Marshall JCM900 bias

Discussion in 'The Workbench' started by Carlos Rajoo, Dec 3, 2019.

  1. Carlos Rajoo

    Carlos Rajoo New Member

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    Hi


    I have this JCM900 Model 4100 which is originally fitted with 5881 tubes. A friend bought 4 new 6l6s and asked me to replace them. After fitting them, when I turn on the amp, there is some low hum from the amp. The bias was at -50V, when I set the bias to minimum (-63V), the hum has reduced very much but it is still there. How do I set the bias. I heard that some resistors value need to be change. If so, which are the resistors and what are the values. Please assist.


    Thanks
     
  2. Dogs of Doom

    Dogs of Doom Moderator Staff Member

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    you should be able to bias it w/ the new tubes. The hum is most likely due to bias being out of balance...
     
  3. mickeydg5

    mickeydg5 Well-Known Member VIP Member

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    Were they a matched quad?
    You need to measure the idle bias current of each and compare values.
    As Dogs mentioned the two sides of the push-pull power amplifier should be balanced within a decent margin (few milliamps).
     
  4. Carlos Rajoo

    Carlos Rajoo New Member

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    No, they are not matched tubes. I do not have a bias probe, how do I measure the idle bias current for each tube. If I go from -50V to -63V, am I increasing the bias current or decreasing. I am taking the bias voltage from ground to the intersetion of R26 and R27. Is this correct.

    Thanks
     
  5. South Park

    South Park Well-Known Member

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    Put the old tubes back in. The right ones . You could damage the transformers
     
  6. mickeydg5

    mickeydg5 Well-Known Member VIP Member

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    You either need to get a bias probe or get your friend to take it to an amplifier technician.
    Why is your friend not doing this exercise?

    The negative voltage is the bias voltage only. It only sets the idle bias current of the power tubes and -50VDC is a nominal value in this amplifier designed for 6L6GC/5881.
    Between R26 and R27 is the correct location to take the negative voltage measurement to compare to Marshall's schematic.

    -63VDC would be a colder bias allowing less idle current flow.
     
  7. mickeydg5

    mickeydg5 Well-Known Member VIP Member

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    What do you mean by the right ones?
     
  8. Carlos Rajoo

    Carlos Rajoo New Member

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    Thanks, I just want to know, now by reducing the bias from -50V to -63V, the hum has gone down very much. -63V is the minimum I can set on the trimpot, which is 47K. If I change the value of this trimpot, so that I can bring down the bias below -63V, can I eliminate the hum totally.
     
  9. South Park

    South Park Well-Known Member

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    Did you mod the amp for the new tubes . The bias is the mili amps on the grids .
     
  10. mickeydg5

    mickeydg5 Well-Known Member VIP Member

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    He is measuring -50 and -63 VDC on the bias supply. I believe it is safe to say this amplifier is setup for 6L6GC/5881.
     
  11. mickeydg5

    mickeydg5 Well-Known Member VIP Member

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    No. The hum is due to unmatched tubes and unbalanced push-pull power amplifier.
    Producing and using a bias supply that is too negative will bring the power tubes operating bias point too low causing crossover distortion due them turning off a larger portion of the time which may be why you are hearing less hum in that state. That is definitely not good.
     

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