Ideas For EL34 Pentode Push-Pull Amplifier

[Deco-Doctor]

Steve

Slightly less good news.

The VR65's are MO based, (Mazda Octal) which means they will need the MO bases.

I have been through the stock I can find so far and have found only 4 good ones. I found several with slight cathode to heater leakage, which maybe OK, maybe not. I have some more somewhere ?!?... I'll keep you posted.

[Deco-Doctor]

Cheers Martin, your generosity of time and help is much appreciated.

Steve - No problem. For me it's a small price to pay for the enjoyment I get from reading and learning from your experiments.

I found some more VR65/SP61/CV118. Eight good testing valves will be on the way to you on Monday. Most of them are obviously used so I have no idea how much life is left in them, but they should be good to get you going.

ATB

Martin

[Cressy Snr]

Thanks Martin

[IslandPink]

Good to see you're preparing for Christmas in the normal way Steve !
I'm celebrating by playing around with notch filter values on LT Spice .

[Cressy Snr]

Good to see you're preparing for Christmas in the normal way Steve !
I'm celebrating by playing around with notch filter values on LT Spice .

Oh yes!
Christmas wouldn't be the same without starting another project

As a brief aside, our Ant brought round to test, a balanced, passive preamp he had built for a customer. I was the only person who could test it of course, as I have a fully balanced amplifier.

It contains a couple of Sowter 3575 output transformers, to provide the unbalanced to balanced conversion. It sounded fine and operated flawlessly, but you could tell it was in the circuit, giving a subtle increase in grain and a shrinking of the sound stage, with voices less believable.

Don't get me wrong, it sounded wonderful, and if I had not lived with a phase splitterless, balanced push pull amp for the last six months, I would not have had any complaints.

But I have lived phase splitter free and for me, using a push pull amp in any other than fully balanced operation, is not something I would like to have to do.

[steve s]

I have a few used mo bases if you need them Steve.. now the ef 50 romance has finished

Let me know ill get some in the post

[Cressy Snr]

I have a few used mo bases if you need them Steve.. now the ef 50 romance has finished

Let me know ill get some in the post

Thanks Steve,

You have a PM.
I wouldn't call it a romance. I'm a bit more pragmatic these days when it comes to stuff like that.
No point building something that uses unreliable valves, no matter how good the curves look.

[Cressy Snr]

At the moment, the existing EL34 amp is having a bit of surgery.
I'm converting the input/driver stage from a pair of ECC81, to a quad of EF184 in triode mode. The triode gain of an EF184 is exactly the same as the ECC81, but its linearity is vastly superior.
The EF184 is also completely silent when idling. It made a fantastic driver for the KT120, single-ended ultralinear amp I built.

We'll see what we get later tonight.
Why do this? Well it just seems like a better engineering solution to me, keeping distortion low. (I hope) unless I have things arse about face because of the partial feedback.

[Paul Barker]

No but yes but no but, all your 2nd is cancelled anyway. I thought you liked the ECC81.

[Cressy Snr]

No but yes but no but, all your 2nd is cancelled anyway. I thought you liked the ECC81.

I do like the ECC81 very much, but i'm an inveterate meddler

It's easy to reverse if I don't like what I hear, as only the cathode resistors need changing. Of course it would have to revert to a single 9pin socket, but there's plenty of room to work in the chassis. I made it roomy, because of my predilection for tinkering.

Also I want to see what trioded EF184s sound like driving pentode mode EL34s, as they may end up in the amp with Martin's OPTs if I don't like the old Brit pentode drivers.
On that amp, I intend to make up a pair of plates with holes appropriate for each driver type, so that I can easily swap driver stages. It makes much more sense to me than hacking an existing top plate about. All I have done using the hacking method in the past, is to generate scrap metal.

Once I have got the EL34 amp built with Martin's OPTs on board, I will probably sell the existing one on, but that's for the new year.

[Deco-Doctor]

Steve

VR65's in the post, should be with you tomorrow.

[Cressy Snr]

Steve

VR65's in the post, should be with you tomorrow.

Thanks Martin.

I love it when a plan comes together

[Cressy Snr]

EF184s are in place and sounding good.
The dynamics have a goodly amount of jump factor and the level of instrumental detail is good. The sound is very clean and well extended.
There is a bit too much gain from the system as a whole, so the feedback levels need increasing a bit, but results are promising.

I've also managed to get the channels out of phase internally, but I know what I've done so it will be easy to put right. I'll do it tomorrow, when I see to the feedback levels.

Because of the lower plate resistance of the EF184 in triode mode, compared to the ECC81, to get the same levels of feedback, the feedback resistor values will need lowering down to about 82K from the 133K, we have at present, I reckon.
The trick will be to set the feedback at a level that reduces the gain sufficiently, without killing the speed and dynamics.

[Cressy Snr]

Feedback sorted and phase reversal corrected.

My estimate of the 82K value for the feedback resistors was about right.
I can now open the volume control more than 5 degrees without being blown out of the room, but the dynamism and depth is still there.

Here it is with its 6 pieces of glassware at the front:


Was it worth the changeover from ECC81s? Yes it was, for the extra dynamic qualities and slightly better tone. But I could have lived with ECC81s in the driving seat.

But OTOH I have more glowy things to look at

[Cressy Snr]

As this existing amp chassis was originally designed for KT120s I thought I'd just try them back in there, in place of the EL34.
The trioded EF184s had made a great sound in the single-ended ultralinear KT120 amp, so I was keen to see how they worked with them in push-pull.

They work very well indeed with push pull KT120s.
Now at last month's Owston, James and I were discussing the merits or otherwise of using EF184s in differential pairs, to drive KT120s as part of a fully balanced partial feedback push-pull scheme, and he was talking about building an amp along those lines.

Well I can say James, that trioded EF184s driving KT120s produces a wonderful aural matrix, as Dr Gizmo used to put it. The difference, between ECC81s and EF184s driving those particular output valves is not subtle, the EF184 being clearly superior. With EL34s, the differences between the two drivers are not nearly as marked.

With EL34s, the ECC81 sounds effortless, particularly the Mullard variety, while the EF184 sounds better tonally, but can become overbearing on the wrong material.

With KT120s, the ECC81 seems to run out of puff at higher levels, most notably at Owston, where I felt it wasn't up to driving speakers of real world efficiency; dynamics becoming squashed and the sound muddying up.

The EF184 however is completely without strain, the sound expanding, with awesome bass power and grunt; staying clean and separated.