The filter capacitors may need to be replaced. How old is that amp ? The Fender amp field guide ( http://www.ampwares.com/ffg ) says it was made from 87-92 so a cap job is probably due. In any event, take it to a tech who will clean the pots, replace caps, check plate resistors - a checkup so to speak. If you don't know what you're doing the stored voltages in some components of a tube amp can kill you.