6AS7 Push Pull

[Cressy Snr]

Here is the amp I'll be taking to Whitham.
Not the OTL dissapointingly, but it sounds quite nice. Full-bodied and quite a little powerhouse.

Because I was using the big transcendent power supply I took the opportunity to really crank up the HT on both the input and driver stages.
We have 535V to the first two stages at 100mA capability and 220V at the power valves with the TX winding capable of providing 1.5A if necessary, so plenty of headroom there.

Heaters are fed from three separate centre-tapped windings on the power transformer; one 3A winding per channel for the 6SN7s and an 8A winding for the power valves. All heating in this case is AC rather than the 25V series DC scheme used in the OTL. Therefore that transformer has been disconnected. Also the bias transformer was disconnected plus the preamp transformer.
As I was putting it together I was making it up as I went along due to the different PSU necessitating changing quite a few components from the original scheme. I also left out the balance pot between each section of each power valve, basically because I couldn't be bothered with it, not for any sound engineering reason.
I had a soldering iron in one hand and a copy of the data sheets for the 6SN7s in the other, plus a calculator on the computer screen; real seat-of-the-pants stuff, so it was a bonus when it worked straight away.
I could get to like this way of working.


There is more than enough gain to connect the volume pot straight to the input transformer/phase splitter and powered by that huge PSU, this is quite a capable amp.

I had thought that, in a pp amp, the power supply was not as important as in a single-ended amp. On the evidence I am hearing, things don't seem quite that straightforward.

Steve
Cct below.

[Nick]

Might be me being daft, but have you got the sn7 and the sl7 the wrong way round?

[Cressy Snr]

Ahhh....

Didn't spot that. There aren't even any 6SL7s in there any more. Both stages are actually 6SN7s 'cos I'd sold all my 6SL7s. I had never once used them since the EL34 amp a couple of years ago.

BTW I've just worked out that nearly 95% of the cost of this amp has gone into the power supply even allowing for the prices of the valves used in it if I had had to buy then new.

Steve

[Cressy Snr]

Hi
Parts of this are only a repeat of what is in the "beginners" section where I asked about how to wire up a long tail pair correctly, so apologies in advance.

With help from Paul I modified the diff amp input and driver stages so that they now function as proper long tail pairs.
The sound is much better. The power is still there but the sound of the amp is possessed of more subtlety and finesse in the way it goes about its business.

The outcome of the OTL valve meltdown has therefore had a positive outcome. I have this 6AS7 push pull which TBH is very very nice indeed, so much so that I think I will keep this amp as it is, in the fifties retro chassis and rebuild the OTL on a big breadboard. I have plenty of components left to do this, but I think a few toroidal transformers wouldn't go amiss.

IMO, OTLs of this size are so specialised that they really do need to be breadboarded up first and any bugs ironed out where they can do the least amount of damage to both fixtures and fittings and the nerves of the builder.

OK, yes, I got it working in the end but I wouldn't trust it in the living room without much more extensive testing and tweaking. The PSU in particular I don't particularly like. Hum levels were far too high IMO. The PSU appears to be far too willing to radiate interference. The complementary power supply to the output stage was particularly noisy and spiky despite the capacitor snubbing. Breadboarding up the OTL will also give me the chance to let a few other, more experienced people such as James, Paul and Nick inspect and evaluate it, maybe at the next Owston.

Speaking of power supplies. After Nick had mentioned that he was slightly disappointed with the hum levels in the GM70 monoblocs, I became a bit concerned with the hum levels in the push-pull amp. After all pp amps are supposed to be able to reject hum much better than single-ended designs, yet I had audible hum at idle on mine.

The upshot of this is that I have ripped out the Rozenblit PSU and rebuilt the whole power supply to my own spec using Duncan Amps PSUD and full wave SS rather than bridge rectification.
The amp is now completely silent!

Heaters are AC throughout, one winding per stage. The driver and output stages are fed from separate cap input power supplies with three stages of RC filtering. They supply the input/driver stages with 530V @ 100mA and the output stages with 200V@ 1A.



