset" is throwing me off. What am I missing (where does the DC voltage come in)?
 
bob p 5/17/2018 7:41 PM
Where does the DC voltage come in? It came in in BobM's original post where he talked about elevating the heaters with a bias supply reference.
 
g1 5/17/2018 8:05 PM
Ok. So that term would only be applicable where there is DC elevation, and not in the most common or 'traditional' arrangement (non-DC) that is referred to as 'virtual center tap'. (fender for example)
What would be your preference there?
 
bob p 5/17/2018 8:19 PM
I don't know if grounding the resistors is "most common" or "traditional." If that's the most common method then I'd expect that there are a lot of unnecessarily noisy amps out there. Chances are that the "most common" or "traditional" methods of wiring the heater circuits don't even have any balancing resistors.

My preference is for providing DC offset to the heaters, which is why I refer to them as DC offset resistors. Granted, if you just ground them then you don't get the benefit of the DC offset, and you don't get to call them DC offset resistors either. If you go grounding them then calling them something like balancers or humdingers is probably a better term.

Getting back to the OP's question, I think that DC elevation sounds better than grounding, so I wouldn't even consider grounding the resistors.
 
g1 5/17/2018 9:12 PM
Well that's drifting a fair bit from your original objection to the terminology I think.
I'm pretty sure the bulk of folks here consider 'virtual center tap' to mean resistors to ground like the standard (CBS) Fender arrangement.
I'd even go as far as to wager most here who commented were discussing that exact scenario.

If your objection to the 'virtual center tap' term is only where there is DC elevation, then I guess I missed the plot.
 
bob p 5/17/2018 9:33 PM
I don't get your idea of drift. That I personally wire them up to a DC source and refer to them as offsetters (or elevators, but never alligators) is separate from the objection that the VCT misnomer is misguided. My objection to the VCT has always been that a VCT cannot behave in all ways like a true CT, which renders the CT analogy invalid. Fussing about offset vs. balancing resistors or agreeing to call them this or that, or pretending that voltages are virtually the same, ignores the central issue that the current flow through the secondary windings can never be the same in center-tapped and non-center-tapped windings.
 
Malcolm Irving 5/18/2018 1:59 AM
Quote Originally Posted by bob p View Post
... the central issue that the current flow through the secondary windings can never be the same in center-tapped and non-center-tapped windings.
Sorry, I don't get that. In a heater winding, the current is exactly the same whether there is a centre-tap or not and whether or not that CT is tied to a ground reference or to an elevated DC reference.

The current could only be different if some current is coming in or out via the CT, which is not the case here. The connection to the CT is just providing a voltage reference for the winding, which would otherwise be floating.
 
pdf64 5/18/2018 2:06 AM
Whatever the dc reference point chosen, isn't the point of having the 2 resistors or CT tied to that node is that they force the heater circuit to be balanced with respect the ac 0V?
 
bob p 5/18/2018 4:10 AM
if all you have is a resistor then everything looks like a nail, if you know what i mean.
 
ric 5/18/2018 10:12 AM
Quote Originally Posted by bob p View Post
if all you have is a resistor then everything looks like a nail, if you know what i mean.
This place holds a real high average for intelligent worthwhile information.

Keeping things scientifically accurate is important and commendable.

Fifty years and more ago we always heard that "scientists now believe..." Now everything anybody says about anything is always just stated as fact. Even as we hear the laws of physics aren't what we used to think. Tomorrow who knows.

Just my opinion....for now.
 
Steve A. 5/19/2018 3:28 AM
Quote Originally Posted by g1 View Post
One of the functions of the center tap is to reduce hum. If the resistors do that, how is that so absolutely not the same function? Do both methods not result in a ground reference that would otherwise be non-existant?
I'm certainly ready to be schooled on this, and hope some around here are willing to forgive my ignorance.
Agreed! So what is it called when one of the two filament leads is connected to ground as was often the case before the introduction of the maligned virtual center tap? I always assumed that wiring like that would (or could) be noisier... true or false?

