Ampeg Gemini, Throbbing noise & Red Plate

Discussion in 'Amps/Cabs Tech Corner: Amplifier, Cab & Speakers' started by plainshick, Dec 19, 2009.

  1. plainshick

    plainshick Member

    Messages:
    65
    Joined:
    Mar 27, 2008
    Location:
    Western Nebraska
    hey all,
    Thanks for any help anyone might have with this wacky issue.

    I have had this 1965 Gemini I (G-12) for about 10 years. Recently it started "throbbing" - sort of a motor-boating sound that quickly grew in volume. One of the power tubes glows red on the plates - throbbing in unison with the throbbing sound. I tried new tubes as one did test weak, but the problem is still there. I also replaced the phase-inverter tube without luck. The amp works fine when first turned on - takes about a minute 'til problem starts.

    It has had all electrolytics replaced a few years back.

    I replaced the tube sockets, since I thought arcing might be an issue - didn't help.

    I have replaced or checked about all the resistors in the power section, and replaced some - no help.

    Do diodes go bad often? Any other ideas? Nothing looks burnt. Proper fuse does not blow. No smoke. I always shut it down quick when problem starts, so maybe it would blow fuses or smoke if I let it go.

    Thanks!
     
  2. slider313

    slider313 Silver Supporting Member

    Messages:
    7,481
    Joined:
    Jul 28, 2005
    Location:
    NC
    Has the twist lock cap-can been replaced? The bias cap is in that can. Have you changed or checked you 140 ohm bias resistor?
     
  3. plainshick

    plainshick Member

    Messages:
    65
    Joined:
    Mar 27, 2008
    Location:
    Western Nebraska
    Hey Slider,
    OK - I think it's fixed (hell yea!). The twist lock can had been replaced (with individual Spragues below the chassis). I had not checked the 140 ohm resistor, mainly because it's a big ol' wire-wound type (and they don't usually go bad I'm told). I did check it and came up with a 144 ohm reading (close enough I figure), but I did not even realize that nestled underneath that big old resistor was a little 25uf@30V electrolytic cap. I happened to have a 25uf@25V, so replaced it. The amp crackled and thumped a bit on startup, but evened out, and I played on it for about 10 minutes and seemed good. I think it's good to go.

    Thanks much for your help - VERY much appreciated!
     

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