High Pitch Squeal On 1989 Marshall Jcm 800 2205

Discussion in 'The Workbench' started by jalexquijano, Feb 9, 2019.

  1. jalexquijano

    jalexquijano Member

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    Recently plugged my les paul traditional into my second hand JCM 800 2205 made in 1989 and seem to be getting a high pitch squeal whenever i set the gain control over 8. Is this normal? If not, how can i fix this problem?
     
  2. RickyLee

    RickyLee Well-Known Member VIP Member

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    This one sounds like a microphonic 12AX7 in the first position. Have you tried replacing the 12AX7's, mainly V1? Also clean and re-tension the sockets as well. Make sure amp is OFF and there is no residual voltage on the socket pins 1 & 6.
     
  3. jalexquijano

    jalexquijano Member

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    Can you recommend a high gain ecc83 preamp tube brand and model for v1?
     
  4. RickyLee

    RickyLee Well-Known Member VIP Member

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    Hmmm. I am not too savvy on the high gain tubes these days. I used to hear that JJ offered the highest gain nut i think there is a Ruby branded that is a China made with high gain. I have one in one of my amps and the model is not coming to me. Ruby 12AX7C H4?

    Maybe someone else will chime in. But I like the Chinese made myself for new production stuff. They seem to be the most rugged and produce good midrange detail.

    You don't have any extra 12AX7's to swap into the amp? Try swapping V1 V2 around first. V2 is for the Normal/Clean channel only.
     
  5. jalexquijano

    jalexquijano Member

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    Will tightening or replacing the preamp tube socket eliminate this high pitch squeal produced from 8 to 10?
     
  6. charveldan

    charveldan Well-Known Member

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    All 12AX7's have a gain factor of 100.
    There is no such thing as "high gain".
     
  7. jalexquijano

    jalexquijano Member

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    then why am i having a high pitch squeal when dialing the gain over 8?? I have replaced V1 with 3 different JJ ECC83s and still the same squeal.
     
  8. Chris-in-LA

    Chris-in-LA Active Member

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    It could be the pups on the guitar. Does the amp do the same thing with other guitars?
     
  9. charveldan

    charveldan Well-Known Member

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    Replace all the preamp tubes one at a time with a known good tube. If that doesn't work its time to put it on the bench.
    That amp is 30 years old. If nobody did proper maintenance over the years this is why you're having problems.
    When was the last time you had new power tubes & a bias ?
    It could be many things, hard to diagnose from internet-land.
     
  10. ampmadscientist

    ampmadscientist Well-Known Member

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    It's oscillation in the preamp.
     
  11. ampmadscientist

    ampmadscientist Well-Known Member

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    Which volume are you turning to 8, to produce the oscillation?
    The clean volume or the boost volume?

    Do you have a footswitch for the amp?

    The foot switch jack can be oxidized, and then the clean and boost channels will not be switched correctly.

    The boost tube causing oscillation could be V2 or V3.
    So swap those out and re-test.
    Plug in the foot switch and re-test.

    Plug a guitar cable from effects send to effects return, and re-test.
     
    Last edited: Feb 11, 2019
  12. RickyLee

    RickyLee Well-Known Member VIP Member

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    As Ampmad suggested, try swapping V2 and V3 as well. But if the problem is coming from the Boost channel, V2 should have nothing to do with it as that tube is for the Normal channel only unless . . . . you are having some strange channel bleed issue.

    Also, these amps can tend to pick up noise and interference big time due to the design at the input and going into the first stage. My '88 2205 used to pick up a local radio station LOUDLY when I played it at a certain local bar. I ended up having to mod it with a grid resistor after the input. A shielded input wire might help, but then I can not remember if these amps came with one from the factory?

    Running the gain that high might not jive with certain guitar pickups and this amp as well. You could also try moving the signal wires around inside the amp. But if you have no experience working around live high voltages, it would be best to have a tech check it out.
     
  13. ampmadscientist

    ampmadscientist Well-Known Member

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    First I suspect corroded jacks, because the amp is old.
    The jacks have switches, and when the jack is oxidized, the switches won't operate.
    There is a foot switch jack, and a effects loop jack, both jacks depend on the switch being operational.
    If the switch in the jack corrodes, it could screw everything up.
     
  14. ampmadscientist

    ampmadscientist Well-Known Member

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  15. ampmadscientist

    ampmadscientist Well-Known Member

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    Not really the tube, but the circuit layout itself.
     
  16. ampmadscientist

    ampmadscientist Well-Known Member

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    Do you have the amp inside the cabinet?
    Or is the amp out of the shell?
    Because there is shielding inside the cabinet, or should be.
     

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