Pentode PP Amplifier Complete Set of Drawings

[Cressy Snr]

Here is the complete set of schematics for my "Barbarik" balanced pentode push-pull amplifier

EDIT: There is a better option about three posts down with a simplified VR tube based output stage screen regulator.
IME this gives better final sound quality than the hybrid shunt EL84/opamp based reg in this collection of sheets.

Audio circuit:


Main Power supply:


Bias supply for both channels:

Screen grid regulator for output stage:


The aim is eventually to provide these drawings as a downloadable, multi-page PDF document.
It is now a fully developed tried and tested design, out of beta (so to speak) has been evaluated for sound quality and is easily capable of providing extremely high quality music playback.
If you want to build it, all the info is here.

[Nick]

Just a couple of IMHO things about the screen reg. I would have not bothered with the 220k across the cap i8n the supply, the 100k will do the same job of discharging it, also, I would have merged the 200uf and 47uf caps on the output into one, maybe smaller, why regulate, and then make the regulators job harder with the big cap on its output. I would have also added a 220nf (or so) cap from bipassing the potential divider to the centre tap of the pot to increase the AC regulation. But just thoughts. I would also look at it with a scope to see how well its working, making sure there are not hidden oscillation.

[Cressy Snr]

Looks like it's back in Beta then.
I'll do some fettling and poking about with my probe, at the weekend.
The reg is the only thing about the amp, I didn't design myself.
17 mins as a final design. Is this a record?

[Nick]

It was only IMHO

[Cressy Snr]

Here are the real final schematics for the Barbarik pentode push pull amplifier.

I have condensed the schematics onto two sheets. These give all info required.

Once again, the audio circuit (unchanged):


Power supply details (condensed to one sheet):


There you go.

[Paul Barker]

Steve I see that the Danbury power transformer is 500mA. Have you used one, or is there an full HT supply per channel? I think you are going to say one.

I am trying to recall what made the big difference in the amp design. Was it the use of the zenner screen regulation on the 12SJ7's?

[Cressy Snr]

Hi Paul,
I have just used the one transformer.
There were two things that elevated the amps performance from excellent for a push pull, to rather fab actually.
The first was the Zener regulation of the bias supply
The second was zener regulation of the input stage screens.
Added together, those two mods lifted the performance in a spectacular fashion.
The icing on the cake is the improved isolation of the output stage screen grid supply by putting it behind the two 200uF RC filters.

[shane]

What o/p transformers are you using, Steve?

[Cressy Snr]

Hi Shane,
The output transformers are an Andy Grove design that he did for the WD 300B push-pull monoblocks that came out a few years ago. They are 6K a-a according to the label on the bobbin under one of the end bells.
My particular transformers were originally sold to Martin (Deco Doctor) by John Caswell.
Martin then sold them to me about 18 months ago.
Whether Anyone makes them anymore, I don't know.
Greg or John Caswell might be able to give you a bit more info.

[shane]

Thanks Steve. Just wondered....

[Paul Barker]

Hi Paul,
I have just used the one transformer.
There were two things that elevated the amps performance from excellent for a push pull, to rather fab actually.
The first was the Zener regulation of the bias supply
The second was zener regulation of the input stage screens.
Added together, those two mods lifted the performance in a spectacular fashion.
The icing on the cake is the improved isolation of the output stage screen grid supply by putting it behind the two 200uF RC filters.

This begs the question should you not try the same zenner with 1uf cap combo for the output stage screens as you used for the VA stage screens. Just in case.

It seems odd that the vr tubes couldn't cut the mustard on the VA stage screen supply which was massively improved when changed to zenner with 1 mic cap, yet the output stage sounds best with vr tube regulation. Though it is true that the first has a more critical position in terms of importance in respect of gain. So differences there would be more stark. Which surely we did experience. I was actually quite surprised that a zenner sounded better there than a vr tube, and decided that it must be because you were able to use a 1 mic cap which you could never do with the vr tube.

Anyway... Must be checked to be absolutely sure? If it turns out to work as well, then people could build the amp more easily. Rationalises the design a little.

I'll be using what capacitors that I have so won't have the same storage of power as you, it would be interesting to compare one of these days. There again I may get carried away and make it for the GK71 with my 650v 500 mA transformer. Just because I can.

[Cressy Snr]

This begs the question should you not try the same zenner with 1uf cap combo for the output stage screens as you used for the VA stage screens. Just in case

Anyway... Must be checked to be absolutely sure? If it turns out to work as well, then people could build the amp more easily. Rationalises the design a little.

I've got half a dozen 75V-5W zeners somewhere. So I'll have a go with a string of three and report back. Didn't think about using zeners on the output stage screens; to be honest I was just glad to get the VR tubes back in.

Zener noise could be minmised nicely with a string of 60 x 5V zeners zigzagged across a tag strip. Taps at appropriate points and a switch arrangement means you could switch from 225 - 250 - 300V drop, to give convenient screen voltages for the popular output pentodes.

[jack]

Noise figures for statistical regulators are excellent... not used them myself so will be very interested in how that turns out... are you using a CCS with it...

Interestingly, strings of 6.2V "true" zeners have by far the best noise figures.... on the cusp of true zener and avalanche mode operation...

[Cressy Snr]

Aye... 6.2V that's the figure, I knew it was somewhere near 5V but couldn't quite remember.
That means less Zeners to string up; only 48. Nevertheless, my eyes'll still be on stalks after putting that lot together.

I'm just about to try a normal cap-bypassed zener string, so I'll see how that turns out before going any further.

[Cressy Snr]

Power supply sheet has now been altered to show a 215V zener string shunt reg to the
output stage screens. No need for VR tubes whatsoever.