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Ampeg Gemini 1 issues...

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  • Ampeg Gemini 1 issues...

    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 ďwhooshyĒ 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 Iíll 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 specíed 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. Thatís one thing Iíve 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.

    Iím 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 Iíve had two years of great performance (besides the tremolo), but Iíve 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? Iím not fully convinced itís 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?

  • #2
    Why do we think it is a transformer problem? That is the most reliable part on the amp. The amp worked until you put it back together, so I'd be looking for something you knocked loose.
    Education is what you're left with after you have forgotten what you have learned.

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    • #3
      The chassis are not exactly the sturdiest. Might have flexed something loose. Chopsticking perhaps. Also, Ampeg was known to change things at the factory. Don't be surprised if schematics don't match the amps. Does yours have the schematic attached to the back panel or under the chassis lid by any chance? All of mine did... My Reverberocket 2 also had a hnd-penciled addition of the power tube cathodes being used to elevate the heaters. Really freaked me out that time I found ~25V on my heaters. Before I understood the difference. Maybe they updated the axhematic on your amp with a 390k instead of a 470.

      Justin
      "Wow it's red! That doesn't look like the standard Marshall red. It's more like hooker lipstick/clown nose/poodle pecker red." - Chuck H. -
      "Of course that means playing **LOUD** , best but useless solution to modern sissy snowflake players." - J.M. Fahey -
      "All I ever managed to do with that amp was... kill small rodents within a 50 yard radius of my practice building." - Tone Meister -

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      • #4
        Crisis averted. I traced the input signal through the amp using an oscilloscope. Signal was getting to the PI but not past it, which sucks because they don't make that tube anymore. I only had one spare, but it sounds great now.

        Yes it does have a schematic inside the back panel, though some of it has torn away over the years. I do not see any pencilled-in changes. The trem seems to be a little more "square" with the higher plate resistor.

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        • #5
          Yeah, well, a 390k vs 470k resistor change won't be the cause of no sound anyway.
          Education is what you're left with after you have forgotten what you have learned.

          Comment


          • #6
            Your schematic link didn't work, this one should: https://ampeg.com/support/files/Sche...0(Gemini%20I)/
            "Everything is better with a tube. I have a customer with an all-tube pacemaker. His heartbeat is steady, reassuring and dependable, not like a modern heartbeat. And if it goes wrong he can fix it himself. You can't do that with SMD." - Mick Bailey

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            • #7
              Originally posted by Enzo View Post
              Yeah, well, a 390k vs 470k resistor change won't be the cause of no sound anyway.
              Yes, I know that. I just used that as an example of "Ampeg liked to change things on the fly too..."

              Justin
              "Wow it's red! That doesn't look like the standard Marshall red. It's more like hooker lipstick/clown nose/poodle pecker red." - Chuck H. -
              "Of course that means playing **LOUD** , best but useless solution to modern sissy snowflake players." - J.M. Fahey -
              "All I ever managed to do with that amp was... kill small rodents within a 50 yard radius of my practice building." - Tone Meister -

              Comment

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