Jtm45 Crackle ?

Discussion in 'The Workbench' started by PG000, Jan 14, 2017.

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  1. PG000

    PG000 Member

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    I got a JTM45 and it had no power tubes so I put KT66 in it. I tried it for several minutes and realized when I turn the amp volume up to 10 and turn the volume of the guitar down the amp begins to crackle. I tried several guitars but it was the same with all of them. The crackling happens at both channels. Does anyone have an idea how to fix this ?

    PG000
     
  2. ampmadscientist

    ampmadscientist Well-Known Member

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    Change the preamp tube V1, change the plate resistors.
    Another thing is the input jacks have switches that can corrode. Replace or clean the input jacks.
     
  3. PG000

    PG000 Member

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    Thank you for your fast reply but unfortunately it didn´t help realy. I replaced the plate resistors of V1 and cleaned the input jacks but it didn´t help. I realized it isn´t exactly ckracklin its more like bleeding.

    PG000
     
  4. neikeel

    neikeel Well-Known Member

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    Can you explain bleeding? What era is the amp? Turret board or pcb? Is it original, modded?
     
  5. ampmadscientist

    ampmadscientist Well-Known Member

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    What is "bleeding?" Post a recording of it.

    Oh I think I get it now: the bias is too cold, and the output tubes are shutting down.
    This causes a fizzing kind of sound when the note is lower volume / decaying...
    sounds like it.

    Are you able to get a hold of a bias probe and measure it?

    Or effects loop jacks are corroded....connect guitar cable between send and return jacks, then try it again. Did the noise stop?
     
    Last edited: Jan 16, 2017
  6. PG000

    PG000 Member

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    The amp is an JTM45 with pcb from 1989. It´s not modded.

    Thank you ampmadscientist for the tip that it could be the bias. And you were right, I mesured the bias again and it was way to low. So I tried to find out what the problem is, because the bias pot was turned to max. So I mesured and finaly found out that the power transformer of this amp has two 300V secondary hv coils insted of two 350V hv coils becaus this amp normaly has two 6L6GC wich use less power. So now I need to try a 220k bias resistor instead of the 180k bias resistor (to get a higher bias).

    The amp has no effects loop.

    PG000
     
  7. PG000

    PG000 Member

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    I solved the problem by mesurinbg the screen voltage and finding out that the screen voltage is only 350V so I calculated the bias and the bias schold be about 46ma at 350v screen voltage. I now tested the amp and now one of the screen resistors blew up. Does someone have any idea what the problem was ?

    PG000
     
  8. Len

    Len Well-Known Member

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    In my experience crackling was due to bad tubes, dirty sockets, or cold solder joints at one of the sockets.
     
  9. PG000

    PG000 Member

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    The problem of the crackling was solved by adjusting the bias to 46ma. It Schlund be 46ma because the screen voltage is 350v and plate dissipation is 25W. But then I tried the amp(at full volume) and one of the screen resistors blew up. Now I don't know why that happened !
     
  10. ampmadscientist

    ampmadscientist Well-Known Member

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    Start the reactor... Free Mars!
    The screen resistor (nearly always) blows because of a bad tube.
    It was either the tubes you replaced were bad or the tubes you have now are bad.

    Replace the screen resistors and re-adjust bias.
    Then see if the resistor blows again. If it blows it could be a bad tube or a bad socket.

    Another 2 things is: there is a bad tube socket (burned arcing) or the speaker impedance is wrong.

    Once both resistors work, the tubes should get equally hot, the plate voltage will drop, both tubes will draw about the same current.
    Does it still crackle?
    That's another clue that an output tube socket may be arcing.

    Inspect the sockets very carefully for any sign of burning, with a magnifying glass.

    Keep working on it and don't give up.
     
  11. mickeydg5

    mickeydg5 Well-Known Member VIP Member

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    How are you measuring and calculating idle bias settings?
     
  12. ampmadscientist

    ampmadscientist Well-Known Member

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    Start the reactor... Free Mars!
    with a scope...
     
  13. PG000

    PG000 Member

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    I am measuring the bias with a DMM and a 1Ohm resistor from pin1 and 8 to ground.

    I changed both resistors and I used 5W instead of 1W, re-adjusted the bias and the same resistor blew again.


    I inspected the tube sockets but I didn't see anything. I need to try other tubes.

    The speaker impedance was set correctly but the output transformer was replaced at some time, so maybe it could be the output transformer !

    PG000
     
  14. mickeydg5

    mickeydg5 Well-Known Member VIP Member

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    Ampmad, I doubt he has a scope. :agreed:

    Well you mentioned a screen resistor blowing. Those are stock at 470 ohm 5 watt.

    So you have installed 1 ohm 5 watt cathode bias monitoring resistors. You mentioned they were 1 watt prior.

    So did you blow a 5 watt screen resistor or 5 watt bias monitoring resistor?

    Was this a brand new set of KT66?
    Was it a known good and working set of KT66?

    The 350VDC on the screens seems a bit low. Is that at both screen lugs?
    What do you measure at the plate lugs?
     
  15. PG000

    PG000 Member

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    I think the screen resitor on a JTM45 is 470 ohm 1 watt stock.

    The bias monitoring resistor is only 0.5 watt and I blew one of the screen resistor two times. I now know that the screen resistor blew because the KT66 had a short in the socket, although it was brand new.

    I know that the voltage is a bit low with 350VDC(normaly about 450VDC, I think) but thats because the amp had 6V6 stock and they need less voltage.

    The 350VDC is at both screen lugs.

    Some voltage messurements:

    Plate(pin3): 365VDC
    Grid(pin5): 376VDC
    it is about the same at the 2nd tube.

    PG000
     
  16. PG000

    PG000 Member

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  17. ampmadscientist

    ampmadscientist Well-Known Member

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    Start the reactor... Free Mars!
    Take the tubes out
    1. Turn the power on, leave standby off.

    2. What is the voltage at pin 5 of each output tube socket? should be negative voltage about maybe -40VDC

    3. Turn standby on.

    4. What is the voltage at pin 5 of each output tube socket? should still be negative voltage about the same as #2

    If there is 376 VDC on pin 5 w/ no tubes in, it is definitely messed up.

    "Some voltage messurements:

    Plate(pin3): 365VDC
    Grid(pin5): 376VDC
    it is about the same at the 2nd tube."

    If the PI plate coupling cap is bad, then 376 VDC can show up on pin 5 of the output tubes.

    Pin 5 of output tube should always have minus voltage (bias voltage). About -40VDC... You cannot operate the amp till that minus voltage is corrected.

    Buzzing:
    another possible thing is that bias diode has shorted.
    Bias filter cap may have gone bad, resistor in bias circuit may have failed.

    But whatever, seems like bias power supply has failed, and high voltage on output tube grid pin 5.

    Change 2X PI plate coupling caps.
    Change bias rectifier diode (you could use 1N4007...etc...)
    Change bias filter cap(s)
    Make sure all resistors in bias are still good.

    Then verify that pin 5 of output tube(s) remains negative voltage.
     
    Last edited: Feb 8, 2017
  18. PG000

    PG000 Member

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    I messured everything again

    Socket 1:
    Stby off: Pin5: -34,2VDC
    Stby on: Pin5: -34,25VDC

    Socket 2:
    Stby off: Pin5: -34,3VDC
    Stby on: Pin5: -34,33VDC

    I think I messed something up with the last messurements. Maybee I put the probe at the wrong socket lead.

    PG000
     
  19. PG000

    PG000 Member

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    Schuld I turn the bias voltage to -40VDC ?

    PG000
     

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