Need help- Jcm 800 2210/4211

Discussion in 'The Workbench' started by Crikey, Nov 18, 2019.

  1. Crikey

    Crikey Well-Known Member

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    Ok, just put new preamp tubes. It was working fine before and after the preamp tube change. I didn't use dexoit but used a quick drying elect contact cleaner - sprayed on all preamp tubes inserted and removed a couple times.

    when I take amp off stand by I hear some static and then a hum starts coming from the speakers and its a hum that gets a little louder and stays steady. I can tell it's not a good sound.

    I cleaned speaker input cables and yet it's still doing it. I cleaned the footswtich cable and the plug. I cleaned the instrument cable and plug.

    Thoughts on what this could be? maybe a power tube going out?? I did not change the power amp tubes.
     
  2. ampmadscientist

    ampmadscientist Well-Known Member

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    Start the reactor... Free Mars!
    But was the hum there before you changed preamp tubes?
    You can swap the new ones out one at a time with an old one and see if one tube has more hum than the others.
     
  3. Crikey

    Crikey Well-Known Member

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    Will try that and report back.
     
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  4. South Park

    South Park Well-Known Member

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    Rotate the preamp tubes to isolate the tube with the noice.
     
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  5. NewReligion

    NewReligion Well-Known Member VIP Member

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    Plug your guitar straight into the return of the loop. If the hum is not there then you know it is the preamp section. If it is it is in the Poweramp section.

    Being a 100 watt version. Pull the 2 inside EL34’s. If you have a 16 ohm cab set amp impedance on 8, if 8 ohm cad set amp on 4. These are easy trouble shooting techniques. Roll all tubes.

    Use chopstick test. Use different guitar, cord, speaker cab, cord. Disconnect Reverb tank etc...

    Take notes.

    David
     
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  6. ampmadscientist

    ampmadscientist Well-Known Member

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    Start the reactor... Free Mars!
    I'll make a note of that.
     
  7. NewReligion

    NewReligion Well-Known Member VIP Member

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    Right. Lately it seems people ask for help then do not respond to the most simple logical troubleshooting techniques.

    D
     
  8. Crikey

    Crikey Well-Known Member

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    My apologies on late response. Will reply shortly on pugging into effect loop , then if swapping preamp tubes solves the humm issue
     
  9. Crikey

    Crikey Well-Known Member

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    Plugged into return. No hum. Sounds pretty cool like this. Plugged in footswitch and amp seemed to get a boost of volume.
    Unplugged reverb last nite but no effect on hum. Will try rolling the preamp tubes.
     
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  10. Crikey

    Crikey Well-Known Member

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    Last night also tried different cabs, cables and guitars. Did not stop the hum.
     
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  11. Crikey

    Crikey Well-Known Member

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    Seems to be v1 tube. Its microphonic. What tube should be in the v1 ,closest to input jack?
     
  12. Crikey

    Crikey Well-Known Member

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    Replaced tube 5, phase inverter and all is well. Thanks for the help and education!!!
    I mixed up the pi tube with others. Any good way to find out which one is the phase inverter?
     
  13. Crikey

    Crikey Well-Known Member

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    Well i think it was the V1 and V5 tubes. Right now running 12ax7 in V1 and V5 and this gainy monster is like a racehorse crashing into the starting gate... wanting to be turned loose. lawd it screams and has a little squeel to it if I don't play something. sounds like RAWK!
     
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  14. myersbw

    myersbw Well-Known Member

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    Awesome! You'll always find extra tubes are always handy! :) Unfortunately, I've found a few new ones bought...crap right out of the box. It happens.
     

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