Ampeg Gemini II: Pot inside chassis?

[livelydan]

Hi, & thank you for your help. What is the pot inside of my Ampeg Gemini II for? I turn it and nothing seems to happen. Thanks, Dan

[Bruce / Mission Amps]

Hi, & thank you for your help. What is the pot inside of my Ampeg Gemini II for? I turn it and nothing seems to happen. Thanks, Dan

There is a little filament supply hum balance pot in some of those... look to see if it is across the filament supply line.
Sometimes a blown power tube will take those hum balance pots right out so yours... (if that is what it is)... could be burned open.

[livelydan]

There is a little filament supply hum balance pot in some of those... look to see if it is across the filament supply line.
Sometimes a blown power tube will take those hum balance pots right out so yours... (if that is what it is)... could be burned open.

thanks, bruce... this is a full-sized pot with solid shaft, don't know where the filament supply line is, but this is located just to the right of where one wire from the 'on' light is soldered. when i tapped it, the signal went out, so i guess i need to resolder it's terminals. could it be for anything else? the amp is very quiet, except when the reverb is full on there is a little noise. thanks...

[gmoon]

My Gemini II has a hum-balancing 100 ohm rheostat wired between the power tube cathodes (wiper to gnd.)

Quite a few variations on the Gemini II, though. I've got the 7591's, and the schematic for this version isn't easy to find (on the web, anyway.) The "Joe Piazza" schematics, for instance, don't show the rheostat--neither the 7591 or 6L6 versions.

Is the POT you describe mounted on the back? You can see a couple photos here.

[livelydan]

My Gemini II has a hum-balancing 100 ohm rheostat wired between the power tube cathodes (wiper to gnd.)

Quite a few variations on the Gemini II, though. I've got the 7591's, and the schematic for this version isn't easy to find (on the web, anyway.) The "Joe Piazza" schematics, for instance, don't show the rheostat--neither the 7591 or 6L6 versions.

Is the POT you describe mounted on the back? You can see a couple photos here.

i apprreciate the help guys... no, it's inside, soldered right on, no lead wires. thanks.

[gmoon]

Soldered on the eyelet board?

Some G-15's have a POT to adjust the bias voltage. Look closely at the second photo on the G II thread link above--there's a small trim POT with a blue plastic collar ("knob.")

And yep, that's "right close" to one lead from the indicator lamp.

If that's what your seeing, it's the adjustment for the power tube bias. You won't necessarily hear much difference when fiddling with that POT, but it will definitely effect the operation. Bias it too warm (closer to GND), and you'll shorten the tubes life (redplating, even--which will quickly fry the tubes, and can lead to other problems); too cold (more negative in respect to GND), and it might sound sterile or harsh.

It can be a subtle difference in any case...you're not likely to hear much audible change at lower volume levels...

[livelydan]

Soldered on the eyelet board?

Some G-15's have a POT to adjust the bias voltage. Look closely at the second photo on the G II thread link above--there's a small trim POT with a blue plastic collar ("knob.")

And yep, that's "right close" to one lead from the indicator lamp.

If that's what your seeing, it's the adjustment for the power tube bias. You won't necessarily hear much difference when fiddling with that POT, but it will definitely effect the operation. Bias it too warm (closer to GND), and you'll shorten the tubes life (redplating, even--which will quickly fry the tubes, and can lead to other problems); too cold (more negative in respect to GND), and it might sound sterile or harsh.

It can be a subtle difference in any case...you're not likely to hear much audible change at lower volume levels...

this pot is full-sized: looks just like a regular one, not a little trim pot... if i could i'd take a pic... if it is a bias pot, how do i adjust it? thank you for helping. it is a sweet sounding amp. i just turned it into a head because the cab was in a flood and water damaged at the bottom.

[gmoon]

The size of the POT wouldn't matter--if that's indeed what you've got.

I'm not certain yet you've got the same Gemini II setup (7591's, 7199 PI, etc.) Not that 6L6's couldn't have a bias voltage POT, of course. I think a pic would help here.

There's also a schematic for this particular 7591 G-15 on the that other forum page--it's not an easy one to find (mostly the Joe Piazza schematics are posted online); it shows both the bias POT (marked BIAS, just left and below the bottom power tube), and the approx bias voltage, noted as APP. -22V (based on 1965 line voltages .)

I don't suppose you marked the original POT setting before you turned it...?

As far as biasing, there's learning curve. I'll point you to RG's Tube Amp Faq...

[livelydan]

here's how the stickers inside read:

model G-15
serial no. 021245

Tube Location (from left to right)
7591 7591 7199 12AX7 6CG7 12AX7

thank you!

[Enzo]

Let's try it this way. Pots have three legs, or three solder lugs if you prefer. EAch will be connected to something. I am assuming by wires in this amp, short ones or long ones. To what do each of the end leg wires connect? And the center leg? If the end legs connect to a pair of wires twisted together that runs from tube socket to tube socket, then it is the heater balance pot Bruce mentioned. If the wires run to a pin on each power tube socket, then it is likely a balance pot. Or measure for DC voltage on its legs while the amp is running. If you find a negative voltage there, it is a bias adjust.

If the pot solders right to an eyelet board, there will still be other things sharing the eyelets - wires, transistors, something.