|Austin||8/19/2012 1:03 PM|
|Power pentodes as preamp tubes?|
I am thinking of building my amp with all power tubes from the input. Is there any reason I shoudn't? I have all these octal slots available and alot of power tubes at my disposal along with huge supply transformers.. Like 6g6g 6f6g etc, I could triode strap them still have plenty of gain I think. Like using the 6g6 as a pentode as input voltage amplifiers and using a 6f6 as a cathodyne phase inverter then using two more 6f6 as dc coupled cathode followers to drive two 6L6 as power stage. Makes sense to me but why doesn't everybody do it? I saw a data sheet listing the curves for ab2 operation when looking at the 6L6 data sheet last night.. I guess I will try it and see how well it works. Any thoughts anybody?
|Steve Conner||8/19/2012 1:18 PM|
|Well, the classic tube amps were being manufactured and sold at a profit, they were subject to economics. You wanted the most gain per dollar in the preamp, and preamp tubes were designed to deliver that.|
Now tube amps are an artform and you can do anything you want.
My Ninja Corvette essentially uses an EL84 as a preamp stage in high power mode.
|Austin||8/19/2012 1:58 PM|
|Gregg||8/19/2012 2:48 PM|
|I've seen Hi Fi amps using EL84 and other power pentodes as preamp tubes so it's been done already. Whether they are suitable for guitar amps is another story.|
|Austin||8/19/2012 3:15 PM|
|Steve Conner||8/19/2012 3:26 PM|
|It's worse than pentode mode. The EL84 drives the bases of a couple of hefty power transistors through a Valve Jr. OT, which is used as an interstage transformer in this case. It makes about 40W and sounds remarkably good for what it is. I got the idea from RG's writings on the Thomas Vox amps. You can google "ninja corvette" for more details.|
One problem I had is that the transistor output stage also amplifies the microphonics of the EL84. Preamp tubes are designed for low microphonics, power tubes aren't, as they are usually not operated with gain between them and the speaker.
I tried a few EL84s to select a quiet one, and also ended up with a silicone gasket from a coffee maker wrapped round the tube to damp it.
|Alan0354||8/20/2012 3:23 PM|
|One question. What about the noise performance? I was just reading about the grid current( from RDH4 where there is current even at -ve bias), that would be a source of noise |
In^2=2q(Ig)(BW) where In is the noise current, Ig is grid current and BW is the bandwidth.
I don't have the noise model of vacuum tubes, since the electrodes are all metal, I assume there is no significant resistance inside the tube to generate thermal noise. So my assumption is the grid current is the main source of noise refer to the input. Sounds like the power tubes has more grid current and has more shot noise. Question is would that be a problem. This is particular important if there is a large grid stopping resistor at the front end as was talked about in a post not too long ago in radio interference and how to stop it.
Also, the electrode capacitance and the inter electrode capacitance must be a lot higher for power tubes that might affect the performance.
|loudthud||8/20/2012 4:08 PM|
|Austin||8/20/2012 5:59 PM|
|Alan0354||8/20/2012 8:45 PM|
Gain of the stage u'=gm(rp//RL)
Where RL is the load resistance. So
Gain =u'= gm (rpXRL)/(rp+RL)
For 12AX7, (rpXRL)/(rp+RL)=37.5K
For 6L6GC, (rpXRL)/(rp+RL)=23K
So gain for 12AX7 is gm(rpXRL)/(rp+RL)=.0016X37500=60
Gain for 6L6GC is 0.0047X23000=108.1
From my calculation, the 6L6GC is about double the gain of 12AX7. Please few free to check and challenge my calculation, I don't know any of these 3 days ago!!! Just learnt it, fresh out of the oven!!!!
|Alan0354||8/20/2012 9:08 PM|
|Still back to my original, how about the grid current. I have no data on this, this cause noise. Common sense tells me that the grid of a big pentodes is going to be a lot bigger, if there is any electrons from cathode or plate or others hitting the grid, you get more surface area and therefore more grid current. That will raise the noise. If you have a high gain design, that might give you a lot of hiss.|
|Austin||8/20/2012 9:20 PM|
|Alan0354||8/20/2012 10:23 PM|
So by itself, it is not smooth, it is bumpy. So there is a noise involve in it as long as there is a current flow through. This is called shot noise ( vs thermal noise that only from resistance). and it is represented by a noise current In where:
Where Ib is the current flow ( grid current in this case), q is charge per electron which is 1.6X10EE-19 coulomb per electron. BW is the bandwidth of interest. In your case for guitar amp, BW=5KHz.
