Jcm 900 Bias Issue

Discussion in 'The Workbench' started by Tim Parsons, Nov 28, 2017.

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  1. Tim Parsons

    Tim Parsons New Member

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    Hi all

    I have a 100 watt JCM 900 Dual Reverb head with a serious bias issue. Power tubes (6L6s) in V1 and V2 were red plating. Done all the usual tube swaps to rule them out.

    I took a measurement on pin 5 of each socket with the tubes removed. Valves 3 and 4 were a perfect -30 and -50 with the bias turned to either extreme. Valves 1 and 2 were unreadable. The multi meter would not keep still long enough to take any sort of reading. Furthermore, when I turned the power off after each reading the meter steadily droped to zero on 3 and 4 but kept going mental on 1 and 2 for a long time after the IEC had been pulled.

    Just to clarify....by 'valve 1' I'm referring to the one closest to the pre amp valves with 2, 3 and 4 following in a line.


    Ideas??
     
    Last edited: Nov 28, 2017
  2. ampmadscientist

    ampmadscientist Well-Known Member

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    Turn the power ON.
    Turn the standby to OFF.
    DO not turn the high voltage ON.
    Filaments are ON, but standby is OFF.

    You can troubleshoot with the standby OFF until you solve the problem. This will prevent the tubes from red plating while you measure the voltages.

    Trace back from pin 5 to the bias split resistors. There are 2X 220K resistors for EL34. They form a "V."

    (these 2 resistors may be 150K for 6550 or other tubes...)

    The common point of the 2 resistors is the bias voltage on one side.
    Then you have the split of the V. One side of the split goes to power tubes 3,4. (working)
    The other side of the V goes to power tubes 1,2 (mental).

    Now you need to decide resistor it self is bad, or if the coupling capacitor is leaking high voltage from the Phase Inverter.
    (high voltage only turns on with standby ON)
    OR if the circuit path is broken, which goes to pin 5.

    Do you read bias voltage on BOTH sides of the 2 bias split resistors?
     
    Last edited: Nov 28, 2017
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  3. mickeydg5

    mickeydg5 Well-Known Member

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    It would be a good idea to pull the power tubes, turn the amplifier ON and take it out of STANDBY so that the pin #5's can be checked for voltage DC.
     
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  4. Tim Parsons

    Tim Parsons New Member

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    LEGEND!!

    R27 was the culprit. Replaced and now working as good as gold. Thank you very much.
     
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  5. Fender

    Fender Active Member

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    awesome diagnostic from afar ! :D
     
  6. ampmadscientist

    ampmadscientist Well-Known Member

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    If R27 is blown then, one of the output tubes could be bad OR one of the coupling caps is shorting...
    don't forget that the resistor won't blow by itself....(rarely)
    something is making that resistor draw excessive current.
     
    Last edited: Nov 29, 2017

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