Rivera Chubster with problem on E string

[bentonar18]

My buddy has a Rivera Chubster combo (the one with EL34's) that has a nasty problem. Everything sounds great until you play the bass E string: then a problem begins. It sounds as though the E string is feeding back, but it's not an octave or two higher: it's the same note as an open E. Only way to stop it is to hit the Stand By switch. This happens on both channels. One other problem (the same? related?) is the Presence knob screetches when turned down to 0.

We swapped power tubes. None of the preamps tubes are microphonic. Pulled the chassis out to check for obvious burned components, and while there is a faint hint of an electrical odor there is no smoke, and no caps bulging or burnt. Your opinions, please guys!

John

[selloutrr]

Check the caps/ resistors in the EQ section does it still make the sound with the bass eq knob turned down?

[Firestorm]

That's an "R" series amp, right? A few years ago, Rivera got a bad batch of 100K resistors. The ones that went south were on the drains of the FETs. Maybe look there?

[jjman]

Maybe someone switched the OPT primaries triggering positive feedback?