Amped Gemini II g15

[delux1959]

a friend is bringing me his amp. He tells me the vibrato is not working. Does anyone have experience with these amps who could share what I should check first? In other words are there common failures I should look for?

[PRR]

Basic-basic trouble shooting: does it need a pedal? Does the pedal work?

Change the reverb tremolo tube. A weak signal tube is just less loud. A weak oscillator tube won't shimmy at ALL.

Cathode cap is equally important. If not fresh, replace on general suspicion.

After that, basic troubleshooting. Voltages are a good start. We may like a link to the plan, we don't have all amps in our tiny memory.

[92Volts]

I did quick and dirty maintenance on a Gemini I G-12 to get it safe and working... 3-wire cord, power supply capacitors, whichever other caps I could replace.
When I was "done", it played fine but... the vibrato still didn't work!
Don't let that discourage you... I didn't replace caps in the vibrato circuit because I didn't have the right value (smaller than coupling caps), and I bet that would have helped.

Why did I replace others and why am I so suspicious of the remaining ones? Not sure what type the original caps were, I think paper-in-oil (electrolytic in the power supply of course). All were discolored, some misshapen, some appeared blackened from heat. The coupling caps could leak DC and cause bias problems leading to major sound issues or bias leading to tube failure so I considered replacing most caps to be a safety/reliability priority... but they also looked terrible and I believe they're a type known to wear out after 50+ years.

So, be ready to replace almost every cap, especially in the problem area (vibrato circuit)... if that maintenance hasn't been done before.

Deoxit (Home Depot sells generic electrical contact cleaner if that's easier to get) should improve pots. Like caps, they need to work and be roughly the right value for the vibrato to do its thing. This also improves audible scratchiness or cutting out heard with audio pots.

Resistors (other than pots) "can" go out-of-spec over time but I feel they're unlikely to be so bad the amp, or a feature like vibrato, no longer works.

This is the same advice I'd give for any problem in an old amp: safety stuff if needed (3-wire cord), capacitors, clean pots. Resistors and bad tubes are a possibility but IMO towards the bottom of the list.

PRR gives other good suggestions with less drastic solutions.

My recommendation would depend on what other work has or hasn't been done on the amp in the past but if basically nothing has been replaced since the 60s, a lot of "preventative maintenance" is due or overdue regardless of which problem is actually happening right now.

[sluckey]

The opto-coupler TM-1 has to be high on the suspect list. Check the voltage on V7 pin 6. It should appear erratic and always changing if using a DMM. If the voltage is steady the oscillator is not working. Try a good strong tube and examine the components around V7B. If the voltage is changing, the oscillator is working and the opto-coupler is the most likely suspect.