Here is the cct repeated from the beginners section



Seems I have ended up with my own amp instead of someone else's again.

I have not by any means given up on the OTL. The sound I got out of it running on four valves was very seductive and it would be a shame to let it go. But this pp amp will do very nicely thank you.

Steve.

[Nick]

Seems I have ended up with my own amp instead of someone else's again.

Always should work out like than IMHO.

[Cressy Snr]

The great thing about these low mu, low resistance power valves is the fact that the builder can specify an output transformer with a low turns ratio, which makes it much easier for the transformer manufacturer to get extended bandwidth.

To this end I have today ordered a pair of size F custom output transformers from my fave manufacturer Sowter. These have a 600 ohm primary, so I should be able to get a bit more power out of the 6AS7s.

The lowest I could get Philip's very kindly loaned Hammonds to, with my speakers, was 2k5, which is over 8XRa.
This is unavoidably compromising the maximum power I can get from the 200V of HT I have available. I could not go any higher anyway even if I wanted to.

The only disadvantage I can see with the custom transformers is that they will be practically useless for anything else except maybe 6C33Cs or the EL/PL series of TV horizontal line amplifiers.

The transformers have been specced to take 250mA per side. Overkill perhaps in a push-pull TX but in view of possible future experiments I thought I had better get them robustly made.

It is interesting that some posters on other forums deem these types of valves not worth the effort to make them work properly, when there are plenty of good established audio valves that are much easier to work with. I say bo***cks to that!

"Me too" products are great when you start out but when you get the experimental bug, you want something a little different. Well I know I do anyway.

Steve.

[Max N]

Hi Steve
That's a nice looking circuit
I think there may be a small error in the way you've drawn the input stage - or it may just be me being thick
Should the secondary of the input transformer be grounded at the centre tap, as shown? It seems to me that this will bias V1 with 17K which will be too much. Should the centre tap actually go to the junction of the 2K and the 15K?

If you do decide to try a ccs in the tail, you could try the modules from www_kandkaudio_com
click on accessories
(can't post a link cos I am a noob, so you'll have to replace the underscores with dots)

[Cressy Snr]

Hi Max

The circuit has actually been altered since I put that one up. I've been too idle to post it.

The 6SN7 on the input has been changed to a 6SL7 in order to get a bit more gain. I bought a nice pair of RCA (1950) ones the other day. The problem at the moment is that the pair of Hammond 125D output transformers kindly loaned by Philip (Pre65) are causing the amp to be quite a bit down in power (>8XRa is a bit high with only 200V of HT at the output valves)

The 6SL7s have boosted the gain to give a nice increase in dynamics and drive. It'll be even better when I get the 600R output transformers wound.

The very high HT has enabled me to put a nice big 300K resistance at the anodes for plenty of voltage drive.

With the 3K cathode resistors I can get a massive voltage swing right up to 500V. I have measured zero difference between the two halves of the input stage diff amp. The anode and cathode resistors were matched into pairs with the meter before being fitted.

So in short I have, for the moment, dispensed with the tail resistor on the input stage. The choke tail pair driver stage has been left well alone though.

Here is the latest version of the cct with the relevant driver stage voltages as measured today.



Simon came round today for some 6080s to use in his current PSU experiments and had a listen to the amp as it is now. We both agreed that it sounded very nice indeed driving the Metronomes. The speakers seem to have responded very well to an amp with a bit more poke than the SE amps I have been using up to now.

As to why the amp sounds so nice, I would cite its relative simplicity and fact that the phase splitting is carried out before anything else and is done passively. There is also the fact that the input and driver stages have been done using valves with inherently low distortion, and that the diff amps used have inherently good balance and common mode noise rejection, giving a very low noise floor. The triode output stage produces very little odd order distortion and runs in class A so there is no need for feedback to linearise the amp. All we need now are the new output transformers to let the flavour flood out. When I first lashed it up, the 12B4A preamp section of the OTL was still in the circuit. It nearly blew the windows out; so I'm looking forward to making some noise with the Sowter OPTXs.


