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So I opened up my Gemini this weekend to swap out the tremolo optocoupler (bought from Fliptops). That operation went great, beautiful throbby trem when I tested last night, and generally healthy sounds otherwise.
After I installed the amp back in the cabinet this afternoon however all I get is “whooshy” static and a very faint guitar signal. Ok, so we take it back out of the cabinet and test pin voltages. Everything looks pretty good (using this schematic for reference: https://ampeg.com/support/files/Schematics/G Series/G-12 (Gemini I)/G-12 Schematics 2.pdf), my B+ is about 20V high, but I’ll chalk that up to modern vs vintage wall voltage.
I will note here that the only change I made besides the optocoupler was to put a 470k resistor on the plate of the tremolo tube. 470k is spec’ed on the schematic, but mine had 390k in there.
The biggest clue I can find is that there is no crackle induced when probing the power tube grids. That’s one thing I’ve noticed working in many amps: probing the power tube grids will make the speaker crackle, and usually lets me know that any problems lie before the power amp. No such crackle here.
I’m somewhat suspicious of the OT, as it has been wired incorrectly when I got the amp. Someone had wired the speaker to the wrong secondary taps, making the output very low, and popping (like physically exploding) the power tube cathode bypass cap when cranked. When I fixed that (two years ago) I also replaced all electrolytics and the power tubes. Since then I’ve had two years of great performance (besides the tremolo), but I’ve always wondered if the OT had been damaged.
The only test I know to do is measuring the DC resistance on the primary: one side shows around 80 ohms and the other 71 ohms. Anything else I can test? I’m not fully convinced it’s the OT as it was working fine last night, but maybe it got bumped when I installed the amp into the cabinet?
If I am looking at a new OT, is there any reason to spend twice as much on the Mercury as the Fliptops version?
After I installed the amp back in the cabinet this afternoon however all I get is “whooshy” static and a very faint guitar signal. Ok, so we take it back out of the cabinet and test pin voltages. Everything looks pretty good (using this schematic for reference: https://ampeg.com/support/files/Schematics/G Series/G-12 (Gemini I)/G-12 Schematics 2.pdf), my B+ is about 20V high, but I’ll chalk that up to modern vs vintage wall voltage.
I will note here that the only change I made besides the optocoupler was to put a 470k resistor on the plate of the tremolo tube. 470k is spec’ed on the schematic, but mine had 390k in there.
The biggest clue I can find is that there is no crackle induced when probing the power tube grids. That’s one thing I’ve noticed working in many amps: probing the power tube grids will make the speaker crackle, and usually lets me know that any problems lie before the power amp. No such crackle here.
I’m somewhat suspicious of the OT, as it has been wired incorrectly when I got the amp. Someone had wired the speaker to the wrong secondary taps, making the output very low, and popping (like physically exploding) the power tube cathode bypass cap when cranked. When I fixed that (two years ago) I also replaced all electrolytics and the power tubes. Since then I’ve had two years of great performance (besides the tremolo), but I’ve always wondered if the OT had been damaged.
The only test I know to do is measuring the DC resistance on the primary: one side shows around 80 ohms and the other 71 ohms. Anything else I can test? I’m not fully convinced it’s the OT as it was working fine last night, but maybe it got bumped when I installed the amp into the cabinet?
If I am looking at a new OT, is there any reason to spend twice as much on the Mercury as the Fliptops version?
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