Ideas For EL34 Pentode Push-Pull Amplifier

What people are working on at the moment
Cressy Snr
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#46

Post by Cressy Snr »

Further refinements to the design today.

The proposed output transformers have been changed to Danbury VT-1040V, 50W with a 5K a-a primary.
These are a better bet for an amp of this power and give a more efficient class AB set of loadlines at the proposed HT than would a 6K6 OPT.
See below:

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Cressy Snr
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#47

Post by Cressy Snr »

Here, the schematic has been modified.
HT is slightly lower, at 400V a-k and the new OPT is shown:

Image
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Deco-Doctor
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#48

Post by Deco-Doctor »

SteveTheShadow wrote:Here, the schematic has been modified.
HT is slightly lower, at 400V a-k and the new OPT is shown:
Steve

I'd be interested to know if you have ever considered trying cathode feedback with a PP pentode design.

I'm seriously considering try these in my EL34 PP amp, interesting reading, my only reservation is how good the R cores will be at the bottom end.

http://softone.a.la9.jp/english/RX-80-5/RX-80-5.htm
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Paul Barker
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#49

Post by Paul Barker »

Having studied the page you linked, no problem with bottom end if you keep dc balance to < 2mA.

Fantastic that they provide feedback windings. Would definitely use those.

I have two pairs of Softone SE output transformers and have no issues with performance of either. One pair double size the other pair. the smaller pair better sounding. Purely my subjective experience YMMV.
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#50

Post by Paul Barker »

My interpretation of the price page is Euro 420 excluding import duty surface mail to the UK boarder. So it may pay to check Sowter et al products. A product of this value is bound to attract HM Customs charges which are also levied on the shipping and the post office levy their own charge for collecting the charges.

Check British companies.
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Deco-Doctor
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#51

Post by Deco-Doctor »

Paul Barker wrote:My interpretation of the price page is Euro 420 excluding import duty surface mail to the UK boarder. So it may pay to check Sowter et al products. A product of this value is bound to attract HM Customs charges which are also levied on the shipping and the post office levy their own charge for collecting the charges.

Check British companies.
Thanks Paul, your the only person I have found with experience of the Softone output transformers. I'm encouraged by your comments.

Sowter have the 'Experimenters transformer'

http://www.sowter.co.uk/specs/8774.htm

However it does not have separate cathode windings, only a centre tapped cathode winding which means a shared cathode resistor. The Softone has two separate CFB windings which is favorable for me as I have separate cathode resistors. The Sowter's will come about about £460 with VAT and shipping, so the Softone may work out a little cheaper even after getting robbed by Customs and Excise.

Apologies to Steve, I don't mean to hijack your build thread!
Last edited by Deco-Doctor on Sun Dec 15, 2013 4:45 pm, edited 1 time in total.
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#52

Post by Deco-Doctor »

SteveTheShadow wrote:Further refinements to the design today.

The proposed output transformers have been changed to Danbury VT-1040V, 50W with a 5K a-a primary.
These are a better bet for an amp of this power and give a more efficient class AB set of loadlines at the proposed HT than would a 6K6 OPT.
See below:
Steve - I have a pair of original WAD 300B PP output transformers here doing nothing. 5K anode to anode primary, single 8 ohm secondary. I bought them from John Caswell a while back and still not got round to using them.

You would be welcome to borrow them if you like, and if you want to keep them I would be asking what I paid for them, £120. Let me know if your interested.
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#53

Post by Cressy Snr »

That sounds like a plan to me Martin. :)
I'd like to borrow in the first instance.
You have a PM.
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Cressy Snr
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#54

Post by Cressy Snr »

Well.....
That's the amp under way sooner than I thought. :)
Better get some EF50 red valves and sockets ordered.

I'll still need eventually, some of Mike's OPTs as the ones in the ECC81 driven amp are, of course, borrowed ones.
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#55

Post by Deco-Doctor »

Transformers on the way. I found a couple of old used 9 pin Loctal sockets if you need them. They are the old paxolin wafer type, but you may prefer something nicer. I'll put them in the post if you need them.
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#56

Post by Cressy Snr »

Thanks Martin,
You're a Gent :)

I've got some ceramic sockets coming, courtesy of ebay seller Big Daddy, they were only a couple of quid each, but thanks for the offer of the paxolin ones, I'll not need them.

Can't wait to get going on the build, when those TXs arrive; thanks again mate :D
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Cressy Snr
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#57

Post by Cressy Snr »

Deco-Doctor wrote:
SteveTheShadow wrote:Here, the schematic has been modified.
HT is slightly lower, at 400V a-k and the new OPT is shown:
Steve

I'd be interested to know if you have ever considered trying cathode feedback with a PP pentode design.

I'm seriously considering try these in my EL34 PP amp, interesting reading, my only reservation is how good the R cores will be at the bottom end.

http://softone.a.la9.jp/english/RX-80-5/RX-80-5.htm
I don't know how I missed these posts from yourself and Paul.
I have not thought about cathode feedback with these amps at all.
i sort of did it a couple of times with the SE amps, including the KT120, by including the secondary of the output transformer, in the the output stage cathode circuit. It sounded very good on the KT120 SE amp.
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JamesD
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#58

Post by JamesD »

Steve,

You may, or may not, have stability problems with multiple feedback loops around the same stage - it can get quite complicated to plot the stability criteria when it includes reactive components within the loop such as OPTs wrapped around shorter loops such as the Schade feedback. It also gets complicated when you apply them to different input ports of the active device as there can be different phase conditions and hence stability conditions attached to the differing inputs.

Not saying not to try it - just that you need to be prepared to cut the power quickly if it takes off!

Of course it gives you more things to compare on a one at a time basis :-)

James
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#59

Post by Cressy Snr »

The balanced plate-to-grid feedback works so well with straight pentode
operation, that introducing more complication is not on the cards.
I was just talking in the context of any future designs. :)

I'm enjoying the present amp a lot. The combination of Mullard ECC81s and the Shuggie EL34B is rather good. Where Mullards, didn't quite gel tonally with KT120s, they are great with EL34s.
I have found that 6201, 6060, 13D9 and Telefunken ECC81 work best with KT120s.
The Chinese EL34s appear to need around 100hrs on them, to settle in fully.
they sound good out of the box, but give them around three weeks and there is a sudden step up in sound quality. The midband tone, has opened out, and both ends of the frequency response have extended and improved their texture. Other users have also commented on this, so I'm confident the effect is a real one.

An order will go in today for a set of EF50 plus the four gas tube stabilisers required for the screen regulators on input and output stages.
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#60

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