When you calculate noise, it is not at all obvious what is the cause of the noise problem. It all depend on the input impedance, the input current etc. That's the reason I kept challenging in the post about the grid stop resistor. Because until you have all the data, it is not obvious. You cannot lump everything into thermal noise generated by resistors.
In your case here, you still need to find the voltage noise of the tube. Just because I don't see where the resistance can be from a tube don't mean there is none. What do I know about tubes? That's the reason I asked more than once whether anyone has the noise model of the tubes.
Also, the calculation I gave on the gain was based on the gm given in the operating condition of the 6L6GC at the given plate current. At the low plate current that you are trying to run, is the gm still 4700? If it drops off, then you loss the advantage.
|Austin||8/20/2012 11:29 PM|
|Well I didn't decide how much current to let the tubes run at but isn't it true that it will be more linear working into a higher resistance? I have no shortage of tube slots or tubes or power supply so gain is not an issue at all. Right now it has 1 6v6 1 6g6g 3 6f6g and two 6l6gc in there per channel and I was going to use the 6v6 as a plate load ccs and use the 6g6 as the preamp and use one 6f6 as cathodyne phase inverter and two 6f6 as driver tubes into tthe 6L6 power stage. Everything triode strapped for now just to get it working until I make a regulated screen supply. I will try and post some pics tomorrow.|
|J M Fahey||8/21/2012 12:33 AM|
|Not to rain on your parade, but 6L6 (or any other power pentode) transconductance at, say, 1mA plate current (you are feeding it through 100K after all) is WAY lower than at usual power tube current levels.|
You want a very rough, no graphs voltage gain estimation for a 6L6 ? (EL34/84 can be calculated in a similar way):
Just consider this:
Idle: plate voltage=V+ , say, 430V ; grid voltage (bias)= -52V
Saturating: Plate, say, 60V ; grid: 0V .
(Delta means "variation")
Plate variation (delta voltage)= 430-60=370V
Grid variation (delta voltage) = 52-0=52V
Effective gain: 370/52=7 ..... incredible, isn't it?
So, what about that huge transconductance, is it a lie? Not at all, but is is the name to describe a *type* of Physical phenomenon: that when voltage changes, current changes.
In this case , it does not mean huge voltage gain but huge current swing for a relatively small (grid) voltage swing.
So, in a nutshell, use small triodes (or small pentodes if you wish) for gain and big pentodes for power.
Can't go wrong.
Hasn't been wrong for the last 90 years, go figure
|Alan0354||8/21/2012 12:57 AM|
|Merlinb||8/21/2012 4:29 AM|
|Alan0354||8/21/2012 9:12 AM|
|Alan0354||8/21/2012 9:13 AM|
|Austin||8/21/2012 9:33 AM|
|So I should keep something like 30 ma through the tube to keep the transconductance up then? |
Also the 100k grid stopping resistors are alot bigger than I intend to use. Why shouldn't I use smaller less noisy values so a bit of grid current wouldn't be a problem? I am not sure but it makes sense that the grid of power tube has some grid current even while at less potential than the cathode if even the 12ax7 does.
|Austin||8/21/2012 9:54 AM|
|Steve Conner||8/21/2012 10:27 AM|
|Any parameter not specified on the datasheet will do whatever it takes to ruin your day. That's one of the corollaries of Murphy's Law of electronic design.|
It follows that power tubes used as preamp tubes are likely to be somewhat noisy, probably for reasons similar to what Alan0354 set out. Noise figure wasn't on the datasheet, so the factory wouldn't have had quality control in place for it.
I say this with a hint of sadness as I've been looking for discrete transistors specified for low noise in the audio band, but I can't find any in current production, except for the high priced dual arrays, MAT01 and so on.
|Austin||8/21/2012 11:24 AM|
|It won't hurt to try and see what happens I spose.. I'm cobbling something up right now....[ATTACH=CONFIG]19746[/ATTACH]|
|Steve Conner||8/21/2012 11:30 AM|
|Whoa, I can feel the vintage mojo right there!|
|Alan0354||8/21/2012 12:17 PM|
|Alan0354||8/21/2012 12:18 PM|
You need high beta to lower the base current Ib.....which lower the shot noise.
