Massive current in SS Gallien Krueger 200MB

[jaret]

Hi there,

I have a Gallien Krueger bass amp (schematic attached) on my bench that was distorting on low notes (on a bass guitar). The bias was a tad off so I adjusted it to 5mV and was checking other voltages inside the amp when I noticed that the power transformer has started buzzing way more than usual and the secondaries from it were actually shaking from what I presume is a massive current draw. I shut the amp off, disconnected the secondaries from the board and powered up the tranny again- the secondaries were testing fine at about 30VAC. Diode testing on the bridge rectifier seems fine, too. The filter caps seem to charge up on a resistance test and nothing looks burnt, except the 0.33R 5W resistors all look a little yellow, but they test fine and it's a 23 year old amp, after all.

This is a 240V amp so I connected it to a 120V stepdown transformer to reduce the chance it'll burn up when I power it up to take voltage readings. However, even at half mains voltage, one of the 0.33R resistors gets sizzling hot within a few seconds of turning on. This is the one connected to the emitter of Q11 so I did some tests on that transistor and its paired Q12 vs. Q21 and Q22 on the opposite 43V rail and here's what I found for resistance and diode checks in circuit:
Q11= 0.3ohms B/C to E, 262ohms B to C, 0mV B/C to E, 262mV B to C
Q12= 1.1ohms B/C to E, 262ohms B to C, diode test same as for Q11
Q21 & Q22= 10ohm B/C to E, 560ohm B to C, 6mV to E, 272mV B to C.

So I'm thinking Q11 is shorted, hence it's low resistance and the massive amount of current through its 0.33R resistor, right? Q12's emitter resistor doesn't get as hot. Does this sound plausible to anyone?

I'm thinking of getting a new pair for Q11 and Q12 and replacing all the tired-looking 5W resistors while I'm at it.

I'd appreciate some advice or suggestions.

Thanks,
Jaret

[jaret]

BTW, I haven't become too familiar with it yet, but I used my scope to trace voltage at the transformer secondary under no load (1), with the power on + no speaker load (2), at the -43V rail under half mains (3) and at the output on half mains with no load (4).

[Structo]

Duh, sorry disregard.

[jaret]

I agree- but this is a transistor/solid state amp, so no output transformer to worry about!