jcm800 2205 hum

Discussion in 'The Workbench' started by louied, Jan 25, 2020.

  1. louied

    louied Member

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    My '84 2205 has a constant 120hz hum, regardless of channel 1/2 or knob settings. Otherwise, it sounds great. None of my other amps have this hum on my house power, just this amp. I've swapped tubes and I don't think they are the problem. Any ideas? Or should I just take it to a tech?
     
  2. ampmadscientist

    ampmadscientist Well-Known Member

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    OK take out the phase inverter does the hum stop?
    Do you know which tube is the PI?
    Do you have a volt meter?
     
  3. louied

    louied Member

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    Is the PI in v4?
     
  4. louied

    louied Member

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    OK, PI is v5. I pulled the tube and fire up the amp. The noise is gone. Yes, I have a volt meter.
     
  5. ampmadscientist

    ampmadscientist Well-Known Member

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    Start the reactor... Free Mars!
    Put V5 back in.
    Take out V1 V2 V3 V4.
    Is the noise still there?
     
  6. louied

    louied Member

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    I pulled them individually.

    v1: noise still there
    v2: no noise
    v3: no noise
    v4: noise still there
    v5: no noise
     
  7. ampmadscientist

    ampmadscientist Well-Known Member

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    I'm not getting what I need.
    Pull all the tubes V1 V2 V3 V4 except the PI V5 and tell me if the noise is gone. Yes or no?
     
  8. Gunner64

    Gunner64 Well-Known Member Premium Member

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    And remember v4 is reverb in this one. (Not trying to get in the way and I'll shut up after this post :)) is the reverb on or off during this humming?
     
  9. ampmadscientist

    ampmadscientist Well-Known Member

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    You made a good point.
    I'm just trying to narrow it down.
    It can be filament, it can be PI, it can be reverb yes...definitely could be the reverb malfunction.
     
  10. ampmadscientist

    ampmadscientist Well-Known Member

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    Start the reactor... Free Mars!
    OK so you know the hum doesn't originate from the power tubes...
    Now step 2
    put the PI back in, and pull the other V1 V2 V3 V4 preamp tubes.
    Now test again.
     
  11. louied

    louied Member

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    I pulled v1 v2 v3 v4 and left v5 in. No noise.

    With all preamp tubes in, the reverb knob does not change the hum. It's constant on both normal and boost channels with reverb set to 0 or 10.
     
  12. ampmadscientist

    ampmadscientist Well-Known Member

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    Start the reactor... Free Mars!
    Now install V5, V4 and leave V1 V2 V3 out.
    Re-test and see if the hum comes back.
     
  13. MickeyJ

    MickeyJ Well-Known Member

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    if you've both swapped pre-amp tubes, and used the isolation method of finding the noise ( pulling tubes to see which stage the noise is coming from ), and,
    if v4 is the reverb return/ send and return , then, it looks like this is a job for the tech as it appears the problem is associated with v4 as the hum starts when pulling that tube working from the power amp backwards.
     
  14. louied

    louied Member

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    With v1 v2 v3 out, no noise.
     
  15. ampmadscientist

    ampmadscientist Well-Known Member

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    OK
    Now V5, V4, V3 in, and V1 V2 removed.
    Did the hum come back?
     
  16. ampmadscientist

    ampmadscientist Well-Known Member

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    Wow, did you get this: Kobe Bryant killed in helicopter crash this very morning.
     
  17. louied

    louied Member

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    With v1 v2 out, the hum is back.
     
  18. louied

    louied Member

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    Tragic news about Kobe
     
  19. ampmadscientist

    ampmadscientist Well-Known Member

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    V1, V2 out.
    So if you pull V3, the hum goes away.
    And if you install V3, the hum comes back?
    Verify please. Just double check....
    I'm looking at the schematic now...gimme a minute it's early :drunk: time for warm ups.
     
  20. ampmadscientist

    ampmadscientist Well-Known Member

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    I'm shocked and saddened of this news. He was a hero to many.
     

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