Then the FETs which has the advantage in noise performance in RF, sucks in audio frequency as the 1/f noise is going to swamp all the other sources. Still the good old tried and true BJT.
In fact, I am waiting for 4 of my pcb coming back, those are for pedals that I have been working on. I came here to talk about tubes few months ago and totally dropped it. It was only because last month my Marshall JCM900 blown!! $#X&!!! I have no choice but to talk out my old beat up Fender Bassman 100 and rip out the whole front end and designed my own clean and dirty channel in. That's got me started again on the tube stuff. I like the result which promp me to start looking into the power amp. So far all I did was lower the grid voltage to bring the power tube more towards class A by drawing more quiescent current. Then I changed out the big OT with a Magnetic Components 40W OT for a pair fo 6L6GC instead of 4.
|Alan0354||8/21/2012 6:45 PM|
|This is a good explanation of grid current:|
|Austin||8/21/2012 8:10 PM|
|exclamationmark||8/21/2012 11:10 PM|
|I'm fairly sure it just sums with the plate current and can be measured through the cathode. I recall measuring the characteristics of some 6AS7G tubes and wondering why the voltage was different (not a huge difference but still significant) across each respective 1 ohm plate and cathode resistor when the grid was driven hugely positive. The DC cathode follower arrangement found in a ton of amps also seems to support this, as the cathode follower 'steals' current from the previous stage to achieve proper bias.|
|Merlinb||8/22/2012 5:05 AM|
|Steve Conner||8/22/2012 5:30 AM|
|Good idea Merlin. The transistors have to be surface mount, but we already use the FZT751 in other places, a surface mount version of the ZTX751. I'll try those out for noise.|
It doesn't really address my original gripe, though: nobody makes a transistor actually specified for low-noise audio amplification any more. Diodes Inc. could decide to add some excess noise mechanism to the FZT751 tomorrow, it would still pass their quality control, and there would be nothing I could do about it. Not very convenient for mass production.
I wonder what makers of mixers and mic pres do nowadays.
|R.G.||8/22/2012 5:46 AM|
The thing that helps is that on average, transistor processing is much better than it was in the 70s, which was pretty much the last hurrah of the discrete transistor, and the poor processing that made the worst of noise issues in discretes has been washed out with the semiconductor tide that's lifted all boats.
Still not a pointed design for low noise, but better in general.
|Merlinb||8/22/2012 9:10 AM|
This guy apparently measured the BC337 at 30 ohms http://www.janascard.cz/PDF/Design%2...amplifiers.pdf
Modern opamps are getting pretty darn good though, with noise specs that are approaching those of discrete transistors. Eventually there may be nothing to choose between them. (I recently picked up some free samples of the OP07D for a high source impedance application. It has some impressive specs without being absurdly expensive.)
|Alan0354||8/22/2012 11:06 AM|
Look at the audio frequency performance, if you can find one better, I would love to use it.
There are three sources of noise, base spread is only one of the three. At audio frequency, 1/f noise is more important than others. There is no particular way to predict 1/f noise, you look at the data sheet on the referred input noise voltage and current to determine, not just by base spread resistance. Base spread only tell you about the thermal noise. High beta lower the base current and lower the shot noise. You have to look at your application to determine which is the best.
I look at 2N4403, again there is not noise spec. How come both of you keep looking at transistors that are not specialized for low noise application and keep talking about noise?
|Alan0354||8/22/2012 11:06 AM|
|Alan0354||8/22/2012 11:13 AM|
This one has very good low frequency performance.
|Steve Conner||8/22/2012 11:52 AM|
|Nice! I didn't know about the 2N5087. There is a surface mount version, the MMBT5087.|
|J M Fahey||8/22/2012 11:54 AM|
|Being in an "European Standards" Country I use BC559C. Cheap, available and *very* good.|
The PNP has less noise than the equivalent NPN.
|Alan0354||8/22/2012 1:44 PM|
Go to Digikey, they are cheap. $0.22 ea.
|Merlinb||8/22/2012 2:10 PM|
|Steve Conner||8/22/2012 2:15 PM|
|I went through a load of transistor datasheets looking for noise data. I just didn't happen to find the 2N5087. (Thanks Alan! ) The 2N4403 used to have noise data, but it was removed in the latest version. I don't think the BCxxx series are available in surface mount, and I want to avoid through-hole for this project.|
Merlin, my point is that if you use it for something that's not on the datasheet, you can't complain to the manufacturer when it stops working. For instance, every transistor I ever tried, the base-emitter junction makes a nice 7.5V Zener. But if I used them for that purpose in production, I'd soon be in trouble.
