hi, my bass amp (SWR) puts out a high pitched noise wich gets higher and lower as I turn the frequency knob. I opened it to see if something was burned or something. Nothing was.. When I put it back together the sound was gone but it still doesn't amplifies my bass. When I turn up the gain the sound of the bass turns up a bit but very faint until it changes to the high pitched noise again (also the preamp clip led switches on). Does anyone recognize this problem?