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Help: JCM900-4501 issues

Discussion in 'The Workbench' started by JCM900_User, Sep 16, 2021.

  1. JCM900_User

    JCM900_User New Member

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    Hii.
    I'm new to the forum and need some help with my JCM900-4501.
    The Tubes were Red-plating and the bias Ran away when I bought the Amp.
    I fixed this issues by exchanging the Infamous C15 Cap in the bias circuit, the bias was way to cold when maxed out, so I lowered the value of R7.
    After that i could bias the pair of new el34s properly, but the max Amp Volume was still way to Low on both Channels, since the Amp was completely Stock I replaced all electrolytic caps on the Power- and Preamp board.
    Still the Amp is not loud enough, of course I checked the FX-loop with a Patch cable > Same result.
    The Voltage on the Powertubes seems resonable to me:
    3rd Pin: 450V; 4th: 450V; 5th: -32V (on both Tubes)

    Now i dont know what Else could be the Problem, do somebody have some Suggestions what to Check next?

    Thanks a lot in advance!
     
  2. Dogs of Doom

    Dogs of Doom >>> Moderator <<< Staff Member

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    Hi, :welcome:to the forum...

    I'm going to move this to workbench, as that's more of a troubleshooting forum...

    :cheers:
     
    Crikey likes this.
  3. johan.b

    johan.b Well-Known Member

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    Since bias network failed, it could be the o.t. but let's not start there...

    So, what does work?... you got light and can bias the power tubes.
    Do you get any hum/hiss through the speakers?
    Does the preamp tubes light up?
    If you plug into the fx return do you get any through the output?
    I assume you have a multi meter?
     
  4. JCM900_User

    JCM900_User New Member

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    Hi.
    Yes, all Tubes Light Up and are working.
    No suspicious Hum or Hiss. If plugging only in the FX Return i can barely hear my guitar, but its there.
    Besides the Low Volume, the Reverb doesnt Work (Not that Importamt to me) and the Bass Control is Not working (I Think i Just Need to replace the Pot).
    The Amp is loud enough to Bring my Tele Into Feedback but it would Not be enough when playing with a Drummer (my 22W Bugera Amp is louder than the 4501 at the moment).
    Yes I have a Multimeter.
     
  5. JCM900_User

    JCM900_User New Member

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    Thx!
     
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  6. johan.b

    johan.b Well-Known Member

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    So, you get sound loud enough to feed back but not enough to hang with drums.... can it get clean or is it always dirty?
    You don't happen to be in "triode" /half power mode, do you ( stupid q, perhaps, just have to ask) and are the screen grid resistor OK?
     
  7. tschrama

    tschrama Well-Known Member

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    try to isolate the problem to either power amp or preamp…Can you measure the Vac on the grid with the volume maxed out?

    Or ise another source, plugged into the FX return, maybe a cd player?

    -32V is pretty low to get propper bias (35mA?). Are the tubes warn out?
     
  8. johan.b

    johan.b Well-Known Member

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    Good point.... it's this the same tubes that redplated or a fresh set?
     
  9. JCM900_User

    JCM900_User New Member

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    Its a new Set of Power Tubes.
    I will check tomorrow the resistors and the voltages again.

    To be honest, I dont hear any difference If the Power selector is in High or Low mode...
     
  10. KraftyBob

    KraftyBob Well-Known Member Gold Supporting Member

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    That's an interesting observation. Do you know if the power switch is actually working and switching from low to high?
     
  11. JCM900_User

    JCM900_User New Member

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    I wanted to Take some measurements,
    after switching the Amp on, there was a short "noise" and now nothing works anymore.
    Something Must be shorted I think, None of the Tubes Light up anymore.
    Both fuses are ok, could it that a Transformer is Shot now?
     
  12. JCM900_User

    JCM900_User New Member

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    Problem fixed, hat to resolder the blue Lead from the Power Switch to the Transformer, Tubes Light Up again...
     
  13. KraftyBob

    KraftyBob Well-Known Member Gold Supporting Member

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    Do you still have the original problem of low volume, or was that related to the transformer wire bad connection?
     
  14. JCM900_User

    JCM900_User New Member

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    It was related to the Wire of the Transformer.

    I checked the Power selector on the Back, it seems to Switch correctly (checked with Multimeter) but i cant hear a difference between Hi / Lo setting. But the impedance Switch seems fishy (all 6 Points have a Connection), I will throw Out the selector and hardwire it to the 8 Ohm setting (maybe this is related to the Low Volume...).
     
  15. 66 Kicks

    66 Kicks Member

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    The Points seem to be connected, but that is because the secondary winding looks like a straight piece of wire to a multi-meter.
     
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  16. cozmacozmy

    cozmacozmy Active Member

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    Lot of JCM900 problems lately... must be that time of year.
     
  17. johan.b

    johan.b Well-Known Member

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    Yes, we had "resurrect a 900 week" a while back... did you miss the advertising?... :fever:
     
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  18. cozmacozmy

    cozmacozmy Active Member

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  19. JCM900_User

    JCM900_User New Member

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    Sry Guys for Not replying, the Last Couple of weeks were very busy.
    I Hope to continuing the Investigation since I really want that Amp to Work properly...
     
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