Anyway, enough of our little three-legged fuses. Erm, friends rather.
|Alan0354||8/22/2012 2:20 PM|
You look at the general noise graphs, they rise at the low end, that's 1/f noise. Unless the data sheet specified for that frequency, don't take for granted even you did characterization on a few devices. Yes, if you are willing to hand pick device, it will work, if it does not have 1/f noise, then that device is good. But short of hand picking, don't count on it.
There are plenty of transistor providing all the noise data you need to do all the designs. The two I provide have everything you need to design the low noise front end. What yours are equivalent to 12AT7 or 12AU7 in the tubes, they are medium power transistors that are not meant for low noise front end.
It is not obvious which one to pick, it all depends on the operating conditions, your input impedance, power requirement and all. That's what the noise figure contour graph is for, showing you the noise figure at different impedance.
If it is not because of 1/f noise, all the FETs and the group III and V advanced semi-conductor will out perform BJT hands down, it's not even close. For one, FET has very low shot noise for very obvious reason, they don't draw gate current!!! Problem is their 1/f noise is so high that it is hard to use it in audio application. The Group III and V type has 1/f noise well into MHz. But it works for RF because you use small coupling cap to block the low frequency noise. As 1/f stated, noise is inverse proportional to frequency, noisiest at low frequency.
|Alan0354||8/22/2012 2:38 PM|
Transistors sure are more friendly to me, don't you notice the tubes are kicking my behind right now!!!?
I agree with you 1000% even on the post before that unless it is specified in the data sheet, don't take for granted unless you are willing to hand pick device.
|J M Fahey||8/22/2012 10:21 PM|
|BC559/560 *are* very good and alive and kicking.|
The exact same die packaged in SOT23 is BC859/860
Go to Datasheet & application note database, pdf, circuits, cross reference, data sheet, datasheets | Datasheet Archive and in the search window paste "bc859"
Out of the many datasheets offered, a good one is:
BC859 Fairchild Semiconductor PNP Epitaxial Silicon Transistor
5 pages, 58.94 Kb
Covers both, 859 and 860.
I use them to build potted mini custom gain blocks ("hybrid ICs" if you wish)
|Alan0354||8/22/2012 11:52 PM|
|Merlinb||8/23/2012 4:45 AM|
|J M Fahey||8/23/2012 7:41 AM|
|Same here |
Think of this as a collaborative effort
Love this Forum.
|Austin||8/23/2012 10:19 AM|
|So say I configure my input pentodes as triodes by connecting the screens to the plates with 100 ohm resistors just to get it working, if I add screen supply afterwards would I then maybe have too much gain?|
|Alan0354||8/23/2012 10:22 AM|
|J M Fahey||8/23/2012 11:22 AM|
If it's a "Metal" amp, there is *never* such a thing as "too much gain".
