Gibson GA-20RVT reissue about 2006

[TwinBeech]

I am working on a friend's Gibson amp that has very weak tremolo. The slow speed oscillator seems to be working but the depth pot does not vary the strength of the weak signal coming from the LDR. Gibson tells me they do not sell amp parts and suggested aftermarket stuff. The problem is I cannot get the part number for the opto-coupler. I found one for a Fender and changed it with no change in the tremolo.

Does anyone have a suggestion or two? I will attach the schematic I have although it may not be 100% accurate. There is a small pulse at the input of the neon tube that is shown on the output of the LDR but is not varied by the depth pot.gibson_ga20rvt_amplifier_schematic.pdf

[TomCarlos]

See if this is correct and easier to read - Gibson GA20RVT.

One easy thing to do... Check the tube, is it the correct type? Pull the tube, make sure the socket is clean. Do you have another tube to try?

[TwinBeech]

That schematic is for the original amps. The schematic I have is for the reissue. The schematic is dated 2006 which is about when the amp was purchased. I have swapped the tubes with no joy. I have tested the neon tube in parallel with the new roach and it flashes. I think my problem is with the proper opto coupler or something else. Thanks.

[TomCarlos]

Ok...

As background reading, you might take a look at this post from "The Amp Garage."

[g1]

The opto is reported to be VTL5C1.
This model (re-issue) is noted for having weak tremolo, apparently by design.

[52 Bill]

Interesting, I have not seen that style of trem before. Instead of varying output bias or shunting audio signal to ground, they are basically shunting the two input grids of the power tubes together.

You should be able to read the resistance across the photocell in circuit and see if it is reacting to the neon lamp flashes. Check to see that it is reacting. Check the workings of the depth pot and the resistance of the relay contacts to be sure that they are not adding unneeded resistance to the circuit. Finally check to see that the value of the 6800pf cap is correct.

It would seem to me that the 6800pf value would be too small to make a significant signal cancellation of the out of phase signals, so you could try a larger value there instead.