Peavey 6505 gain channel not working

[handyman11]

Borrowed my brothers 6505 , which he has'nt used in years ..and the clean works great ...gain channel volume is really low , can barely hear it , even at max volume ...I'd love to fix it , just don't want to get caught up with a big repair bill ..any chance chance it is just tubes , or something simple ? thanks in advance ///

[peterdjp]

Preamp tube. Swap out the 12ax7's one at a time till you find the bad one.