Steve A.
 
Malcolm Irving 5/19/2018 5:40 AM
As pdf mentioned, when the CT or artificial CT is ac grounded, the two wires of the heater supply are balanced. When one wire is instantaneously at some positive voltage the other wire is simultaneously at an equal negative voltage (with respect to ground). The electric field generated by the two wires cancels out, if the two wires are in the same place. It is therefore good to twist the wires together to put them as close as possible to being 'in the same place'. The current in the two wires is always the same, but travelling in opposite directions, so again we have good cancellation of the magnetic field, although the CT has no effect on the magnetic field.

You should therefore get less hum with a CT, or artificial CT, than you would with just grounding one side.
 
Malcolm Irving 5/19/2018 5:58 AM
A 'humdinger' can give even lower hum by allowing the two resistances (in an artificial CT) to be adjusted away from being equal. I think this works because the most sensitive point or points in the signal circuit, where the hum is being picked up, will be closer to one of the heater wires than the other.

Another possibility is that hum from the unbalanced heater wiring is out of phase with some other hum source.
 
Malcolm Irving 5/19/2018 6:11 AM
Possibly one of the reasons that humdingers are not seen more often is the power dissipation. 6.3Vac across 200 ohms gives 0.2 watts - maybe a bit hot for a typical preset pot.

For those of us who like to fiddle about with such things, how about a chain of four 50 ohm resistors - giving a choice of 5 possible points to ground (or DC elevate) - find the point with least hum by trial and error.
 
Leo_Gnardo 5/19/2018 8:37 AM
Quote Originally Posted by Steve A. View Post
Agreed! So what is it called when one of the two filament leads is connected to ground as was often the case before the introduction of the maligned virtual center tap? I always assumed that wiring like that would (or could) be noisier... true or false?

Steve A.
Disclaimer: I don't know what it's called, Steve. But I'll flap my fingers across the keyboard nonetheless, as I do.

A single sided filament line, with the chassis used as the current return, doesn't make much difference in a simple low gain amp like a Champ. I used to think it did, and was disappointed when altering the scheme to the balanced style didn't reduce hum. In Champs and Champ-like amps now I concentrate on filtering the high voltage better, and that works a treat. Hi gain amps with multiple cascaded preamp stages, there's where you need a balanced filament supply, or even better go DC for the gain stages.

Malcolm, nice idea for the 4 x 50 ohm resistor and switch. Problem is, the sweet spot for hum reduction is more often found with a smaller difference between balancing resistors than can be achieved with your suggestion. In some cases I've put in an 82 and 120 ohm fil balance pair, or 82 and 100, or 100 and 120. By the time I've finished faffing, I could have installed a humdinger. Best pot for this I can easily find is a wire wound 100 ohm from Antique/CE. It's not ideal, with a sort of flimsy splined "stick." Best would be a pot like Fender used on their 70's large amps, with an inset screwdriver adjust, leaving nothing sticking out that could be accidentally broken off or twisted off its mark.
 
Chuck H 5/19/2018 8:59 AM
Mouser sells a better looking CTS pot for a couple more bucks. Rated at 5W and 500V!

https://www.mouser.com/ProductDetail...aBSSW89g%3d%3d

I've never played with the humdinger design or incorporated one in a build. I've been lucky (?) than any hum in my builds so far was able to be eliminated with ground scheme refinement. But with modern tubes (where balancing on power tubes is just "ok" and preamp tubes sometimes hum like they don't know the words) it'll probably come up at some point.
 
