High pitched squeal.

[Chubsman]

When I jump the Low input normal to the high TMB Channels and have the normal volum allthe way and the tmb channel 7-10 i get a high pitched squeal that goes away when I lower 1 of the 2 pre-vols.

it's like it's pushing the tube too hard... BTW this is with or without a guitar plugged in.

Any ideas...

[novemberrain]

Small animals trapped in yer amp?

Sorry, I couldn't resist.

Probably high frequency oscillation. I'm guessin' one channel doesn't "bleed" (crosstalk) enough to make the amp oscillate but two channels do. If you look at oscillator circuits they don't differ much from amplifier circuits.

Drive it into the squeal zone and poke around its innards with a scope for a visual clue to the exact location of the oscillation. If you don't have a scope you're pretty much guessin' but you can move grid wires around and/or shorten the signal path to the grids as much as possible. Shield the grid wires although the tiny bit of capacitance in shielded wire will rob you amp of a tiny bit of "brilliance". Sometimes changing cathode bypass values on the preamp tubes will kill some oscillation. Grid blocking resistors will get rid of oscillation.

There's a few other ways to deal with it but those suggestions top my list.

[Chubsman]

More poking around has led me to the fact that the Treble control will ad or remove the squeal.

Would making the 8.2k grid resistors a higher value help with the oscilation?

Also, if I have a jack cord just plug into the input, but no guitar... it's REALLY bad!.

Thanks