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PP KT88 Amps
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Author:  David McGown [ July 18th, 2020, 12:31 pm ]
Post subject:  PP KT88 Amps
I have delayed posting about this project after a certain amount of embarrassment, since my first attempt of getting this amp working was using a Dynaco Modified Williamson circuit that just did not work for me, despite getting great advice by Grover, Charlie, and others (apologize to the non-credited). However I was able to get a working and nice sounding amp using a modified Mullard topology.

The amp is a Push Pull KT88 using DynaParts A-431 output transformer, an Angela Instruments universal power transformer, and a big honking 10H/300mA Hammond choke on top. Power supply is all motorrun capacitors (you see two 100uF caps poking thru the topplate, there are more underneath). Tube rectified, using a Mullard GZ34. B+ is around 450VDC

The circuit is a modified Eico HF-89 circuit, i.e. Mullard, with a 5751/12AX7 input amplifier, direct coupled to a 6SN7 Schmitt phase splitter with a CCS on the cathode, cap coupled to the KT88 grids. Using fixed bias, with adjustable bias level for both tubes (from underneath the amp), and balance adjustment on the top of the amp, similar to the arrangement on the HK Citation V.

Tube lineup currently is a Genelex Gold Lion (reissue) 12AX7/B759 in the input position. A Brimar 6SN7 (CV1988) for the phase splitter, Genelex Gold Lion (reissue) KT88.

The amp is built on a blank Hammond 5U front panel. Chassis is Birch veneer plywood that I had around, I built these during the early part of the shutdown, so did not want to run to a lumberyard to find nicer wood. It turned out pretty well.

Coupling caps are 0.33uF Miflex KPSU-01 Copper film and oil. Poor man's Dueland. I just ordered (and should be receiving) a quad of the newest silver oil Dueland bypass (0.01uF) caps for bypassing the Miflex. This is an arrangement that Charlie Phelps and Jim Gerfin found to work great, though both of them have the Dueland coupling caps in their amps.

I have been listening to them for several weeks, and it is nice to have the power reserve on my Elsinore speakers. The PP amp has great control at the bottom, and sounds uncolored through the entire range. Reproduces a nice 3D soundstage that is expansive. Nice detail and timbre. Very happy with the results.

David

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Author:  Jim G [ July 18th, 2020, 3:01 pm ]
Post subject:  Re: PP KT88 Amps
(Thumbs up emoji) Nicely done David. Great use of the time. Looking forward to having a listen...soon.:-)
Author:  FerdinandII [ July 18th, 2020, 6:27 pm ]
Post subject:  Re: PP KT88 Amps
Nice work, as always David. :thumbup: :thumbup: :thumbup:

BTW, the rest of us are quite often embarrassed by the quality of your workmanship in comparison to our own, so don't feel bad about not being able to make someone else's Frankenstein Feedback circuit achieve stability.
Author:  Grover Gardner [ July 19th, 2020, 1:00 pm ]
Post subject:  Re: PP KT88 Amps
Very nice! There's more than one way to skin a cat! :-) Those Miflex caps are quite nice.
Author:  David McGown [ July 19th, 2020, 8:01 pm ]
Post subject:  Re: PP KT88 Amps
Grover,

I never could get the Williamson version to stop oscillating and overload the output section. The problem was the amp pulling so much current that I feared I would redplate the tubes and/or burn up the output transformers. Therefore, never really could run it more than a few seconds at each change I made, which was not enough time to really do any live troubleshooting or using a scope to determine what the oscillation was and how to defeat it. I decided to build the other monoblock using the modified Mullard circuit, and it worked great at first turnon, absolutely stable. I then rebuilt the front end of the non-working Williamson to the Mullard and called it done. Hooked them up, and outside of upgrading coupling caps and some tube rolling, they have been in my system ever since.

I do not know what my problem was, but Dynaco modified Williamson circuit was more complex than the classic Williamson, with an additional feedback loop and a weird resistor bypassing the coupling cap to each output tube. Perhaps the classic circuit you used, with vintage iron, 6L6s, and cathode bias would have been the way to go. There is no doubt the Mullard is the more forgiving circuit, though it does have a good amount of gain, I have had to make adjustments in my sources (setting the Chord Qutest down to 1V output, and some tube changes in my phonostage). I wish I could find an input tube that sounds as good as the Genelex 12AX7 (it is REALLY good), my idea would be something around a gain of 40 to 50. Perhaps try a 12AY7...

David
Author:  Roscoe Primrose [ July 19th, 2020, 8:07 pm ]
Post subject:  Re: PP KT88 Amps
David McGown wrote:
with an additional feedback loop and a weird resistor bypassing the coupling cap to each output tube

With that resistor bypassing the coupling cap, are you sure you actually had a negative voltage on the grid?