Leo_Gnardo 5/19/2018 9:14 AM
Quote Originally Posted by Chuck H View Post
Mouser sells a better looking CTS pot for a couple more bucks. Rated at 5W and 500V!

https://www.mouser.com/ProductDetail...aBSSW89g%3d%3d
Thanks for that lead Chuck! I'll have to include a batch on my next Mouser order.
 
nickb 5/19/2018 10:12 AM
Quote Originally Posted by Malcolm Irving View Post
Possibly one of the reasons that humdingers are not seen more often is the power dissipation. 6.3Vac across 200 ohms gives 0.2 watts - maybe a bit hot for a typical preset pot.
It's worth noting that the actual current required to make the humdinger work is small and therefore a higher value pot say 1k to 10k 0.5W (which are readily and cheaply available) will do the job. You have have to provide an alternative path to ground for fault currents. This can be done using a TVS diode. These 5KP12A ones give a good voltage margin and will take 400 amps for long enough for the fuse to blow. Less expensive than a 200 ohm 5W (to handle the fault current) pot, I should think. Just connect it with the cathode (stripe) to the heater supply and anode to ground.
 
Chuck H 5/19/2018 10:38 AM
Quote Originally Posted by nickb View Post
It's worth noting that the actual current required to make the humdinger work is small and therefore a higher value pot say 1k to 10k 0.5W (which are readily and cheaply available) will do the job. You have have to provide an alternative path to ground for fault currents. This can be done using a TVS diode. These 5KP12A ones give a good voltage margin and will take 400 amps for long enough for the fuse to blow. Less expensive than a 200 ohm 5W (to handle the fault current) pot, I should think. Just connect it with the cathode (stripe) to the heater supply and anode to ground.
Well... Not an engineer so it goes beyond my wee mind, but I always just assumed that the 100R value for the balance resistors was relative to circuit impedance. If a pair of 5k resistors "will do the job" then, are there any disadvantages to the higher value? I mean, the 100R value WAS chosen at some point by guys that knew what they were doing, right? Also, if the 1k to 5k value is suitable I would think it could mitigate those components blowing in the event of a plate to heater short, which Pete indicated as undesirable. Seems like a win/win if there's no downside.?.
 
nickb 5/19/2018 10:47 AM
Quote Originally Posted by Chuck H View Post
Well... Not an engineer so it goes beyond my wee mind, but I always just assumed that the 100R value for the balance resistors was relative to circuit impedance. If a pair of 5k resistors "will do the job" then, are there any disadvantages to the higher value? I mean, the 100R value WAS chosen at some point by guys that knew what they were doing, right? Also, if the 1k to 5k value is suitable I would think it could mitigate those components blowing in the event of a plate to heater short, which Pete indicated as undesirable. Seems like a win/win if there's no downside.?.
The low value and high power rating was chosen to handle fault currents, not for the humdinger current which is in the microamps. And don't forget they didn't have TVS diodes back then either.

The high value pot on its own is no use as the voltage under fault conditions would be too high so you could damage the pot and all the tubes.

BTW, I recently applied 600VDC to a Sovtek 12AXLPS both ways from cathode to heater. The insulation did not break down. No leakage at all.
 
Leo_Gnardo 5/19/2018 11:05 AM
Quote Originally Posted by Chuck H View Post
Well... Not an engineer so it goes beyond my wee mind, but I always just assumed that the 100R value for the balance resistors was relative to circuit impedance. If a pair of 5k resistors "will do the job" then, are there any disadvantages to the higher value? I mean, the 100R value WAS chosen at some point by guys that knew what they were doing, right? Also, if the 1k to 5k value is suitable I would think it could mitigate those components blowing in the event of a plate to heater short, which Pete indicated as undesirable. Seems like a win/win if there's no downside.?.
Why 100 ohms? Why indeed? I've seen some Fenders with 47 ohm, and once or twice factory installed 82 ohm. If they ran out of 100's, gotta punt to fill those orders. For humdingers, I've used up to 500 ohm with no ill effects. I got a batch of Ohmite 250 and 500 ohm mil spec pots for dead cheap, but they're long since used up. Will have to experiment with higher values.

