I've been told you can put a microphonic tube in this spot and generally it
won't be a problem. I'd put a decent tube in there, but wouldn't waste a
Tele or Mullard. The tone gets made before the PI.


Not true unless your amp is strictly a preamp overdrive type with a
class AB2 or class B output stage. Every tube in the signal path has an
effect on sound quality. A poor quality tube used as the phase inverter
will cause the amp to lack life and punch. It is the number one reason I
have seen that Marshall and Fender amps sound weak. I have seen so many
people change out output tubes and preamp tubes thinking the phase
splitter tube is there just for the ride. It is the heart of a push pull
stage. In fact, a funky phase splitter circuit can cause your amp to
loose as much as 50% of it's power. Bad resistors, a bad tube and bad
coupling capacitors all contribute to phase splitter imbalance and the
result is a lousy sounding amp. I rebuild guitar and hifi amps for a
living and I don't know who started the 'urban legend' of putting your
junk tube in the phase splitter socket. If anything, this should be your
best tube. Also you should always use the correct tube in your phase
splitter socket. Do not try different types of tubes. If it calls for a
12AT7 use a 12AT7.
Bill B.