Roscoe
Author:  Grover Gardner [ July 19th, 2020, 8:43 pm ]
Post subject:  Re: PP KT88 Amps
David McGown wrote:
Grover,

I never could get the Williamson version to stop oscillating and overload the output section. The problem was the amp pulling so much current that I feared I would redplate the tubes and/or burn up the output transformers. Therefore, never really could run it more than a few seconds at each change I made, which was not enough time to really do any live troubleshooting or using a scope to determine what the oscillation was and how to defeat it. I decided to build the other monoblock using the modified Mullard circuit, and it worked great at first turnon, absolutely stable. I then rebuilt the front end of the non-working Williamson to the Mullard and called it done. Hooked them up, and outside of upgrading coupling caps and some tube rolling, they have been in my system ever since.

I do not know what my problem was, but Dynaco modified Williamson circuit was more complex than the classic Williamson, with an additional feedback loop and a weird resistor bypassing the coupling cap to each output tube. Perhaps the classic circuit you used, with vintage iron, 6L6s, and cathode bias would have been the way to go. There is no doubt the Mullard is the more forgiving circuit, though it does have a good amount of gain, I have had to make adjustments in my sources (setting the Chord Qutest down to 1V output, and some tube changes in my phonostage). I wish I could find an input tube that sounds as good as the Genelex 12AX7 (it is REALLY good), my idea would be something around a gain of 40 to 50. Perhaps try a 12AY7...

David


Are you using any feedback in this version?
Author:  David McGown [ July 19th, 2020, 9:35 pm ]
Post subject:  Re: PP KT88 Amps
Yes. Global feedback, around 20 dB or so. Basically using the feedback network from the Dynaco Mark 3 and it seems to work fine. Made sure the divider at the cathode of the input tube was the same, and used a bypass cap around the rest of the cathode resistor needed to bias the stage. I probably should fine tune it, but the amps are so sounding fine as is, been hesitant to tweek them any more.

David
Roscoe Primrose wrote:
David McGown wrote:
with an additional feedback loop and a weird resistor bypassing the coupling cap to each output tube

With that resistor bypassing the coupling cap, are you sure you actually had a negative voltage on the grid?

Roscoe


I tried it with and without, and had the same problem. That had me running in circles for a while.

David
Author:  Grover Gardner [ September 6th, 2020, 3:25 pm ]
Post subject:  Re: PP KT88 Amps
David McGown wrote:
Yes. Global feedback, around 20 dB or so. Basically using the feedback network from the Dynaco Mark 3 and it seems to work fine. Made sure the divider at the cathode of the input tube was the same, and used a bypass cap around the rest of the cathode resistor needed to bias the stage. I probably should fine tune it, but the amps are so sounding fine as is, been hesitant to tweek them any more.

David
Roscoe Primrose wrote:
David McGown wrote:
with an additional feedback loop and a weird resistor bypassing the coupling cap to each output tube

With that resistor bypassing the coupling cap, are you sure you actually had a negative voltage on the grid?

Roscoe


I tried it with and without, and had the same problem. That had me running in circles for a while.

David


David, as an update on this, I just built the same thing, only using the Acro TO-330 outputs and EL34s. I will say that that bias circuit is tricky. I had to fiddle with it quite a bit to get the output tubes stabilized at the right operating current, about 65mA each. And if it's any comfort, this "stable" Williamson is anything but! Of course, the Acros are notorious for HF instability, whereas the Dynacos are probably a lot calmer. Even so, at first measurement I couldn't attach a probe to the input without the thing going haywire. I modified it with my all-purpose, rather aggressive HF rolloff filter at the input (1200pF + 820) and it was stable with .22uF across the 8 ohm load. Here are some pics, first the stock circuit at 10kHz, then the stabilized version, and a frequency plot of the stable version. You can see it's still "singing the mass" out to 100kHz, but it's flat to 20kHz. The red line represents the phase shifts.

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Author:  SoundMods [ September 6th, 2020, 3:41 pm ]
Post subject:  Re: PP KT88 Amps
Roscoe Primrose wrote:
David McGown wrote:
with an additional feedback loop and a weird resistor bypassing the coupling cap to each output tube

With that resistor bypassing the coupling cap, are you sure you actually had a negative voltage on the grid?

Roscoe


Excellent question! Me thinks the builder mistook a grid stopping resistor as a resistor installed across the coupling cap. That would place +DC on the power tube's grid. Not good. :o

The tube would pull full current until self-destruction. :shock: Either fixed bias on the power tube grid of say -50-vdc (a S.W.A.G.) or cathode bias.

But under no circumstances bypass the inter-stage coupling cap.
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