Ampeg Jet II J-12T Tremolo distortion

[drewl]

Got one of these reissues that when you crank temolo intensity past half way the output has a nasty raspy distortion.

I had increased the cathode resistor from 120 to 200 ohms to lessen stress on the EL84's, but it still has this problem with the original value.

Any ideas?

[drewl]

Hmmm, possibly because cranking the tremolo like doubles the voltage across the output tube cathode resistor?
Limiting the tremolo signal with a resistor from the intensity pot to ground removes the distortion, and the bias voltage only increases a little.

So basically cranking trem intensity is causing twice the output tube current draw.

[Helmholtz]

So basically cranking trem intensity is causing twice the output tube current draw.

That alone won't cause distortion.
But a tremolo signal amplitude > 11V will cause distortion by grid conduction and cut-off.

[Enzo]

Scope the trem signal, scope the power tube grids. Is the trem signal hitting a wall anywhere? The schematic calls for 35v p-p at the plate of the trem tube. How does yours compare?

I recommend finding out what it happening rather than guessing and changing parts.

[drewl]

I'm seeing around 80v p-p at v1 pin 6.

Helmholtz, where would that 11v be?

Note: the amp had two 12AT7's in it I replaced them with the correct ax7's.

[Enzo]

I believe what he is saying is that according to the drawing, the stock cathode resistor, you should have about 10.4v on the cathode, so 10.4v bias on the power tubes. If your trem signal exceeds that, it can cause grid conduction. If I am not mistaken that equates to a 22v p-p trem signal at the grids.

[drewl]

Fascinating, any idea what could cause that?

Lowering the plate resistor to the tremolo amp lowers it's gain and also cures the problem.

[g1]

Lowering the plate resistor to the tremolo amp lowers it's gain and also cures the problem.

So you have 80V p-p where there should be 35?
It says there should be 156VDC at the plate with pin7 grounded, did you check that?

[drewl]

Yes, the dc voltages of the trem oscillator are where they should be.
All the resistor values are okay.