As far as mitigating damage from high voltage shorts, that's not going to work. Any resistors or pots with values in this range would be vaporized by a short from a tube amp's B+. One possible solution: remember those mid 60's Ampegs that used a capacitor between their filament center tap and ground? Sometimes that's the best way to go and certainly worth a try if you're flailing around trying to minimize filament induced hum/buzz. So - try a good quality film cap from your humdinger pot wiper, or resistor junction, to ground. A cap with 600-630V rating will safely keep the pot/resistors from passing fault current to ground and wrecking them. Caps 0.022 to 0.1 uF are usually used in this application, but you can try other values and maybe find one that crushes that awful hum. Granted this isn't the solution in every case. Different amps require different solutions, and eliminating hum/buzz - despite application of standard electrical truths - still seems to be a "black art." That's not to say it's magic. But often the best solutions require experimentation, and what works in one amp may not work well at all in another.
 
J M Fahey 5/19/2018 6:40 PM
Quote Originally Posted by bob p View Post
ongoing curmudgeon rant:

Is pseudo really any better than fake? I think that both terms are inappropriate because they continue to use the "center tap" analogy, and perpetuate the premise which is founded on ignorance. While those resistors do provide a DC offset, they don't actually do anything in the way of emulating the functionality of a center tap. In that respect any kind of a reference to a center tap is technically incorrect.

This is a case where there is no real equivalence in function between the "virtual center tap" and a real center tap -- the only reason that anyone is using the term is because the picture just looks superficially similar to someone who either isn't paying attention or just doesn't understand the difference.
Sorry but you are quite wrong
And to make it clear itīs just not a typo, you repeat same wrong concept in different ways

The center tap, whether real (copper wire tap in the middle of the winding) or a halfway point between 6.3V wires created with two same value resistors is NOt just a "DC reference" (even that is wrong, itīs actually an AC reference to chassis, no DC there) BUT a way to generate 2 x 3.15V **balanced** voltages, which to boot are preferrably carried on *twisted* wires (ever thought about the reason for that?) so they cancel each other and donīt induce hum on nearby wiring.

In fact, some tubes have spiral wound filaments for the same reason, but of course they again need balanced filament voltage to achieve that.

And since balancing may not be perfect, some amps have a wirewound "Hum Balance" rheostat to fine tune Hum reduction.

But said balanced voltage can be created either by a center tap or two resistors, same thing.

In fact, on some old amps which actually had a copper wire center tap itīs often suggested to remove and tape it out and instead use a couple resistors, only for the very good reason that resistors can easily be found with 5% tolerance (or better, but itīs not necessary) , while a winding "center tap" may actually not be in the exact center.
Since those are low turns count windings, maybe one turn is "too low" while the next one is "too high", you have definite "voltage steps" from turn to turn, and if, say, you have an odd number of turns there is actually NO exact center tap.
Suppose you have 13 turns (0.5V per turn which gives a 6.5V secondary) , you either pick the turn #6 or the #7 one, flip a coin, both are "wrong".
While 2 resistors may be much closer.
 
jmaf 5/19/2018 7:53 PM
Quote Originally Posted by bob p View Post

Does anyone know where that "virtual center tap" term originated? It's totally wrong, but it seems to have caught on so that people use it anyway. I'm not sure why. Maybe it's just because it looks like a center tap if you draw it that way, but electrically that's not what's going on.
It originated from the fact that the 2x100 ohm resistor trick is simply a poor man's center tap for amplifiers where you didn't have 3.15 + 3.15 VAC. Like almost everything else in guitar amplifiers, the terminology is street based. A lot of the amp modders and builders are self taught and don't have a fancy EE degree from MIT and last I checked they don't deploy tube amps at NASA, so folks use the terms they see which is fine IMO, everyone knows what is being referred to here. I don't understand what the issue is, virtual center tap has a nice ring to it.
 
bob p 5/19/2018 9:08 PM
Quote Originally Posted by J M Fahey View Post
Sorry but you are quite wrong
And to make it clear itīs just not a typo, you repeat same wrong concept in different ways
Maybe you're not following parts of the conversation that haven't been explicitly fleshed out with schematics. Maybe the problem is that you jumped to a conclusion when you read post 22 on page 1 and you didn't bother to read posts 39-44 on page 2, which clarified those things that you falsely accused me of not understanding. As if I can't tell the difference between AC and DC! Come on!

