JMP-1 - Noise on high note decay - old tubes?

Discussion in 'The Workbench' started by MikeyV, Jul 30, 2020.

  1. MikeyV

    MikeyV New Member

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    Hi everyone,
    I got a JMP-1 a while ago, and I seem to be having a noise issue with it.

    When you strike a high note and hold it, the decay is lumpy after a few seconds, and there is a little radio static sound in the background of the note.

    This thing has the original Marshall branded tubes.

    I've heard that the tubes in these things don't really do much, but what sound issues can be caused by old tubes? They must be 30 years old.

    My signal path is (in stereo) JMP-1 > G-force > Hush Super C > Marshall 9100 > old 1960 cabs.

    I hear this sound with the G-Force Bypassed and the Hush bypassed. I hear it in both channels of the 9100 and in both speakers (panning the 9100 volumes left to right).

    To me, that means that the sound must be coming from the JMP-1.

    Also, haven't done this yet, but I will disconnect the Hush and the G-force and cable directly from the JMP-1 to the power amp. See if that makes a difference.

    Thanks for any help!
    Cheers,
    Mike
     
  2. Michael Roe

    Michael Roe Well-Known Member

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    If you have some other known good tubes laying around then give them a shot and see if you still hear the noise.
    It is a bit of a pain to get to those tubes but they do make a difference. One of the tubes works for the clean chs and the other works for the OD chs.
    You could also swap the existing tubes to the other position to see if that makes a difference.
     
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  3. Dogs of Doom

    Dogs of Doom Moderator Staff Member

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    be careful using the effects loop, that you do not overload the JMP-1 on the return. It will distort badly, as described.

    Make sure, you use dry kill on the unit & make sure you are not running too hot of a signal back in regardless...
     
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  4. MikeyV

    MikeyV New Member

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    Thanks for the replies, guys. I do have some known good tubes.

    Also, maybe I'll give it a good listen on both clean and dirty channels to see if I hear the same sound on both.

    As for the FX loop, I tried that first, and it didn't work very well. I switched it to having the FX all after the JMP-1.
    Maybe I can try it the other way and see if the sound is still there.

    But good call on being careful on the levels.
     
  5. MikeyV

    MikeyV New Member

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    Update: I've definitely isolated it to the JMP-1.

    Initially, I took the tuner out of the signal chain, then disconnected the G-Force so it was just JMP-1 > Marshall 9100 > 2x 1960 cabs. Sound still there.

    So just to be sure, I hooked the JMP-1 up to completely different rig.

    Different guitar, different cable, JMP-1 straight into an old Fender Showman into the showman cab and the sound is still there.

    I hear it on the distortion patches mostly, and I feel like any of those patches are dirtier/overly distorted than they're supposed to be.
    Like an overall muddy-ness. Along with the harsh decay/static noise on held high notes.

    Next step, a tube swap. I'll report back.

    Cheers!
     
  6. MikeyV

    MikeyV New Member

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    OK, swapped in some known good EH 12ax7s. Les Paul > cord > jmp-1 > M9100 > 1960 cab

    The sound is still present and unchanged from before. Present on both channels of the M9100.

    I guess the next thing would to maybe try to make a sound sample.

    The sound is kind of a pulsating harshness that cycles once per second or so, has a gritty pulsating sound that effects held high notes during the decay, the B string on the 12th fret for example.

    Happens with multiple guitars. I would say that I hear it mostly on the distortion patches. All of them. Not really on the clean channels, but I have heard some stuff on those too...muuuuuch less pronounced.
     
  7. MikeyV

    MikeyV New Member

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    Can an old battery cause things like this?
    The battery is soldered in (right? I think it's soldered, can't move it. ) Looks like it may never have been changed.
    I"m also going to try a factory reset.
    Cheers
    Mike
     
  8. Dogs of Doom

    Dogs of Doom Moderator Staff Member

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    some of those, the transformer goes bad & gets noisy...

    The battery wouldn't do that. The problem when the battery goes, is that it won't store programs, leaving the machine unprogrammable. Usually, the LED will blink.
     
  9. Michael Roe

    Michael Roe Well-Known Member

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    I would take it to a tech and have it checked out. Like @Dogs of Doom said, the transformers can go bad. I had to have mine replaced a few years ago and it wasn't that expensive to have done. Mine didn't make noise but it just lost power on the OD channels. The battery would probably only cause a loss of saving presets. Oh, and yes, the battery is soldered in. I had my tech change that to a removable battery when he installed the new transformer.
     
  10. MikeyV

    MikeyV New Member

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    Tired a few more things.

    Have old house - I tested the plugs with a tester, found the one I was using was wired wrong...found a good one.
    Sound was still there, although, it seemed like it took a few minutes to start hearing it. I thought it was gone, but then it started again. Seems so start once the thing gets warmed up.

    Anyhow. Sounds like shit. It's effecting the overall distorted sound making it muddy and flabby.

    I live in the Bay Area, CA. Does anyone know a tech around here that can work on this thing?
    I don't think Hal at Guitronix works on PCB stuff, otherwise I'd go straight there, in SF.
    I will ship it if I have to.

    Any recommendations would be appreciated.

    Thanks!
    Mike
     
  11. MikeyV

    MikeyV New Member

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    Could this be from old filter caps?
    I"m gonna try to make a sound clip.

    I feel helpless. I can build and repair (to a good degree) PTP tube amps, but this thing looks like a compute inside! Yikes!
     

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