I recently purchased a "new to me" Bob built VTA ST-120 amp and have been loving it so far, but it seems to have to much gain. I am pretty sure it has what is referred to as the "VTA Classic" board (probably 2010 vintage) that uses 12AT7 tubes. I see that the current production uses a lower gain 12AU7 tubes. I have also read on this board that the "Classic" board can be retrofitted with new resistors and 12AU7 tubes. So, my question is this:
Can I retrofit just the center tube to an 12AU7 to reduce the gain but keep the 12AT7 phase splitters? The reason for keeping the 12AT7s is that I have a bunch of 12AT7 tubes and would like to not "waste" them--plus I have some nice Mullards.
Is this possible?