|Austin||8/24/2012 5:35 AM|
|J M Fahey||8/24/2012 9:27 AM|
|Because of your nickname and Avatar, I *never* thought you were a "metal" guy|
|jbefumo||11/23/2015 5:07 AM|
|A bit late to this thread, but I encountered it by wondering if I could use a 6BM8 as a preamp tube. Not trying just to be different. the 6BM8 is, to my knowledge, the only pentode/triode back in current production. I'm using a pair of them in a lunchbox amp with the triode sections (5751 Equiv, I believe) for phase inverter, and the pentode sections (EL82 equiv) in push pull, fixed bias, for around 12W power section. Have used EF86s with good results in several builds (following Merlin's guidelines closely), but obviously, pentode and triode in a single socket would be an advantage. Hence, using the 6BM8 would be advantageous if it sounds good, and can be configured so as not to stress the power supply too much. Were it not for the space issue, I would be inclined to use them in multiple spots, i.e., in the first stage for the touch sensitivity, in the OD for its driven tone.|
|jbefumo||10/12/2017 9:04 AM|
|I actually have a reason for investigating this question, other than just trying to be different 8^) I've used a variation on the Dumble OD scheme (i.e., a 2-stage bypassable overdrive circuit), in which the second stage is an EF86 (configured for relatively low gain), and like the sound. A bunch of guys in another forum do something similar with triode/small-signal-pentode tubes, to reduce part count. Unfortunately, there are no such tubes in current production, and I prefer to avoid designing to obsolete components. The only current production triode-pentode I know of is the 6BM8, which is a high-mu triode and a power pentode (they work great as a 1-tube reverb-driver/recovery, btw). I guess the only way to judge the tone will be to go ahead and try, but would be interested in any contraindications before wasting time ....|
|Enzo||10/12/2017 9:57 AM|
|Tube circuits are SOOOO lacking in criticality. Try it, even if it doesn't "work" on paper, it may well work fine in your application. If my mom liked driving around in a cement mixer, she could use one to go shopping, even if it didn't "make sense". We have working circuits with 12v on triode plates, we have TV sweep tubes in hifi amps, you can "abuse" tubes all over the place and they still function. it either sounds pleasing or it doesn't. Give yourself permission to screw around with it.|
As to obsolete components, there are plenty of small pentodes NOS that will be in large supply for years to come. Certain ones commonly used in amplifiers may have been snatched up already, but unless you are going into production, a box of say 10 of some odd type ought to be a lifetime supply for a one-off amp, obsolete or not.
I haven't bought tubes in a while, apparently Sovtek gave up on the 7199. triode-pentode.
Considering cost of the new 6BM8, I might trade a couple parts counts for saving $20 per tube.
|Gnobuddy||10/13/2017 12:25 AM|
I have been tinkering with 6JW8 (triode pentodes) in guitar preamps, with promising results, but there are still some issues to iron out. It would be interesting to see if anyone else is using the same valve, and if so, what sort of circuits they're building around it.
But a small-signal triode is a small-signal triode; it doesn't matter what it was originally designed to do, it will amplify audio! And a properly used preamp tube will probably last a couple of decades, so it's pretty likely that if I have one or two in stock, they will outlast me.
Another way to look at it: buy twenty NOS $1 triode-pentodes, and you can have twenty times as much fun experimenting as you would get from one new-production $20 valve!
|Mick Bailey||10/13/2017 1:26 AM|
|The 6BM8 works fine configured as a small signal amplifier. I've used this tube a fair bit in hi-fi builds as a preamp/driver tube. There's also a commercial amp from the 60s, the Telewatt V-112, that used the tube as the final stage of a preamp (pentode side) and PI (triode side).|
The real bargains are the TV versions of tubes. Here in the UK the 6BM8 is known as an ECL82. The same tube with 16v heater voltage is PCL82 and are way cheaper for Mullard/Brimar NOS.
|Old Tele man||10/13/2017 10:21 AM|
|ANY multi-grid tube can be used as a preamp as long as it produces gain, but there are reasons why some tubes are better than others:|
1) Multi-grid tubes tend to generate more odd-order harmonics as the number of grids increase; so triodes are preferred for lowest distortion.
2) Multi-grid tubes can be configured to provide "adjustable" gain, triodes can't; so for amplification flexibility a multi-grid tube is preferred.
3) Multi-grid tubes have much higher dynamic plate load values than triodes and thus present LESS loading to subsequent circuits.
4) Multi-grid tubes behave much like constant-current sources making them easier to interface to / from.
5) Multi-grid tubes can utilize much more of their Eb-Ib curve and thus are more efficient at power utilization.
6) Multi-grid tubes require more support components than triodes; so triodes are simpler to use, circuit-component wise.
|jbefumo||10/13/2017 8:34 PM|
|J M Fahey||10/13/2017 10:03 PM|
|Just as a side note: during WW2 here in Argentina we had *terrible* supply problems on a lot of stuff.|
NO car/truck tires because Japanese grabbed Far east rubber plantations, all of Brazilian production was bought in bulk by USA and UK, for "some reason" we had no access to German synthetic rubber (BUNA) made at their advanced plant in Auschwitz , so in emergency, some cars and trucks even run on wood or steel wheels, go figure.
