Small Push-Pull Amp.

What people are working on at the moment
Cressy Snr
Amstrad Tower of Power
Posts: 10552
Joined: Wed May 30, 2007 12:25 am
Location: South Yorks.

#1 Small Push-Pull Amp.

Post by Cressy Snr »

I like playing around with valves.
Not that this is going to replace the SEOTL, but I do fancy a play around with the idea of another small, push-pull valve amp.
Not decided yet what valves to use, but I have a drawer with quads of EL34, KT66 PCL86, PL36 and 6V6.
The last push pull amps I built disappeared up their own backsides with added complexity; lesson learned, so simple is the way to go.
I also have a pair of proper Telefunken ECF83 triode/pentodes, I got from God knows where and rejected years ago because the damned pentode section is remote cutoff instead of sharp. Now I know a bit more, they might be of use as an input stage/cathodyne phase splitter, or maybe not; not worked out how to use them yet.

Anyway we’ll see.
Sgt. Baker started talkin’ with a Bullhorn in his hand.
User avatar
Ali Tait
Eternally single
Posts: 4373
Joined: Fri Jun 08, 2007 8:10 pm
Location: Galashiels

#2 Re: Small Push-Pull Amp.

Post by Ali Tait »

I remember liking that EL34 PP Nick made for one Owston.
User avatar
Nick
Site Admin
Posts: 15705
Joined: Sun May 06, 2007 10:20 am
Location: West Yorkshire

#3 Re: Small Push-Pull Amp.

Post by Nick »

Ali Tait wrote: Wed Jul 17, 2019 3:26 pm I remember liking that EL34 PP Nick made for one Owston.
I had to think, do you mean the one I made using Collins output transformers and the sowter interstage phase splitter?

Still have most of that somewhere.
Whenever an honest man discovers that he's mistaken, he will either cease to be mistaken or he will cease to be honest.
User avatar
ed
retired
Posts: 5384
Joined: Thu Jun 21, 2007 4:01 pm
Location: yorkshire
Contact:

#4 Re: Small Push-Pull Amp.

Post by ed »

You guys have much better memories than me, but.....

didn't our man in Bedford make a little push pull el34 years ago? I seem to vaguely remember it being red and everybody liked it at the time...it may have been the time he played Stravinsky's 'firebird'....but then again it might not have...
There's nowhere you can be that isn't where you're meant to be
Cressy Snr
Amstrad Tower of Power
Posts: 10552
Joined: Wed May 30, 2007 12:25 am
Location: South Yorks.

#5 Re: Small Push-Pull Amp.

Post by Cressy Snr »

ed wrote: Wed Jul 17, 2019 4:42 pm You guys have much better memories than me, but.....

didn't our man in Bedford make a little push pull el34 years ago? I seem to vaguely remember it being red and everybody liked it at the time...it may have been the time he played Stravinsky's 'firebird'....but then again it might not have...
I think that was 45s and a load of Majestic transformers.
Sgt. Baker started talkin’ with a Bullhorn in his hand.
Cressy Snr
Amstrad Tower of Power
Posts: 10552
Joined: Wed May 30, 2007 12:25 am
Location: South Yorks.

#6 Re: Small Push-Pull Amp.

Post by Cressy Snr »

Having perused data sheets all afternoon, and given that I have 5K a-a OPTs with a single 8R speaker winding, the only configuration that works bang-on with the transformer characteristics, is Class AB, triode connected EL34s with cathode bias and a bypassed pair of cathode resistors- 400V on the anode. That’s yer lot. With my OPTs, PCL86 is not a contender.

I’ve got plenty of EL34s, so that’s not a problem.

It is, I suppose, possible to do KT66s in Class AB triode at high voltage on a slightly shallower loadline than the 4K a-a GEC specify, but I don’t want the the used, grey glass ones I bought off Steve to be subjected to that sort of hammering.

Having re-examined the spec sheets, gawd knows what I was thinking with the EL34 pentode monoblock amps I built.
Pentode in fixed bias needs 3K5 and in cathode bias needs 6K5 at high volts or 3K5 at lower volts. Ultralinear requires 6K5.

The EL34 amp I sold Paul was fine as it had big 3K5 OPTs and fixed bias. The monoblocks were fixed bias at 5K. Small wonder they were so weak and wimpy.

Bloody hell. :lol:
Sgt. Baker started talkin’ with a Bullhorn in his hand.
Ant
Shed dweller
Posts: 2332
Joined: Mon Jul 31, 2017 6:45 pm
Location: Yorkshire

#7 Re: Small Push-Pull Amp.

Post by Ant »

What about 6as7 and those sowter 600r pp output trannies that you had.
3575 on the input, 5751 driver? Nice and simple
Also starring Rex Hamilton as Abraham Lincoln

www.bte-designs.weebly.com
Cressy Snr
Amstrad Tower of Power
Posts: 10552
Joined: Wed May 30, 2007 12:25 am
Location: South Yorks.

