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RE: "if I was buying the kit"

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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