Marshall Jcm 900 4500 Crackling

Discussion in 'The Workbench' started by Grady Cook, Oct 2, 2017.

  1. Grady Cook

    Grady Cook New Member

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    I've had this JCM 900 4500 for a month or so now. While i was recording for a project today, I had all of my levels set perfectly, nothing clipping whatsoever. However, when I hit record and listened back, it sounded like it was clipping in random areas. I had no idea it was the amp until i wasn't playing and i heard it again.

    My first suspicion is that the tubes need replacing maybe? I'm far from an expert though so i'd prefer a more experienced opinion
     
  2. Jethro Rocker

    Jethro Rocker Well-Known Member Silver Supporting Member

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    Firstly does it sound like that when NOT recording and playing back?
    It did this crackiling when not playing?
    That could indicate a tube, yes, possibly the PI tube, maybe V1, could conceivably be a power tube problem. Could potentially be a dirty pot.
     
  3. Grady Cook

    Grady Cook New Member

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    It happens when i'm not recording, yes. I just realised it the most when I was recording. I noticed it slightly a while ago on the clean channel but now it's the same for the dirty channel too.

    I've heard a bit about it potentially being a dirty pot but I was under the impression that it'd make that noise from turning one of the pots like the volume. Forgive me if i'm mistaken. thank you for your suggestion!
     
  4. Jethro Rocker

    Jethro Rocker Well-Known Member Silver Supporting Member

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    Buddy's 2203 was thin, weak, quiet. Thought it was a tube. Turns out it was dirty MV pot. Completely changed the sound. But the pot usually crackles when you rotate it, yes. If it doesn't then replace PI tube, the one nearest the power tubes, first. Then do 1 at a time to narrow it down. Past that, I'm out of here!!
     
  5. ampmadscientist

    ampmadscientist Well-Known Member

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    THERE IS these THINGS:
    Plug a guitar cable between effects send and effects return, then re-test.
    If the problem goes away, it's a bad effects loop jack. (the switch contacts in the jack are corroded)
    This is very common on the 4500.

    If you never use the effects jack: it doesn't matter.
    The switch inside the jack MUST work, even if you do not use it. Remember this.

    2. Buy 1 new 12AX7
    Change (swap out w/ new tube) the preamp tubes one at a time, and re-test each time.

    No you do not need a whole set of tubes....just swap w/ 1 new one.

    3. Cracking / clipping...
    The bias is set too cold. Make sure it's set correctly.
    People frequently put the wrong tubes in...without changing the bias. Then it sounds terrible.
    Happens all the time.
    YES cold bias will cause clipping / distortion. Bank on that...

    4. Turn the reverb down to ZERO. OR UN-plug the reverb tank.
    Now re-test.
    A broken reverb spring will cause that clipping/cracking also.

    5. Test the amp w/ different speakers.
    The speakers could be blown.
    Don't ignore this possibility...VERIFY.
     
    Last edited: Oct 5, 2017

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