Oscillation help

[drewl]

hey all,
I have a Super Reverb clone, no trem circuit with a post phase master- A dual 1meg pot with 4 .1uf caps

I've cleaned up alot of the bad wiring, got rid of most of the noise and oscillations, but with the master pot turned to it's last 1/4 to full, the amp produces a farting, high freq. oscillation that sounds buzzy, or distorted.

All the tubes have been checked/replaced.
The 4 .1uf caps in the master and the master pot have been changed, any other ideas where to check?

It seems like the master circuit is acting like an oscillator/sig generator.
At one point of rotation it produces it's own signal!

Putting a scope probe on the caps feeding the output tubes produces motorboating, and sometimes probing output tube inputs (pin 5) will also produce motorboating/oscillation.

The output tranny plate leads are correct, I checked that.
Removing the master circuit has no effect.

I'm thinking I need to move the speaker jack, it's too close to some of the preamp stuff.
Shoot me some other ideas to check out please, thanks Andrew.

[loudthud]

1 meg is too high for a post PI MV. Try a 200K or 250K.

[drewl]

That's what Marshall used for several different models.
It's not in place of the bias feed, it's in between the phase inverter and output tubes.
Can't hurt to try though, thanks.

[Chuck H]

1M is correct for THIS type of master vol. If it were the type where a dual gang pot replaces the power tube grid loads then a dual 250k would be correct. What I'm getting is that as signal across the MV leads is increased the amp becomes unstable. The location of the MV, the length of the leads and their relative proximity to other signal leads is where I would start. If the MV is too close to other preamp leads you'll need to move the preamp leads. Shorten the leads to and from the MV if possible. Perhaps change the location where the MV is grounding signal to a different ground point.

Chuck

[Enzo]

I would suspect all the wires running to and from the thing are being little antennas and causing the problems.

[loudthud]

The instability is caused by the new high frequency roll off you have introduced inside the feedback loop. Just because a 1M was used in some other amp with different tubes and OT, doesn't mean it will work in this amp. You might be able to cure it with lead dress or a change in the feedback factor. My suggestion of a 250K pot would raise the roll-off frequency.

[drewl]

After a week vacation, I'm back futzing with this mess.

I swapped the dual master for a dual 100k pot which didn't help.

I think the power supply might be a cause.
All the filters were replaced before me, so the supplies look pretty clean, but the output plates of the phase inverter seem to have some noise of some sort on them, and the voltage seems likes it's unstable at the plates, even with the PI tube removed.

I got it clean on a scope with signal applied, but a strong note from a guitar will cause the oscillation/distortion.

I was told a Fender tech went all through this amp and couldn't find the problem either.
The cabinet says Thomas Amplification....don't know if it's a kit or what...I'll have to look it up.