Hi -
Had one of those a long time ago.
I don't know what's causing the problem, but new tubes might work.
I would check out the peavey forum - there was a guy there who had like 3 of them and knew everything about them. I betcha he has the schematics.
http://forums.peavey.com:8080/~amps
Good luck and sorry i couldn't be more of help.....
jeff