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Mesa Boogie Mark 4 (B) bias issue

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  • Mesa Boogie Mark 4 (B) bias issue

    Good morning to you

    Ok This i think is more about understanding this and whats going on

    Mesa boogie mark 4 (b) Looking at the diagram for both A and B versions mine is closer to the B than the A
    It came in for bad tubes and hum
    One checking the bias voltage on the 6L6GC tube bases without tubes , the bias voltage was missing.
    Traced it to 2 x open circuit 220k. Could also see that the 2 x 10,000uf 16v were leaking, Replaced all items
    Tested without tubes, I had bias voltage on the tube bases , but its lower on V8 - V9
    I can see by the circuit (-53V) supplys R322-321 then to V7-V6 but the supply to V8-V9 goes through
    another 220K for each V8 - V9


    On this amp there are a couple of differences the 220k for V8-V9 is 150K and there is No 2.2k This is all original with no sign of any changes since manufacture. I have changed the bias caps as a matter of course

    Amp is set for 240V line voltage
    Bias voltage @ -53v
    HT @ 430v
    so what i have on the tube bases is V8-V9 is has about 44v and V6-V7 has about -47v ..without tubes
    I have new tubes to fit , but at this point, im using a test set of 6L6's and the amp powers up and sounds ok,,

    Without checking it im guessing that the bias will be higher on 2 tubes than the other 2 ??

    Looking at other amp circuits most have 220k supplying all tubes either in pairs or quads
    So why is this one different and for what reason or do i have an issue ???

    many thanks
    Mr A
    Attached Files
    Last edited by Arbutt; 10-09-2020, 09:56 AM.

  • #2
    There are countless little variations on those amps. You should find both pairs of 6L6s working differently.
    The basic difference is established by the two 2M2 resistors between the grids and ground. That pair usually conducts 8-10 mA more than the other at idle.


    • #3
      Thanks, so its ok then is it.. I just wanted to check..

      Thanks I have learnt something today
      Mr A


      • #4
        Simul class amps will have different bias voltages on the pairs.They were designed to use 2x 6L6GC and 2x EL34


        • #5
          ...and there is No 2.2k
          Don't forget to install those grid stoppers as recommended on the last page.