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Old 11th Oct 2021, 6:33 pm   #1
G6Tanuki
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Default He's been playing around again: self-driving P-P valve output stage!

I've always been fascinated by the idea of 'something for nothing' and when I came across the idea of replacing the classic 'see-saw' common-cathode push-pull phase-splitter with actually driving the push-pull power-valves as a see-saw, it appealed to me.

So I kludged together the following circuit. Two [unmatched] 6V6 with the cathodes commoned, one has its control-grid grounded, the other's control-grid is the input.

Suppressor-grids commoned to a shared feed-resistor and decoupler.

I used one of the RS "Universal" 3.5-Watt output-transformers as a load; these are rated for 40mA primary current but I reckoned that was more a single-ended consideration and in this application the two anode-currents would balance out to avoid any core-magnetisation.

So I built it - with a 2-position switch which in one position wired a bypass-capacitor across the 270-ohm cathode-resistor anr in the other position switched this capacitor out and added a second parallel 270-ohm resistor.

The idea being that in one position, if I pulled-out the grounded-grid 6V6 I could flick the switch and give the remaining 6V6 its traditional 270-Ohm bypassed cathode rsistor, in the other position - with the grounded-grid 6V6 in circuit the pair then got 140-ohms of unbypassed cathode-resistor about which to see-saw.

So I could do a quick-and-dirty switch between self-balancing push-pull and single-ended.

The result - well, though I happily say I don't have 'golden ears' andI was never expecting any kind of 'HiFi' experience, the two-6V6 solution definitely __sounds__ louder.

Yes, I know the differing anode-impedances between the grid-driven and cathode-driven parts of the push-pull part _should_ introduce all sorts of odd non-cancelled-out-by-the-push-pulledness issues, but _I_ cant hear these supposed artefacts.

Thoughts? Ideas?? Suggestions? My [non-golden] ears are open! What say you, people??
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Old 11th Oct 2021, 6:48 pm   #2
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Default Re: He's been playing around again: self-driving P-P valve output stage!

That is very similar to something I did, but on a much smaller scale. I built my first audio amps using wired 6021 valves 3 of them.

I basically followed a circuit called the Powerman, but rather then let it overload I added negative feedback to keep it cleaner. i did ask members on the forum for comments about the see-saw type final outputs. As to Golden ears, well it sounded good to me for my first valved audio amp and did for a few months.

So the idea has definitely been used in small guitar practice amps.

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Old 11th Oct 2021, 7:25 pm   #3
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Default Re: He's been playing around again: self-driving P-P valve output stage!

It's the valve equivalent of a long-tailed pair.

You'd get better push-pull symmetry with a constant current source in place of the 135 ohms. Ideally the cathode effective R to ground should be >> 1/Gm, i.e. >> 270.
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Old 11th Oct 2021, 7:38 pm   #4
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Default Re: He's been playing around again: self-driving P-P valve output stage!

In a similar vein, anode and screen grid derived (starting at book page 585):

http://www.tubebooks.org/books/rdh4.pdf

Lawrence.
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Old 11th Oct 2021, 9:02 pm   #5
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Default Re: He's been playing around again: self-driving P-P valve output stage!

A young friend did this.
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Old 12th Oct 2021, 1:43 pm   #6
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Default Re: He's been playing around again: self-driving P-P valve output stage!

Quote:
Originally Posted by John_BS View Post
You'd get better push-pull symmetry with a constant current source in place of the 135 ohms. Ideally the cathode effective R to ground should be >> 1/Gm, i.e. >> 270.
Agree! Though, i don't doubt that the Post #1 circuit works, and encouragingly well, too!

Quote:
Originally Posted by G6Tanuki View Post
Yes, I know the differing anode-impedances between the grid-driven and cathode-driven parts of the push-pull part _should_ introduce all sorts of odd non-cancelled-out-by-the-push-pulledness issues,
It won't with a truly constant-current 'tail' because then whatever one valve increases its current by, the other decreases it by the same amount.

With the resistor this ain't quite so because the 'tail' current is not quite constant.

The circuit Greg has put up is an embodiment of John_BS suggestion, and I've got no doubt that it is capable of good results. I've not met the author yet but she's uncommonly-well switched-on...
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Old 12th Oct 2021, 3:05 pm   #7
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Default Re: He's been playing around again: self-driving P-P valve output stage!

Quote:
Yes, I know the differing anode-impedances between the grid-driven and cathode-driven parts of the push-pull part
Are they really different? I can't see the mechanism.
 
Old 12th Oct 2021, 3:43 pm   #8
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Default Re: He's been playing around again: self-driving P-P valve output stage!

There is an article by Melvin Leibowitz over on DIY Audio projects forum from 1961 in its simplest form. Here:http://diyaudioprojects.com/Technica...wer-Amplifier/

And Bruce Heran's ''Oddwatt'' with the addition of a CCS (using an LM317 in the cathode resistor position) here: http://diyaudioprojects.com/Tubes/EL...Pull-Tube-Amp/
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Old 13th Oct 2021, 5:52 pm   #9
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Default Re: He's been playing around again: self-driving P-P valve output stage!

Thinking about a constant-current-circuit for the common cathode tesistor, and not wanting to do it using semiconductors, I dug out ftom the attic a Parmeko 10-Henry 200mA choke, whose DC resistance was 96 Ohms, so needed a bit of resistive padding to get the bias back to where it should be.

[My thought being that the behaviour of a choke is essentially to oppose changes in the DC current flowing through its winding].