It seems that there are a lot of people reading this thread who are wearing blinders that cause them only to think about hooking up resistors to the secondary and wiring the junction to ground, which is actually not such a great way to build your amp. The fact that the two types of secondary windings behave similarly in the case where resistors are attached to them has made some people ask if the windings will behave the same way when something else -- like diodes -- are attached to them. Of course that won't be the case. Current will flow through the center tap. There is no equivalent circuit in that case with non-center tapped secondary, so the simplified banter that the two secondary designs are equivalent in the case of hanging resistors on them is based upon flawed logic as it only considers a narrow subset of transformer behavior. This is precisely why center-tapped and non-center-tapped secondaries have to use different rectifier topologies.

The center tap, whether real (copper wire tap in the middle of the winding) or a halfway point between 6.3V wires created with two same value resistors is NOt just a "DC reference" (even that is wrong, itīs actually an AC reference to chassis, no DC there)
We've already clarified that I never said it was a DC reference in that case -- what I said is that I call them DC offset resistors when I use them, because I provide a DC offset in my builds. Again, it seems that you've made the misteak of imagining a schematic that's different than what I'm talking about. In the posts on page 2 I clarified this -- the resistors provide an AC reference + balancing when you connect the resistor junction to ground, and provide a DC reference + balancing when you connect the resistor junction to an elevated DC source, as the OP asked about the original post, which everyone seems to have ignored. For some reason everyone seems to be ignoring that part of the discussion, and limiting their thoughts to the two resistors to ground model. It's time to take off the blinders.

It seems evident that some people keep reading this thread with one specific schematic in their mind, and are ignoring other topologies that have been discussed. I didn't think I'd have to draw pictures to make it clear, but I guess I was wrong on that. My bad.
 
jmaf 5/19/2018 9:39 PM
Quote Originally Posted by bob p View Post
It seems that there are a lot of people reading this thread who are wearing blinders that cause them only to think about hooking up resistors to the secondary and wiring the junction to ground, which is actually not such a great way to build your amp. The fact that the two types of secondary windings behave similarly in the case where resistors are attached to them has made some people ask if the windings will behave the same way when something else -- like diodes -- are attached to them. Of course that won't be the case. Current will flow through the center tap. There is no equivalent circuit in that case with non-center tapped secondary, so the simplified banter that the two secondary designs are equivalent in the case of hanging resistors on them is based upon flawed logic as it only considers a narrow subset of transformer behavior. This is precisely why center-tapped and non-center-tapped secondaries have to use different rectifier topologies.
I've read this 4X (for real) and I have no idea what he's trying to say here. CT and non CT secondaries have to use different topologies due to the limitations of the era, back in the day rectifiers were 2 plate tubes, then we got flooded with cheap silicone diodes. They're completely different circuits but as with any circuit, they obey the same laws.

Dangling two 100 ohm resistors across any power supply is not rocket science and it quiets most hum. What's the big deal here? I couldn't imagine a 2 page thread about something so simple TBH. Are we really discussing OHMs law and 2 100 ohm resistors here?
 
Bob M. 5/21/2018 11:35 AM
I'm glad that Malcolm pointed out that there's virtue in using 'virtual' in electronics.

I'm glad to see that G1 brought this argument full circle.

Obviously, some people are in the DC offset bias camp; others are just grounding their resistors or pot. I decided to try the resistor grounding idea first and my amp is as quiet as quiet can be (that's an unscientific term) so there's no reason to try anything else. Of course, part of my good luck has to do with shielded grid wires and a very good grounding scheme.

Now, I'm trying to decide if I like the 5V4 rectifier better than the stock 5Y3 and I'm experimenting with a couple of single ended replacement output transformers that I have in stock. For those who have replaced the output transformer on their Champ, VC and/or Bronco amps, what are some successful good choices for an upgraded improvement of this amp's sound?

Thanks all for your input and knowledge,

Bob M.