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In Reply to: RE: To be honest posted by 1973shovel on July 01, 2021 at 12:38:07
"I'm not familiar with a pentode input stage, but it sounds interesting. "
I'll assume you meant "pentode output stage," being as that was what I was describing.
Essentially, you would disconnect the screens from the screen taps on the output transformers and connect the screens at each channel together. The screens at each channel would then be connected to B+ through a common screen grid resistor with a value as indicated in the data sheet. Decoupling at the screen pins is not required in this configuration.
The above modification places the amplifier output stages into pentode mode. The gain of the output stage will increase considerably, as will available output power. Below is a chart showing this operating condition with voltages similar to those in the ST-70.
Several things to note...
The value of the common screen grid resistor is 1K. It should be rated 4W or more. Maximum output in the chart is 55W. This is based on the point at which the tubes produce 5% distortion. 20 dB of NFB will reduce that to 0.5%, and it will be less if output power is constrained to the original 35W rating. At listening levels of a few watts, distortion won't be audible at all, except maybe to a golden-eared few. The additional global feedback can be accomplished by adjusting the series resistor in the feedback loop, and this will help to normalize the gain of the amplifier relative to its previous UL operation. All else will stay the same. No changes are needed to the bias circuit or driver, and unlike triode conversions, the same OPT impedance as is used for UL is appropriate for this mode.
You've tried everything else. This is all that's left, and it's a modification that's easy to implement and easily reversed if you don't like the sound.
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Buy Chinese. Bury freedom.
Follow Ups:
My mistake. I read what you wrote, but was focused on something else.
I'll save what you wrote, and the next time the VTA is on the bench (which may be a while), I'll try connecting the output in pentode. You're right, I've got nothing to lose by trying it.
As always, thanks for your time and expertise, TK.
Well, I know what I said, but using EL34s and those output transformers in straight pentode mode is much more than just a last ditch effort. I want to be clear about this. It's exactly the way I'd go if not for the vintage iron I'm currently using up, and I'd do it with or without an actual ST-70 amplifier. Some of the replica Dynaco transformers on the market look very good, and I wouldn't hesitate for a second to build a straight pentode amp around them. If you ever get a chance to do this, I think you'll be pleasantly surprised.
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Buy Chinese. Bury freedom.
Thank you for the suggestion. It's the first time I've had any enthusiasm for doing any work on the amp in quite a number of years.
My friend would email me, telling me all the things he'd been trying, to coax music out of his amp, to no avail, and I mentally kept pushing it further and further down my list of interesting projects. "Pleasantly surprised" would be nice.
Thanks again, TK!
We can walk through this while you're doing the work if you want. Offline is OK too. Just say the word.
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Buy Chinese. Bury freedom.
As I mentioned to you elsewhere, I have a PP 6BQ5 build which interests me more, but after that's done, I'll put the ST-70 on my list.
Thank you for the offer of help, that's greatly appreciated.
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