#8 Re: Small Push-Pull Amp.

Post by Cressy Snr »

That’s a good idea.
I’ve still got a few 6AS7s, including four coke bottle Russian ones courtesy of Mr I. :)

Should do a nice 10WPC.
Sgt. Baker started talkin’ with a Bullhorn in his hand.
User avatar
izzy wizzy
Old Hand
Posts: 1496
Joined: Fri Nov 02, 2007 7:02 pm
Location: Auckland NZ
Contact:

#9 Re: Small Push-Pull Amp.

Post by izzy wizzy »

You could use EL34 or 6V6 into the 5k OPT. Class A of course coz AB is pooh and it keeps the power supply nice and smooth and not bouncing all over the place. OK, it might be a bit low Z but ...

The sound of the thing will probably be dominated by what you do before it and how you deal with splitting phase and how you maintain balance. I don't think you can beat a transformer for the splitting. As Ant suggested, a nice Sowter 3575 on the in. And then a diff amp into the finals. That diff amp will be the key and you can flavour it however along with the coupling method.

Cheers,
Stephen
User avatar
Ali Tait
Eternally single
Posts: 4373
Joined: Fri Jun 08, 2007 8:10 pm
Location: Galashiels

#10 Re: Small Push-Pull Amp.

Post by Ali Tait »

Nick wrote: Wed Jul 17, 2019 3:43 pm
Ali Tait wrote: Wed Jul 17, 2019 3:26 pm I remember liking that EL34 PP Nick made for one Owston.
I had to think, do you mean the one I made using Collins output transformers and the sowter interstage phase splitter?

Still have most of that somewhere.
Yes that sounds about right I think Nick. I do like an EL34.
User avatar
Ali Tait
Eternally single
Posts: 4373
Joined: Fri Jun 08, 2007 8:10 pm
Location: Galashiels

#11 Re: Small Push-Pull Amp.

Post by Ali Tait »

Cressy Snr wrote: Wed Jul 17, 2019 4:54 pm
ed wrote: Wed Jul 17, 2019 4:42 pm You guys have much better memories than me, but.....

didn't our man in Bedford make a little push pull el34 years ago? I seem to vaguely remember it being red and everybody liked it at the time...it may have been the time he played Stravinsky's 'firebird'....but then again it might not have...
I think that was 45s and a load of Majestic transformers.
Yeah that was a cracker IMO.
Ant
Shed dweller
Posts: 2332
Joined: Mon Jul 31, 2017 6:45 pm
Location: Yorkshire

#12 Re: Small Push-Pull Amp.

Post by Ant »

I think the red one that people remember was that el84 with an enormous power supply wot MI dad built. Had a massive transformer and a 22000uf cap in it iirc
Was nice that one
Also starring Rex Hamilton as Abraham Lincoln

www.bte-designs.weebly.com
Ant
Shed dweller
Posts: 2332
Joined: Mon Jul 31, 2017 6:45 pm
Location: Yorkshire

#13 Re: Small Push-Pull Amp.

Post by Ant »

This one is the thread I think
http://www.audio-talk.co.uk/phpBB2/view ... =11&t=4821
Not sure why there aren't any images
Also starring Rex Hamilton as Abraham Lincoln

www.bte-designs.weebly.com
Cressy Snr
Amstrad Tower of Power
Posts: 10552
Joined: Wed May 30, 2007 12:25 am
Location: South Yorks.

#14 Re: Small Push-Pull Amp.

Post by Cressy Snr »

Trawling through my valve box upstairs, I find a NOS pair of Tung-Sol 6336A. Now that’s interesting :wink:
A stereo, single ended amplifier in one tube, or a push pull with two of them.

Now, I have actually built an SE amp with one of these before. It was only a two-stager with just about enough gain to wheeze its way through a pair of my original FE108EZ Fostex Metronomes, but I was pretty crap at amp building then and didn’t have a clue about gain structure.

I could get a 6-8W stereo SE amp out one of these tubes now. The curves are very, very good looking, and a 2K5 loadline looks just the job for SE; therefore a 5K a-a transformer will also be just the job for a reasonably powered, push-pull, indirectly heated, true triode Class A amp; none of that triode strapped pentode stuff.
Sgt. Baker started talkin’ with a Bullhorn in his hand.
User avatar
Paul Barker
Social Sevices have been notified
Posts: 8862
Joined: Mon May 21, 2007 9:42 pm

#15 Re: Small Push-Pull Amp.

Post by Paul Barker »

Nowt wrong with triode strapped valves. Some prefer gk71 over gm70. I liked 813 triode connected as much as I loved 211 for their unique stand out extra not heard in most amplifiers. Entirely different to each other yet only mood or tastes depending on which way your preference lies.
"Two things are infinite, the universe and human stupidity, and I am not yet completely sure about the universe." – Albert Einstein
Post Reply