2203 Oscillation (and solution).

[TimmyP1955]

Our GM handed me a 1980 2203 and said "I'm borrowing this, check it out". Tubes tested OK and I set the bias.

I noticed a tiny bit of fuzz on the output, so I flipped the scope to FFT mode. When cranked the amp was oscillating at about 30kHz and IIRC 40kHz, and sometimes other frequencies.

In the past, someone installed a loop right before the MV. So I ran into the return and there was no oscillation.

I got the scope probe out and found that V1B was clean on the plate, V2A was clean on the grid, and there was 30kHz on V2A's plate. (This apparently was perturbing the amp and thus causing the additional frequencies.) I swapped V2, which made no difference.

I tried putting different caps across V2A's plate resistor, and when I got to 1000pf/.001uf, the oscillation went away.

Not to worry about loosing any top end, it can peel paint if you want it to, and sounds quite good when adjusted "properly".

[nsubulysses]

I may try this now. I have a 1979 JMP 2203 that squeels when the presence is up past 8. Thanks for sharing.

Edit: Yup I just did this and it worked a treat. Before installing the cap I would get oscillation up to 20W output with all knobs on 10!!

Thanks Timmy

[TimmyP1955]

On another forum, it was suggested that an iffy supply filter cap could be a contributing factor.

I believe the reason would be that the filter cap having higher than normal series resistance or inductance would allow signal from one circuit to make its way into another, instead of being dumped to the power supply.