For the same reason, NO tubes available, neither European nor USA nor Japanese, all were fighting and had none to spare, so we being resourceful people used *any* tube available. anywhere as needed.
I remember yellowing Electronics magazines saying so, specially that any tube could be used, except rectifiers.
|Mick Bailey||10/14/2017 10:24 AM|
|I guess at a pinch an output tube could also be pressed into service as a rectifier. You'd need two for full-wave, though one of my contacts has some dual 30W triodes that would do nicely.|
|J M Fahey||10/14/2017 10:50 AM|
|Going further with the thread derailing, this is what WW2 soldiers in their foxholes and even concentration camp inmates used when needing an "audio diode" and Mouser was still 40 years into the future: enter the mighty razorblade_and_pencil diode |
|Gnobuddy||10/14/2017 12:18 PM|
|Old Tele man||10/14/2017 1:12 PM|
|Parallel a 100KΩ input load with a 2KΩ triode output and you get something less than 2KΩ upon the input load.|
Parallel that same 100KΩ input load with a 33KΩ pentode output and you get about 25KΩ...more than 10-times LESS loading upon the input load.
It's easier to generate a voltage GAIN across a larger resistance/impedance than it is across a lower resistance/impedance, because it requires less current to create that GAIN.
|Gnobuddy||10/14/2017 1:25 PM|
Parallel a 100k input load with a 2k triode output, and you get 98% of the signal voltage making it through the voltage divider - this is called very little loading (only 2% of the voltage is lost). The load (the subsequent stage) has very little effect on the output voltage of the preceding (triode) stage.
|Old Tele man||10/14/2017 1:50 PM|
|"Following" circuits are not always amplifying tubes, quite often they're RL/RC-tone shaping circuits, which attenuate passing signals QUITE a bit, so more loss means less output to subsequent circuitry, which has to MAKE UP (and usually exceed) those losses. A good designer (employed) uses as few components as necessary because of cost.|
|pdf64||10/14/2017 2:00 PM|
|It seems topsy turvy to describe outputs as loading inputs?|
|Old Tele man||10/14/2017 2:35 PM|
|Loading is NOT a unilateral event, it goes BOTH directions.|
|J M Fahey||10/14/2017 2:41 PM|
|Sorry, still not the best description of whatīs going on.|
Parallel a 100KΩ input load with a 2KΩ triode output and you get something less than 2KΩ upon the input load.
It is driving the next stage, which has a 100k grid resistor.
Also applies to passive elements, next stage might have no tube whatsoever but consist of just a 100k resistor (passive load) to ground and nothing else.
Or a capacitor or inductor or any combination which, at least at that frequency, shows 100k impedance.
Any of these is loading the triode (generator) which has 2k internal impedance so in principle itīs lightly loaded.
The 2k is not "loading" anything, itīs the generator internal impedance, and definitely is not loading itself.
Parallel that same 100KΩ input load with a 33KΩ pentode output and you get about 25KΩ...
To be more precise:
* "a load" goes from generator output to ground.
* "internal impedance" is in series with generator output.
Clearly these two concepts are very different.
more than 10-times LESS loading upon the input load.
A load loads a generator.
If you put 2 loads in parallel (such as 2 speakers in parallel) would you say one is loading the other?
|pdf64||10/14/2017 3:01 PM|
Is the hill forcing the cyclist to pedal harder, or is the hard pedalling creating the inline?
|Old Tele man||10/14/2017 3:15 PM|
|The control-grid of a triode is 'normally' a HIGH impedance, so it presents a HIGH-Z load to whatever is sourcing/driving it. But, as soon as that grid goes positive enough to draw current (even slightly) it promptly becomes a LOW-Z load back to what is driving it...it has loaded its source beyond what it was originally intended to do.|
As investigators like to say: "...Follow the current (instead of money)..."
|Gnobuddy||10/14/2017 3:28 PM|
It is, of course, entirely up to you what you want to do about that.
|pdf64||10/14/2017 3:35 PM|
Whatever, I donít understand how that relates to your hypothesis of Ďa source impedance loading the following inputí?
|Old Tele man||10/14/2017 3:51 PM|
|Go back to my original post to the OP question. I was describing why multi-grid tubes CAN be used as preamps, but not always for the better.|
|cjenrick||10/17/2017 4:28 AM|
|those big plates might rattle around a bit more than a RCA 7025,|