1960s era ampeg jet j-12-a has tremolo problem

[Enzo]

I suggested a 1 meg pot, not resistor. Wire the wiper to the grid resistors. That way you can dial down th intensity . As you found out, just raising that resistor does nothing. If the trem signal is too hot, it can run your grids into the bias voltage. I mean the bias level acts like a wall the thing hits.

Have you scoped this? How hot is the trem output? And what is the cathode bias on the tubes?

[g1]

They can be matched tubes yet still not matched in the amp circuit. You need to check idle current for each power tube socket.

[Helmholtz]

A leaky 0.02µ coupling cap or a missing screen voltage could cause an imbalance.

[lectricmandan]

Checked the idle current on the 7591 tubes - got 154ma on one and 165ma on the other. measured using a 1 ohm resistor on pin 5. Checked the voltages on all the screens and they are all there. The trem sine wave is about 9.5 volts pk/pk at the junction of the 680k resistor.

[g1]

Checked the idle current on the 7591 tubes - got 154ma on one and 165ma on the other. measured using a 1 ohm resistor on pin 5. Checked the voltages on all the screens and they are all there. The trem sine wave is about 9.5 volts pk/pk at the junction of the 680k resistor.

That sounds like massive current to me. Is that with no signal and no trem ?

[Helmholtz]

Checked the idle current on the 7591 tubes - got 154ma on one and 165ma on the other. measured using a 1 ohm resistor on pin 5.

Sure you really measured the voltage drop across the individual 1R resistors and not to ground?

Something like 160mA per tube would melt the plates - or blow the fuse.

[lectricmandan]

I don't know what was wrong but that was the numbers I got, I think there was a problem with the resistors I used. So today I did it the real way with the fluke 87 in series with each one . At V3 I got 42.4 ma and at V4 I got 40.4 ma these numbers look a little better. I've never done this using resistors before and probably not do it again as I will do what I normally do . : )