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Thread: loud growling hum in ab763 clone

  1. #1
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    loud growling hum in ab763 clone

    my first build now amplifies now :-there is a buzz(not worried about that yet), but much worse is a loud, Moog like growl when i turn the amp above about two . If I put a guitar in the noise disappears and the guitar is amplified, but as soon as I unplug it comes back.
    I tried disconnecting the preamp stage from the PI input and the growl still came out of the speaker. I am concluding its not preamp related- so think its either PI, Power stage or Herater related
    The amp is , essentially the vibrato section of a AB 763 , but without the vibrato bit
    All the voltages and currents in preamp are good, I havent measured power amp current ( I dont know how!)
    I am an absolute beginner and the forum helped me get noise from the thing, I have posted this new thread as the issue is different to my original 'no sound' problem
    could this be about a push pull problem, or is it something else
    any help appreciated
    chris
    Last edited by chris shaw; 11-29-2009 at 12:20 PM. Reason: missed space in title and bad description

  2. #2
    Supporting Member Jazz P Bass's Avatar
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    Grounding Jack

    Are you using a grounding input jack?
    It has an extra tab that should attach to ground when no input is inserted.

  3. #3
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    Will try later

    Thanks , will check tonight and respond

  4. #4
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    partial success!!

    thanks jazz p bass the 1 meg input resistor wasnt connected correctly. This has stopped the oscillation at input (how did it get into power stage when pre amp had been disconnected?!)
    nonetheless I am left with the following:-
    a hum/ buzzing tone which comes on when amp is turned on, is quite loud , can get louder as amp volume is turned up BUT if i put a cd player through it , even with the cd player volume turned to zero the buzzing will reduce as I turn up the volume of the built amp (not the CD) to 1 or 2 ,
    if I pull the cd player jack plug out the buzz comes immediately back even if the volume of the amp is set at a level which, with the cd player connected has previously reduced the buzz level to very low

    I am assuming this is a grounding issue ?!!!,

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