Is is just the knob has broken (in which case, buy yourself a new knob and replace the one that's there) or is the potentiometer shaft broken too? That's a much bigger job, that involves pulling the whole amp apart and replacing the pot --- you need to get an electronics tech to do it.
Google mg series knobs to find replacements --- they just push on.
Answers & Comments
Is is just the knob has broken (in which case, buy yourself a new knob and replace the one that's there) or is the potentiometer shaft broken too? That's a much bigger job, that involves pulling the whole amp apart and replacing the pot --- you need to get an electronics tech to do it.
Google
mg series knobs
to find replacements --- they just push on.