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You Make the Call

Discussion in 'Amps/Cabs Tech Corner: Amplifier, Cab & Speakers' started by TheAmpNerd, Sep 17, 2006.

  1. TheAmpNerd

    TheAmpNerd Member

    Messages:
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    Joined:
    Sep 23, 2004
    Location:
    Tejas
    Okay y'all.

    In keeping with the spirit of TGP and knowing how some of you have
    a fit about gutting an amp and rebuilding it.

    We can do this a cap at a time!

    You make the call!

    The amp is a 1966 Ampeg Model J-12D Jet.
    The amp hums like crazy and the tremolo doesn't work.

    So far I've done the following:
    + Replaced the mutisection cap
    + Replaced the bypass caps
    + Replaced the tone pot (original was coming apart).
    + Cleaned and lubed all the pots
    + Cleanded and tightened all sockets
    + Cleaned and tested tubes and tube pins
    + Cleaned the input jacks, including ground and switch

    The voltages and specs are as follows:
    (I'll post them when I find my lab book)
    I have three and the one with these measurements
    is hiding from me.

    Here is a pic of circuit:

    [​IMG]



    That's right, I'll label all the coupling caps from left to right
    and include my measurements (when I find the lab book).
     
  2. Blue Strat

    Blue Strat Member

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    Location:
    Sterling, VA (not far from Washington DC)
    I wouldn't do anything else to it unless it doesn't sound right.
     
  3. WaltC

    WaltC Member

    Messages:
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    Feb 21, 2006
    Location:
    Northern CA (Sacramento area)
    Is this one of the 7868 ones? I just worked on one and had to replace two defective plate load resistors (270K in the one I worked on). I put in higher value resistors to tame some of the difference in modern day wall current (330K in one and 470K in the other) and got the volume up and the tremelo working fine now. Took some time signal tracing and checking voltages in the amp to find the bad (one drifted *way* off, three and a half megs, and the other, the tremelo one, "shorted" to infinity) resistors, but once replace, the amp works and sound very good now.
     
  4. TheAmpNerd

    TheAmpNerd Member

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    Location:
    Tejas
    Trem doesn't work and it still hums. Output is weak
    and almost spitty sounding.

    I think most all the caps are leaky.
    I still need to find my lab book with my notes on it too.

    That is up next, checking the resistors. Thinking all the ones I did check were in spec.
    Walt, this one has the 6Bk11s and the 7591s.



     
  5. rhinson

    rhinson Member

    Messages:
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    Joined:
    Jan 14, 2005
    Location:
    memphis
    you need to replace all the caps in the trem section---i've worked on a couple of these before and they are leaky and a problem. they'll bring down down the power section voltage leaking to ground. just replace them and go from there---i bet voltages and volume pop back to normal when you do, assuming you don't have something else smoked--then you can better assess the rest of the amp. rh
     
  6. slider313

    slider313 Silver Supporting Member

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    Location:
    NC
    And replace the 6BK11 as I believe it is the tremolo tube. If you can't find a 6BK11 I think a 6C10 will work in it's place.
     
  7. WaltC

    WaltC Member

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    Feb 21, 2006
    Location:
    Northern CA (Sacramento area)
    the one I was working on had two 6BK11s and two 7868s. Sounded good when I got done, but was real weak and noisy when I started. Not much break up in there though <G>.

    Walt
     
  8. roknfnrol

    roknfnrol Silver Supporting Member

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    Location:
    Louisville, KY
    You ever get this amp up-and-running? If so how does it sound?
     
  9. Dana-L

    Dana-L Supporting Member

    Messages:
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    Feb 27, 2006
    Location:
    Outer Portlandia
    Did you ever find the root cause(s) and get this amp working again?

    -Dana
     
  10. TheAmpNerd

    TheAmpNerd Member

    Messages:
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    Joined:
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    Location:
    Tejas
    I set it aside.

    I'll get back to it eventually.

    I did replace all the caps in the vibrato section
    along with most of the other coupling caps.

    Most everything elese was in spec, i.e., ressitors, pots,
    and voltages.

     

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