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Small Push-Pull Amp.

Posted: Wed Jul 17, 2019 1:17 pm
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.

Re: Small Push-Pull Amp.

Posted: Wed Jul 17, 2019 3:26 pm
by Ali Tait
I remember liking that EL34 PP Nick made for one Owston.

Re: Small Push-Pull Amp.

Posted: Wed Jul 17, 2019 3:43 pm
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.

Re: Small Push-Pull Amp.

Posted: Wed Jul 17, 2019 4:42 pm
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...

Re: Small Push-Pull Amp.

Posted: Wed Jul 17, 2019 4:54 pm
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.

Re: Small Push-Pull Amp.

Posted: Wed Jul 17, 2019 5:35 pm
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:

Re: Small Push-Pull Amp.

Posted: Wed Jul 17, 2019 6:04 pm
by Ant
What about 6as7 and those sowter 600r pp output trannies that you had.
3575 on the input, 5751 driver? Nice and simple

Re: Small Push-Pull Amp.

Posted: Wed Jul 17, 2019 7:03 pm
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.

Re: Small Push-Pull Amp.

Posted: Wed Jul 17, 2019 7:43 pm
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

Re: Small Push-Pull Amp.

Posted: Wed Jul 17, 2019 8:05 pm
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.

Re: Small Push-Pull Amp.

Posted: Wed Jul 17, 2019 8:06 pm
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.

Re: Small Push-Pull Amp.

Posted: Wed Jul 17, 2019 8:46 pm
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

Re: Small Push-Pull Amp.

Posted: Wed Jul 17, 2019 8:56 pm
by Ant
This one is the thread I think
viewtopic.php?f=11&t=4821
Not sure why there aren't any images

Re: Small Push-Pull Amp.

Posted: Thu Jul 18, 2019 4:53 pm
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.

Re: Small Push-Pull Amp.

Posted: Fri Jul 19, 2019 9:28 pm
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.