Mesa M-Pulse 600 Output BIAS/Current Sharing

[PJay1]

MP600 is not making full power; have to max-out the main/master gain control to get loud. I'm essentially only pushing the preamp more to get any decent volume out of it. In checking the SSP 600B power block, I believe there's a gross mismatch in mV drop across .22 ohm source resistors. For example, Q4 reads 14.8mV, decreasing to 13.6mV at Q1 (22% difference). If that wasn't bad enough, the variance between Q4 and Q2 is 32%. I don't think this is what I should be seeing, as the N-channel devices are sharing current a lot better. Those are all within +/- 10% of each other. I made a JPG chart but can't upload it. Any help greatly appreciated.

[nickb]

MP600 is not making full power; have to max-out the main/master gain control to get loud. I'm essentially only pushing the preamp more to get any decent volume out of it. In checking the SSP 600B power block, I believe there's a gross mismatch in mV drop across .22 ohm source resistors. For example, Q4 reads 14.8mV, decreasing to 13.6mV at Q1 (22% difference). If that wasn't bad enough, the variance between Q4 and Q2 is 32%. I don't think this is what I should be seeing, as the N-channel devices are sharing current a lot better. Those are all within +/- 10% of each other. I made a JPG chart but can't upload it. Any help greatly appreciated.

13.6mV vs 14.6mV is just 8%, not 22%.

How does is behave if you plug into the return jack and set the FX pot to WET?

If still poor, check the input to the power stage. With a 200mVpp sine wave input, what is the Vpp at V2B grid, V2B plate and V2A cathode?

[PJay1]

Sorry, I made a mistake on the mV readings. The difference between Q1 and Q2 is 13.6mV and 11.2mV respectively (22% variance). The 32% variance is correct between Q4 and Q2 (11.1mV and 14.8mV). Unfortunately, I don’t have immediate access to my test gear for waveforms.

[J M Fahey]

In any case, a small (or large) bias current difference between power transistors is completely unrelated to your problem, which is a preamp gain one, or connector related.

Sadly, you´ll need to inject 100mV 1kHz at the input and follow it along the signal chain, there´s no other way to find the problem.
yet at least make this basic test:
a) play your Bass into preamp input, connect Preamp out to another power amp input, even if a humble Loop Return on *any* amplifier you have available, even a Guitar one, does your preamp drive it properly?
b) play into another amplifier, connect its Pre/Line out to Mesa powwer ampinput ... is it loud now?
c) plug a regular guitar cord from Pre out to power amp in or same in Lopop send/receive.did you get your gain back?
That problem is so common that it has its own stickie here, something like "the dreaded swiching jack problem" go figure.

[PJay1]

FX switching jack problems did not have any effect on performance; I cleaned them as preventative maintenance. Next step is to do the swap chaining of preamp/poweramp stages and go from there.