Ampeg ET-2 tremolo will not work! Why?

[Leo_Gnardo]

Do you have any modulation at the plate of V5A?

Have you verified the speed and intensity pots are working?

Old Ampeg vibrato "bugs" are dying off at an alarming rate, as they are in aging Fenders. I've substituted my LED driven bugs for Ampeg's cube shaped bug, but it takes some fine tuning to make them work right. Unlike in Fenders, where the bug's photoresistor attenuates signal, in the Ampegs instead the blinking lamp allows the photoresistor to shunt across the vib depth pot (4 MegOhms ! ), passing signal with each blink. It takes only a very short pulse of current to allow this to work the way you want it to, so the resistor in series with the LED often winds up being a lot larger value than you might expect. Not having done one of these in a couple years I don't recall exactly. But around 2.2 Meg seems to ring a bell.

[guildguy]

Do you have any modulation at the plate of V5A?

Have you verified the speed and intensity pots are working?

The plate modulates between 130 and 131 when tremolo is engaged, that can't be enough modulation{?} A solid 276 when off.

Speed and Intensity pots are working. When intensity is on and tremolo is engaged there is distinct initial volume drop like it's turning on but then goes back to normal volume with no further modulation, likewise when intensity is all the way down and tremolo is engaged there is no volume drop. Speed pot checked resistance between lugs and is operational.

Any other suggestions?

[guildguy]

Old Ampeg vibrato "bugs" are dying off at an alarming rate, as they are in aging Fenders. I've substituted my LED driven bugs for Ampeg's cube shaped bug, but it takes some fine tuning to make them work right. Unlike in Fenders, where the bug's photoresistor attenuates signal, in the Ampegs instead the blinking lamp allows the photoresistor to shunt across the vib depth pot (4 MegOhms ! ), passing signal with each blink. It takes only a very short pulse of current to allow this to work the way you want it to, so the resistor in series with the LED often winds up being a lot larger value than you might expect. Not having done one of these in a couple years I don't recall exactly. But around 2.2 Meg seems to ring a bell.

I believe you maybe talking about a different circuit. Ampeg does not use LFOs, it is biasing tremolo.

[Chuck H]

I believe you maybe talking about a different circuit. Ampeg does not use LFOs, it is biasing tremolo.

Low Frequency Oscillator

There is no trem roach (or bug)

Of course it uses LFO's. Or an LFO as it were. Otherwise there's be no modulation. You recognized this in post #17 above. That triode is the oscillator. The low frequency oscillator. It seems to be engaging and conducting current when on, but not oscillating. Very hard to troubleshoot with just a tight shot of the components because it's impossible to see how they're wired to everything else. But there must be a problem with either how it's wired or any unexamined components related.

[Helmholtz]

Ampeg does not use LFOs, it is biasing tremolo.

That's no contradiction. Any tremolo/vibrato needs a low frequency oscillator (LFO). It's the V5a circuit.

Edit: Double posting.

[Leo_Gnardo]

I believe you maybe talking about a different circuit. Ampeg does not use LFOs, it is biasing tremolo.

What I wrote applies then I guess to the Gemini series. Whooops hadn't looked at the Echo Twin schemo.