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In Reply to: RE: Found It posted by Triode_Kingdom on June 30, 2021 at 20:39:22
Not that you have anything beyond intellectual interest in the VTA circuit, but in a purely self-serving way, I sure wish you had one. I have no deep objective knowledge of what improvements my (pre-CCS) 12AU7 based VTA might possibly need to improve its objective and / or subjective performance. But I can give you a list of the subjective things I've tried in my VTA, which include:
Increasing the value of the capacitance in the "on board" B+ caps over stock.
Using Panasonic, rather than Xixon electrolytics on the board.
Using a variety of well regarded resistor brands and types instead of Xicon metal films, including Dale RN, PRP, and Riken Ohm carbon films, all the same values as the stock schematic calls for.
Several different brands of NOS 12AU7s
12BH7s
Upgraded coupling caps
Output tubes tried included Gold Lion KT-77, Russian 6P3S-E, and original (not reissue) Mullard EL-34
Despite the above, I still find the amp "musically uninvolving" with the only significant change being the use of the 6P3S-E, which were the best of the tubes I tried.
I email another VTA ST-70 owner who has the later, CCS version. He's taken things further. In addition to the some of the above, he's increased and decreased the amount of feedback, removed the additional "on board" electrolytics, and attempted to change the circuit to use the more linear 12AX7, to name a few that I can recall.
Although we both have very different speakers and musical tastes, we're in agreement in that we both have other amps, both tube and solid state, which we enjoy listening to more than the VTA ST-70.
You can't please all the people all the time, so I thought I'd post my results to add some balance to this thread.
Follow Ups:
Wow, you've really put in a lot of time and work. I've also owned amplifiers that were "musically uninvolving," but the vast majority of those were solid state. :)
Back in the day, I was privileged to hear many different tube amplifiers - Scott, Fisher, Dynaco, Heath, McIntosh, etc. I never cared for the sound of the ST-70, but that could have been the result of many things that have been addressed in more modern times, such as the power supply, coupling caps, etc. Nevertheless, when I do hear complaints about this amplifier, I'm reminded of my experiences years ago.
Are you familiar with anyone's work to run the ST-70 in pentode mode, rather than UL? This requires slightly more modification than the usual triode conversion (which I believe is entirely ill-advised), but it might remedy some of the sonic grievances. I have always thought the reduction in output stage gain created by UL operation might be more of a detriment in this particular amplifier than a benefit. In fact, it appears to be one of the reasons aftermarket driver circuitry has gradually morphed into something that bears almost no resemblance to the original product.
Converting the ST-70 to pentode might be counterintuitive, but it's been my impression through the years that pentode operation is more dynamic and engaging. Conversion of the ST-70 output stage would be relatively straightforward, owing to the characteristics of the EL34. The feedback network could remain as-is for initial tests or revised to provide the more typical 20 dB of NFB. In any event, it's not too difficult to undo if the results aren't beneficial.
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Buy Chinese. Bury freedom.
...It really wasn't a lot of time or work. I bought a blank board from Tubes4hifi and stuffed it with the previously mentioned caps and resistors. I didn't mean to imply that I had tried, and then removed all the Xicon components.
To a pure objectivist, I certainly wasn't doing any worse by using (mostly) Mil-Spec Dale RN's over the Chinese Xicons VTA was using, even if they "sounded" the same. The exception to the Dales are Riken as grid stops, Kiwame on the plates, and a few PRP in the B+
The same with the Panasonic electrolytics I built it with. They have low ESR, are rated at 105 ºC, and had higher capacitance than the Xicon value VTA used. Possibly a lateral move, perhaps, but certainly no worse.
The point of all that is, I feel that I've given the stock circuit a fair chance, and then tweaked via tube rolling, if not the passive components.
I'm not familiar with a pentode input stage, but it sounds interesting.
Oddly enough (or perhaps not) my most involving amp is SS, but it's a (Nelson Pass) First Watt F1J, so it's hardly a typical SS circuit. If the late Art Dudley's Stereophle review is accurate, I'd need to use a good 2A3 SET circuit to better the F1. Maybe it's time to explore that a bit further.
"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.
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.
Oh no, not the 2a3. Just when I thought I was out, you pull me back in.Now I have to put back my Bottlehead bookmark.
Edits: 07/01/21
As the Eagles sang, "You can check out any time you want, but you can never leave".
The most sobering aspect of a 2A3 SET for me is its low power, and yet their siren song lures me toward the rocks.
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