Mesa 2:90 power amp channel B distorted

[jvm]

Mesa 2:90 power amp channel B weak output and distorted. I have swapped power tube sets and output trannies between channels with no change. Problem stays on ch B. All voltages ok with schematic on power, preamp and driver tubes.

[jvm]

Signal tracing has my signal clean at pin 2 of preamp but very distorted at pin 1. All voltages ok

[g1]

Is this the schematic you are using?

[bob p]

If that's the right schematic, then it sounds like you're talking V1a (12ax7 input).

Have you checked the cathode cap, and metered the resistors around the tube?

[jvm]

That IS the correct schematic, g-one. Thanks for posting it. I have checked every component around the tube, even comparing reading with the other half components. I replaced a 4.7k that was barely out of tolerance. No change. Everything seems absolutely perfect on the preamp circuit. Any idea on how to unhook the presence circuit? Presence control doesn't seem to be working. Kind of hard to tell with the distorted signal.

[bob p]

I have checked every component around the tube, even comparing reading with the other half components.

I'm not sure what you mean when you say that you checked every component around the tube. I'm not sure what kind of test gear you have. It's not clear if you're using a VOM or if you have a scope and a cap tester, or if you're taking VOM readings off of parts in-situ or if you're doing things like removing caps to test them out of circuit.

Your original complaint suggested that you'd isolated distortion to the V1a gain stage. If that's the case, then after verifying that the tube and control resistors are OK, I'd replace the cap and measure the old one. Ck is cheap and time is valuable. I'd also rule out the coupling caps as being a problem.

Everything seems absolutely perfect on the preamp circuit. Any idea on how to unhook the presence circuit? Presence control doesn't seem to be working. Kind of hard to tell with the distorted signal.

On the other hand, this new information suggests that your problem could be caused in the NFB circuit. I'm not sure if you mentioned that before. If you have a presence control problem then I think you've also got to look at the circuit between "FB>-" and the phase inverter. You could have bad caps there, or more likely, defective LDR. Are all of the amp's switching functions in the NFB circuit working properly?

If the sound of the amp is ambiguous to you because you're listening by ear, or you're scoping with a monotonic frequency generator, try sweeping the input. That should tell you very clearly if there's a frequency response problem and if the NFB/presence circuit is functioning.

[jvm]

What I meant is that all preamp components test as good. I also replaced the cathode cap with no change. I would like to rule out the LDR but have only VTL5C4's on hand. The amp uses a VTL5C9. Can I get away with the 5C4 in just a switching circuit?

[g1]

Unless it's a royal pain, I would swap parts between the good and bad side (for stuff you don't have like the opto).
And you should be checking all your measurements against the good side, Mesa schematics can be different from the actual units.

[jvm]

I , in fact, have been checking against the other channel, g-one. Gotta love stereo amps! It is a Mesa Boogie so everything WILL be a royal pain. You'd think I'd have learned my lesson by now.