AB763 Board noise problem

Discussion in 'Amps/Cabs Tech Corner: Amplifier, Cab & Speakers' started by tmac, Jul 28, 2005.

  1. tmac

    tmac Goldmember Gold Supporting Member

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    Just put a new Hoffman board (from Airtight garage) into my Super Reverb. It works but has a hum/buzz in the reverb/trem channel. When I pull V2 (Rev channel 12AX7) the hum goes away and the normal (V1) channel is perfect with no hum when V2 is pulled. Likewise grounding the plates (pin 6) on of V2 and V4 kills all hum. Pulling the Rev tube (V3) and trem tube (V5) has no effect on hum level...

    Seems that the power amp and filtering is fine and my problem lies between V2 and the PI. All voltages check out ok.

    Any ideas guys? :(
     
  2. tmac

    tmac Goldmember Gold Supporting Member

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    I just read my original post again and maybe I wasn't too clear - I can the amp with all tubes installed except for V2 and she's dead quiet. When I put in V2 I get a hum, grounding out pins 6 or 7 (the volume control side of V2) on V2 kills the hum. Definitely seems something is not grounded properly. Grounding pin 2 of V3 (reverb tube) doesn't help so I don't think it's the 'verb.

    Here's the layout diagram:

    http://www.hoffmanamps.com/images/Ab763.gif

    pretty close to a standard BF Fender
     
  3. Blue Strat

    Blue Strat Member

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    Have you tried a different tube?

    Try disconnecting pin 7 and see what happens. Sounds like you're injecting hum into the grid (pin 7). Maybe something's flakey about the connection at the other end?

    Retrace your wiring.
     
  4. calieng

    calieng Member

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    Check the entire circuit against the layout. I purchased one that had an incorrect underboard connection. The guys that assemble these boards can make a mistake sometimes and never realize it because they are not tested before you get it.

    You may have also wired up your tubes incorrectly.

    Good luck!
     
  5. tmac

    tmac Goldmember Gold Supporting Member

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    Duh! I grounded one of my filter caps in the wrong location (funny how that can make such a difference). The first and 2nd filter caps need to be grounded at the same location by the power tranny. Forgot about that. Anyway, quiet as a little mouse now and sounds very good.

    Good news is that I got it fixed. Bad news is that I got a lightning strike Friday and lost my TV, telephone, and a big pine tree in my front yard! :(

    Thanks to everyone who replied!
     

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