Steve

[Cressy Snr]

Well I put the 12B4A preamp section back in today as the passive arrangement I had seemed to be leaving some of the life behind at the output of the CD player.

Reading about the Amity on Lynn Olson and Gary Dahl's site I could see that the way the input/driver stages of my amp are laid out is very similar to the Amity/Aurora setup even down to the use of a phase splitting transformer right at the front.

Olson talks about the fact that with an input transformer/phase splitter, a passive volume control on the input is not recommended, as the way it interacts with the input transformer can very easily throw away half the top end!
He recommends using a preamp with a low output impedance but advises against cathode followers, so we are left with having to use preamp valves with low plate resistances. So the 12B4as seemed ideal for the job.

The 12B4As have a gain of 5; so for 2 volts output from a CD player, they will put 10V after the phase splitting transformer, into the grids of the 6SL7 differential voltage amps, easily driving them to full power. This is giving the 6SN7 drivers a far better chance of pushing the output valves to a decent power level.

Much better. The speed has picked up and there is a bigger soundstage, with plenty of fine detail being dredged up, and a better sense of the presence of the performers in the room. The top end has improved dramatically, the 12B4As seem to be be a much better match for the phase splitting transformer than the passive control was.

A slight downside to all this, is that hiss has increased at idle. Whereas before there was silence, with an ear to the speaker, there is now a gentle ssshhhh with a tiny bit of hum thrown in.

The advantages though outweigh the slight increase in noise, so I can live with it. Engineering is all about compromise anyway.
Steve

[Paul Barker]

That's a good example of how less is not always better. Each new active stage blended correctly can add a positive overall contribution. It is now long gone the era when I believed that less active stages sound best, though you would be able to dredge up from distant memory times I aplauded fewer stages.

Jim de Dutchman VT52 has always stood against that method, and gradually won most experienced people back with him.

[Nick]

Much better. The speed has picked up and there is a bigger soundstage, with plenty of fine detail being dredged up, and a better sense of the presence of the performers in the room. The top end has improved dramatically, the 12B4As seem to be be a much better match for the phase splitting transformer than the passive control was.

Also (just to be awkward), you have added a single ended stage, maybe you are adding a bit of the 2nd harmonic back that the following balanced stages were doing their best to avoid.

I am not suggesting the top end improvement and speed is due to this, as you said, its probably a driving issue, but all the other terms you have used to describe the sound, ARE those often used to describe the sound of SE after PP

[Cressy Snr]

Yes,

Less stages are fine I suppose where you are using easily driven power tubes but in the case of the 6AS7 I think I have found out just how much sheer welly these things really do take to drive them properly. A difficult amp to implement for someone doing a first push-pull build but a lot has been learned doing it and I have a sound that that has certainly rewarded the effort put into it.

Interesting thought on the single-ended input stage Nick. I think it was definitely a drive issue, coupled with an impedance mismatch between the volume pot and the phase splitting transformer, which took out the very high treble and tilted the response up at the bass end. However there may be something in your theory Nick although you might have meant it a bit tongue-in-cheek.

The amp is after all now a perfect example of Paul's mix, match and blend philosophy. We have a single-ended input stage, transformer phase splitter, high gain differential voltage amp, choke-tail-pair driver stage and push pull transformer coupled outputs.

I've been a bit reluctant in the past as I didn't nuch care for the sound of Black Gate caps but it might be time to play around with improved componentss and all that fancy stuff, but that'll start to cost money.

Here's the circuit now



Steve.

[Nick]

Well, if you can explain why inproving the drive to the transformer can provide "a better sense of the presence of the performers in the room", I will admit to it being only tounge-in-cheek

[Cressy Snr]

Well, if you can explain why inproving the drive to the transformer can provide "a better sense of the presence of the performers in the room", I will admit to it being only tounge-in-cheek

Er.......yes....well

[Nick]

Of course, you missed the obvious, as I cant explain why adding a single ended stage would do that that either