Tubes Asylum Questions about tubes and gear that glows. FAQ

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Which versions?

Posted by 1973shovel (A) on November 10, 2023 at 03:32:49

In Reply to: Amp Comparison - VTA ST-70 vs ST-120 vs M125 (pair) posted by billpoletti on November 4, 2023 at 17:53:54:

Their input boards come in two versions, the noval 12AU7, and the octal 6SN7 versions, or at least the ST-70 and ST-120 do, since they use the same input board.

I installed a VTA board in one of my stock Dynaco ST-70s years ago. I purchased my 12AU7 board from them blank, because I wanted to use 'better' caps and resistors.

I was never in love with the sound of the amp. I tried various 12AU7s. I bought 12BH7s. I changed caps, I changed resistor types (metal film to carbon film) etc. I also tried EL-34, KT-77 and 6P3S-E output tubes, but still found the amp amusical. I'd put it back on the shelf, and pull it out once every few years to listen to it and work on it again, thinking maybe I'd eventually find some magical combination of parts or 'best' 12AU7s, then back in storage it would go.

Another Inmate here also has their ST-70, although he bought his amp new from them. Via email we found we both agreed that we weren't happy with the sound of our amps. He went much further than I did, taking his modifications beyond my cap and resistor swaps, by actually modifying the circuit itself.

I'd read numerous posts here on the Tubes Asylum over the years about what a nonlinear tube the 12AU7 was, but for some now inexplicable reason, I ignored these posts. Then one day Inmate Tre' posted yet again about the non-linearity of the 12AU7, and this time I took note.

Not willing to invest much more money and work into this thing, I bought a 12AU7 (socket) to a 6SN7 (tube) adapter, the 6SN7 being a much more linear tube than the 12AU7. I inserted the adapter into the voltage section (i.e. the VTA board's 'center' position) and finally had an amp I could enjoy listening to music with.

The Inmate with the VTA ST-70 mentioned above tried the 6SN7, with the same positive results. The culprit had been the 12AU7, all along. We posted our results on the VTA forum and Roy Mottram of VTA replied that 90% of his customers who've tried both the 12AU7 and 6SN7 versions prefer the 6SN7. I only wish I'd paid attention to Inmates here years sooner.

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