Earth Sound Research Revival driver stage issue

[jvm]

Earth Revival 2 6L6 combo.One driver transistor, Q7, has low, distorted output. I have replaced both drivers (mpsa42). I then swapped them to be sure I did not have a new faulty transistor. Problem stayed with the Q7 circuit. I have recapped the amp including coupling caps with no change to the driver stage issue. Both drivers have 23vdc emitter, 24vdc base but collector voltages are off. Q6 shows 51vdc collector and Q7 shows 33vdc collector. I can't for the life of me find a reason for the low collector voltage at Q7. All resistors in the driver stage check fine, supply voltages are same for both drivers. Any help appreciated.

[Jazz P Bass]

Change Q6.

[octal]

That tone stack amuses me. Rather than scale the pot and cap values by a factor of 10 or something for an SS amp, just use the stock marshall style values, make a high impedance network and then feed it into a JFET so it's not loaded down too much! Probably works fine, but it seems kind of silly to do it that way.

[Enzo]

And check the collector resistors.

[octal]

On topic: when you say you "changed the drivers" are you referring to Q6 and Q7? They are a transistor long tail pair. C17 and C19 are also new? Installed with the correct polarity? The biasing of the LTP is a little strange in that it's referenced between the low voltage B+ and the collectors. Coupling caps c21 & C22 are new? The negative bias for the output tubes could be dragging down the collector of one of the LTP transistors if c21 or 22 is leaky, but I'd suspect c17 or c19 first. Hope that helps, good luck.

[jvm]

Replaced Q6 as Jazz suggested and now both sides have the same symptom. Low, distorted output. ?? octal, by "drivers" I did mean the Q6-Q7 pair. C17 and C19 are new and I checked for proper polarity, C21 and C22 are also new. I have also now replaced all resistors in the collector circuit. No improvement.

[octal]

Are the DC voltages on the collectors of the LTP roughly the same now that you've changed that transistor?
When you say "low, distorted output"- from the speaker?
Have you checked the output of the driver/phase inverter stage with a sinewave generator and an oscilloscope to verify that the problem is actually in the LTP/PI/Driver stage?

[jvm]

yes, octal, dc voltages are the same since replacing Q6. I traced the signal to the drivers stage using my audio tracer. Strong, clean signal at base of both Q6 and Q7. Low output at both collectors. The collector signal seems "squashed" or "strangled". I checked at C21 and C22 for leakage. No bias leakage there. The problem is also evident through the speaker of course but I am testing with output tubes removed.

[jvm]

Ok, now since replacing Q6, the signal is cleaner at Q7 but still slightly distorted compared to Q6.

[52 Bill]

The problem is also evident through the speaker of course but I am testing with output tubes removed.

You're trying to drive the speaker with no power tubes?

[jvm]

No, 52 Bill. I have the tubes removed for testing. I mean that WITH tubes installed, the distorted signal is also present at the speaker but I have traced it to the driver stage.

[octal]

yes, octal, dc voltages are the same since replacing Q6. I traced the signal to the drivers stage using my audio tracer. Strong, clean signal at base of both Q6 and Q7. Low output at both collectors. The collector signal seems "squashed" or "strangled". I checked at C21 and C22 for leakage. No bias leakage there. The problem is also evident through the speaker of course but I am testing with output tubes removed.

OK, let's clarify something else. When you say "the collector voltages are the same" do you mean that they haven't changed compared to what you said earlier in the thread (i.e. 51V and 33V) or that they match each other now ? I'm hoping you mean the 2nd option. If option #2, what are the voltages now?

You've checked the common 10K emitter resistor? Basically, transistors are going to distort the signal if they are a.) bad or b.) not biased correctly for some reason due to the failure of another part. Since this is an LTP, if there's too much current flowing in one of the transistors in the pair, there's going to be too little in the other one. So a problem on one "side" can cause symptoms on the other if that makes any sense.

Are the MPSA42s the original type of transistor? If not, are you sure the pinout is the same? (i.e. ECB or whatever)

What is the B+ voltage of the solid state section anyway?

[jvm]

The collector voltages are now the same at 33vcd. The 10k emitter resistor reads 9.99k. MPSA42's are what the amp had in it when I got it and looked to be original to me. The driver section B+ is 156VDC at the second 20uF filter cap.

[octal]

Hmm. I'm running out of ideas here if the collector voltages match. The bias on the pair seems to be decided by a combination of things; the bases are pulled down to the low-low voltage preamp supply via the 10K resistors (what is the low-low voltage supply anyway? Based on the 25V rating on the caps, I'm guessing maybe 15-20V?) They are pulled up to the collectors via the 1M resistors, which also seems to provide a sort of local DC negative feedback to stabilize the stage. One more thought; if Q6 is amplifying OK and Q7 is biased the same (as evidenced by the collector voltage matching) perhaps it's a signal coupling issue rather than a bias issue. Q7 works sort of like a grounded base amp. The base is held near AC ground by the network of C33, R36, R38 and C20. The only ac signal there should be a small amount of negative feedback signal from the output stage. You could try grounding the negative terminal of C33 to see what happens. I can't see how multiple components could open up and break the virtual ground, but maybe there's a torn foil trace or a crack in the PCB. Hopefully some other folks will have ideas too.

[jvm]

I replaced both mpas42's with 2N3439's per the schematic. Very little, if any improvement. I then started tracing backwards in the amp and discovered that I have a much hotter signal in the preamp than I do at the 2N5459 input. Oddly, I now also have very little output volume with tubes installed. The amp was functional when I started. ?