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Home build mini JCM800 misbehavin'

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  • Home build mini JCM800 misbehavin'

    Hi all,

    I posted about this a little while ago, but decided to re-post now since I have updated info to share.

    I have an Allen Accomplice I built in 1999. After years of modding and tinkering with this amp, I decided to do a major modification and turn this little amp into a JCM 800 clone. I love the results! I've also done a cap job on it, about a year ago, after (gulp) almost 20 years on the original Illinois E-caps.

    This amp has sounded KILLER for over a year, but now has suddenly developed an issue.

    Tthe volume has started fluctuating, mildly, but just enough that my bandmates and I can hear it. There is also a very faint, extra distortion when a note's volume fluctuates "low". It's minor, and a non-musician may not notice it, but it's there, and it's a new phenomenon. And it's driving me NUTS.

    The noise is most easily re-created when playing an open E chord, specifically plucking the E note on the D string. It is an intermittent, distorted, fuzz-tone type "note" riding on top of the true note - sounds like a fuzz-tone, and sounds like it's a musical third (chuckle, the noise has a good ear and a sense of humor). And as I said, it is intermittent (but easily induced), but most definitely there where it sure shouldn't be;

    I've also noticed an intermittent crackling, as well.

    I swapped out all tubes, preamp, power, and rectifier, without any effect. It's not a bad tube. I'm suspecting a problem with a filter cap, at this point, even though I recently replaced them all.

    Ideas?

    Thanks in advance!
    Last edited by Fred G.; 09-16-2018, 10:29 PM.

  • #2
    Two things I would consider. First would be a cold solder joint. The intermittent nature of the problem and the crackling would be common symptoms. The other would be parasitic oscillation. These can go in and out making it seem intermittent.
    "Take two placebos, works twice as well." Enzo

    "Now get off my lawn with your silicooties and boom-chucka speakers and computers masquerading as amplifiers" Justin Thomas

    "If you're not interested in opinions and the experience of others, why even start a thread?
    You can't just expect consent." Helmholtz

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    • #3
      Chuck mentioned parasitic oscillation, one thing that often accompanies it is crackling. Does the crackling and drop out/fuzz usually occur at the same time?
      Originally posted by Enzo
      I have a sign in my shop that says, "Never think up reasons not to check something."


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