M3 amp oscillating

[Jaim]

This is a continuation of the M3 Perc problem. Amp will start oscillating at 1/3 to 1/2 pedal.

Before I get to deep in this I noticed that the amp in question (AO-29-7 K ) has a 10K resistor and a .002 cap across the 6V6 outputs while another AO-29-7 J only has a .001 cap.

Is one better than the other?

Jim

[David Anderson]

Both low-pass filters work, but your problem is likely elsewhere. Most likely is a bad cathode bypass capacitor or B+ filter allowing two gain stages to couple through the power supply.

[Jaim]

B+ filter allowing two gain stages to couple through the power supply.

Thanks David. I hadn't considered that. The 4 section can is pretty bad. Ordered parts.

Jim

[David Anderson]

De nada. It's the first thing to look at when an amp oscillates. If there is high impedance in the power supply, that's what happens.

[Jyvoipabo]

Before I get to deep in this I noticed that the amp in question (AO-29-7 K ) has a 10K resistor and a .002 cap across the 6V6 outputs while another AO-29-7 J only has a .001 cap.

Is one better than the other?

Jim

Hello,
This is a 'Boucherot cell' or 'Zobel network' used to correct the impedance of the speaker at high frequency.
https://en.wikipedia.org/wiki/Boucherot_cell
The cell is associated with the speaker.
Can be a source of instability like anything in the feedback loop of the amplifier.

JP

[Jaim]

Before I get to deep in this I noticed that the amp in question (AO-29-7 K ) has a 10K resistor and a .002 cap across the 6V6 outputs while another AO-29-7 J only has a .001 cap.

I know what it does. You often see design changes in amps - just wondered why.

Jim

[Jaim]

Replaced both cans and the other e-caps. All is well. Both 20uf sections didn't register any capacitance on the ESR meter. Rather interesting how the can went from bad to worse on the bench. Appears we got that amp swapped out just in time.

Thanks again David and JP for the guidance. Very easy to go down the wrong rabbit hole.:embarrassed:

Jim