Ampeg Gemini G-12 trem, problem or not?

[Randall]

Tremolo works, but it very weak. Someone has rebuilt the trem circuit and dated it 1992. It is a mixture of replaced, and still in tolerance original caps and resistors. The electrolytic cathode cap was replaced by me, and the big .22 cap measures close enough, and is not leaking. A new tube makes no difference. Of the three .05 oscillator caps, two have been replaced with .068 20%, and one is two .047s glued together in parallel and reading .072. I think this is factory, judging from the same brown glue I find everywhere else in the chassis.

This brings me to wonder if there is anything else to fix? The big factory photocoupler could be weak I suppose, but I wonder how strong was this trem when it was new? It is pretty good when the intensity switch is on, but with it off there isn't much.

I wonder about the three caps that are supposed to be .05 on the schematic, do I bother to order up some .047s to try that, or would that even matter? The tube is oscillating, and it follows the speed control.

https://ampeg.com/support/files/Sche...matics%201.pdf

And do I understand correctly how this circuit works? Points X and Y are the photoresistor, and are across the intensity control, so when the control is full off there is a shorted path from V5a coupling cap to the tone stack, but when it is full on the control provides 4M ohm to the tone stack, but the fluctuating resistance of the photoresistor shunts an ever changing increasing/decreasing resistive path to the tone stack.

[Enzo]

Sounds like you understand it.

Turn your trem off, so as I see it, the trem bug lamp stays ON. That should be the low end of the photocell resistance. With the intensity pot at the highest resistance, measure resistance across it. That should be your benchmark low, which would be loudest. If you short the bulb, what change appears? What difference in level does the pot create?

The intensity switch. When you say on and off, does that correspond with closed and open? Or are they opposite. I see the open as not affecting trem, and closed as bleeeding off some of it.

[Randall]

Ok, with trem switched off, I measure 11 ohms across the pot CCW, and 3.9K ohms CW. For the second measurement, are you asking me to short the neon bulb? There's 58v on the + of that bulb.

For the switch, I mean on is closed. When the switch is closed, I get trem, with it open I get very little, if any. I also just realized the switch was intermittant, which was probably confusing things. I got some deoxit in there, and it is good now.

[Dave H]

For the second measurement, are you asking me to short the neon bulb? There's 58v on the + of that bulb.

It's OK to short it as there's a large resistor in series. I can't quite make it out but it looks like 300k.

[Enzo]

Not only is there the 300k resistor in series with it, the plate resistor has 470k additional, so 770k between bulb and B+. B+ looks to be 350v on F node. A short on the bulb would only draw what half a milliamp?

[Chuck H]

Well... You say it's pretty good with the intensity switched on. How is it supposed to work with the intensity switched off??? Not intense? Just sayin'.

[Randall]

With trem switched off, intensity pot on full CW I measure 3.9K across pot, and have full volume. With neon lamp grounded, I measure 5.2M across pot, (same as amp powered down), and with significant loss of volume. Seems like the opto is working, but why isn't it making trem?

With trem on and intensity switch open, I get 66 - 135v on the left side of the 390K ohm resistor coming off the plate of V7B, but on the right side of the same resistor feeding neon + I only see 53 - 58v. That would explain why the opto isn't swinging, but why?

[Justin Thomas]

Is the switch at the far clockwise end of pot travel or far counterclockwise? I'm asking for Chuck's sake because I'm thinking this amp may have the "afterbeat" effect, which is supposed to produce a more "helicopter"-like sound than a regular smooth volume modulation.

Justin

[Enzo]

The neon bulb will have a characteristic voltage across it while glowing, I suspect that is the 50v you are seeing.

[Randall]

"Is the switch at the far clockwise end of pot travel or far counterclockwise? I'm asking for Chuck's sake because I'm thinking this amp may have the "afterbeat" effect, which is supposed to produce a more "helicopter"-like sound than a regular smooth volume modulation.
"
I believe this is the case. The switch is engaged at the far CW position.

[Dave H]

With trem on and intensity switch open, I get 66 - 135v on the left side of the 390K ohm resistor coming off the plate of V7B, but on the right side of the same resistor feeding neon + I only see 53 - 58v. That would explain why the opto isn't swinging, but why?

Just speculating here -
66-135V is above the neon maintaining voltage (50V) meaning the neon is always on so there's no trem.
Closing the switch pulls the voltage down so that it's swinging above the neon striking voltage (70V) and below the 50V maintaining voltage so the neon turns on and off and you have trem.

[Chuck H]

With trem on and intensity switch open, I get 66 - 135v on the left side of the 390K ohm resistor coming off the plate of V7B, but on the right side of the same resistor feeding neon + I only see 53 - 58v. That would explain why the opto isn't swinging, but why?

53V to 58V is also above the trigger (if that is 50V)

Maybe there's just too much voltage in the circuit due to modern wall AC to allow the bulb to turn off for sufficient time (or at all) to achieve much trem.?. Maybe increase the value of the 390k?

[Randall]

I subbed a 1M pot for the 390K, and settled at 470K. After replacing the 390K with the 470K it sounds 'a little' better. I also just noticed there is a 22M resistor that doesn't look like any of the factory ones between the bottom of the 22K cathode resistor switch point, and the right side of the 390K (now 470K), effectively the + of the neon lamp. Maybe the guy in here before was trying to drain some of the voltage off the neon as well? I tried it in and out of circuit, not much difference.

I don't know.