I have a Randall RH150 G3, it has a 12at7 tube driven mosfet power circuit.

The head seems to be lacking gain (could just be the old speakers im using, i've heard that can make a slight difference), but i have heard about people replacing 12at7's with 12ax7's to get more gain due to the difference of gain factor of the tubes, but most of the time thats in the pre amp position, since this isnt really in the pre amp position would this work the same way and give more gain and would it be ok or could it harm the amp?

In your case, all the overdrive tone form in the preamp, when the power amp and its driver work in a linear mode.
You can try and swap the tubes, but:

1. You can overdrive the MOSFET power stage and you won't like the tone or you may overheat the transistors.
2. The circuit built for a 12AT7 can't be directly suited to a 12AX7.
3. 12AX7 can lack current capability to drive the mosfets.

Anyway it's best to have a schematic to mod anything and I can't seem to find one so those are only guesses.
Last edited by Ace Carson at Sep 11, 2011,