To be honest, my 'lead ears' [damaged by an explosion - I can't hear much below 200Hz] couldn't tell the difference between the simple resistor and the choke+resistor combination.

Still, it's fun to experiment with this sort of thing - as they say "We don't stop playing because we grow old, we grow old because we stop playing!"
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Old 14th Oct 2021, 9:47 am   #10
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Default Re: He's been playing around again: self-driving P-P valve output stage!

it's a neat and elegant option! Far better than a resistor itself.

I was thinking about how to calculate the inductor, for the cases where you don't have a handy Parmeko component in their attic, and I came up with a rule of thumb that the inductive reactance should be 10x greater than 1/gm for the valve at the lowest frequency of interest.

So, for a gm of 4mA/V (approximately 6V6), we're looking for 2.5kΩ, giving L = 7.95H. The 10H choke is therefore a good fit!
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Old 14th Oct 2021, 7:15 pm   #11
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Default Re: He's been playing around again: self-driving P-P valve output stage!

Intriguing to see that my instinctive choke-kludge has calculatable merit! And equally, fascinating to look at some of the other links showing that the 'self-balancing see-saw push-pull' output stage has had successful use in the past.

And that the idea has also been done 'Ultra-Linear' !

I'm now pondering whether to rework my design [which uses a couple of 6V6] and see what it can do with some rather-well-used EL84.
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Old 15th Oct 2021, 11:30 am   #12
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Default Re: He's been playing around again: self-driving P-P valve output stage!

Hello,

Just picked up on this thread as I’ve has similar thoughts as regards the use of the Long Tail Pair output valve configuration. Whilst musing this idea I found the Ferguson 289 uses this configuration.

I’ve attached some screen shots from the manual if it helps.

Just my two penn'orth...

Regards
Terry
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Old 15th Oct 2021, 11:38 am   #13
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Default Re: He's been playing around again: self-driving P-P valve output stage!

To drive the 'distant' valve to swing over the current range complementary to the current range of the 'near' valve. So the grid-cathode voltage of the distant valve has to have several tens of volts of swing on it to achieve this.

The only place this voltage swing can come from is the common cathode voltage of a long tailed (or short tailed) pair. So the DC drop on the cathode resistor has to be quite significant.... and the same minimum applies as if it was the lower section of a concertina phase splitthey have a hard enough task to perform, so having some light current frippery doing the phase splitting is a good fit to the job in hand. It feels right, it works right and there is mathematical justification from several directions.

David
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Old 15th Oct 2021, 3:53 pm   #14
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Default Re: He's been playing around again: self-driving P-P valve output stage!

Quote:
Originally Posted by Radio Wrangler View Post
To drive the 'distant' valve to swing over the current range complementary to the current range of the 'near' valve. So the grid-cathode voltage of the distant valve has to have several tens of volts of swing on it to achieve this.

The only place this voltage swing can come from is the common cathode voltage of a long tailed (or short tailed) pair.
True.

Quote:
Originally Posted by Radio Wrangler View Post
So the DC drop on the cathode resistor has to be quite significant....
Ah. But that requirement goes away with G6Tanuki's innovative use of a cathode choke. The resistance (choke + external added R) provides the bias, and the inductance can swing the paired cathodes above and, if necessary, below 0V.

If each valve needs 5V peak to drive it, (thus 10V grid-grid) you just have to drive the available grid with 10V peak, and the choke will take care of the other.
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Old 15th Oct 2021, 4:57 pm   #15
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Default Re: He's been playing around again: self-driving P-P valve output stage!

True.

The choke saves the efficiency loss, but it needs enough Henries to not truncate the LF response and it needs to be meaty enough to take the DC currents of both valves adding together without saturating enough to distort. So the DC component cancellation that the output transformer benefits from, doesn't apply to this ironware.

Can't win 'em all. But the size, weight and price of that choke might fund a lot of small components.

DAvid
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Old 15th Oct 2021, 10:16 pm   #16
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Default Re: He's been playing around again: self-driving P-P valve output stage!

I like the Ferguson 289 method of hum neutralisation - assuming it provides a practical benefit given that there is feedback to assist as well.

With respect to a cathode choke, the incremental inductance achieved would depend on how far out on the BH curve the DC bias point would sit. And how large a swing along the BH curve would depend on frequency and peak signal voltage, somewhat similar to parafeed choke load. If using a topology like the Ferguson with its GNFB then may need to be careful of additional LF resonances for LF stability.
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Old 15th Oct 2021, 11:18 pm   #17
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Default Re: He's been playing around again: self-driving P-P valve output stage!

This thread had what looks like a home build long-tail pair PP output amp that was to be reworked. https://www.vintage-radio.net/forum/...d.php?t=183439

The parts mostly look 1950 or earlier so similar age to the Ferguson.
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Old 16th Oct 2021, 7:11 am   #18
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Default Re: He's been playing around again: self-driving P-P valve output stage!

A variation on the 'something for nothing' theme, only usable if the power stage stays in class A (and the OPT has to have two seperate anode windings). The value of R is given by 1/gm.
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Old 26th Oct 2021, 3:13 pm   #19
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Default Re: He's been playing around again: self-driving P-P valve output stage!

The only benefit I can see with this is that you can use a PP transformer which might be cheaper than an SE transformer (if iron is more expensive than copper). Might also suit a very low-watt circuit. but otherwise I can't see why anyone would do this with decent sized bottles; when you've paid for them and the PT and OT, the cost of a phase inverter is basically nothing and you can run in class AB for more power too.
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