Hi i need some help here to find what is the problem: Well, after i have done some mods to my Marshall dsl 15h the amp now starts squealing when i rotate the gain knob to add gain. If i rotate the gain knob up to 1 the amp doesnt squeal, but after this position if i rotate the knob to add more gain the squeal changes pitch and gets louder. What could be the problem here? if the squeal changes pitch could the problem be a capacitor? The amp has this problem even with anything plugged into the amp. i also have checked the preamp tubes ans eveything is allright.