Jmd1 Floating Bias Values

Discussion in 'The Workbench' started by Oscar Pauly, Jun 3, 2018.

  1. Oscar Pauly

    Oscar Pauly New Member

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    Hello all!
    I am re-biasing my JMD1 100W combo, but I am having some strange issues. I am installing new Mullard EL34 matched quad. Plate-cathode voltage is 475 Vdc. According to Marshall, the cathode current should be around 75 mA. However, for one set of tubes I am getting not more than 20 mA and for the other not less than 90 mA. I tweak the bias pots from one limit to another, but values are not changing much.

    I am measuring both 1 ohm cathode resistors terminals and bias test points, and I get the same results. I have a tube tester and the 4 tubes are matched and fully working.

    Any idea what could be going on?

    ----> Links to schematics in this thread: http://musiciansroadhouse.com/viewtopic.php?t=11599
     
  2. mickeydg5

    mickeydg5 Well-Known Member VIP Member

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    How did the old tubes bias up?
     
  3. Oscar Pauly

    Oscar Pauly New Member

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    Old tubes were failing, so I just threw them away and swapped new ones.
     
  4. mickeydg5

    mickeydg5 Well-Known Member VIP Member

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    If you are checking resistance on 1 ohm bias resistors they will have to be checked hot. Let them heat up a bit then turn the amplifier off and check.

    Continuity (looking for zero resistance) from chassis ground all the way to pins #8 on the power tubes and through CON6 pins #1 and #3 can be verified the same way sometimes. Heat can cause things to act up.

    Swap power tubes around to see if the issue sticks with that one side.

    Clean power tube pins and socket prongs with good electronics contact cleaner/lubricator.
     
  5. ampmadscientist

    ampmadscientist Well-Known Member

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    Is the amp plugged into a speaker?

    Really sounds like you have:
    A. Blown screen grid resistors.
    B. Blown 1 ohm resistors
    C. Bad volt meter / dead battery.
    D. Bad output transformer...
     
  6. Oscar Pauly

    Oscar Pauly New Member

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    How do you test for a bad transformer? I am getting 500 V plate to cathode, which I think is abnormal.
     
  7. Oscar Pauly

    Oscar Pauly New Member

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  8. Micky

    Micky Well-Known Member VIP Member

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    Have you performed any of the tests mentioned above?
     
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  9. Oscar Pauly

    Oscar Pauly New Member

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    Yes, I already have. 1 ohm resistors are OK. Continuity between ground and pins 8 are OK. Screen grid resistors seem to be fine, tested them with a multimeter. The multimeter is doing fine. The only one missing is to test the transformers, but I am not sure of the easiest way to do this. I am using brand new Mullard power tubes.

    Another thing I noticed is that one of the bias does not get to more than 30mV, while the other side is around 120mV. The other day smoke started to come out of the amp! I am not sure if one of the tubes is not being "read" by the amp.
     
  10. Micky

    Micky Well-Known Member VIP Member

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    Most importantly, have you swapped the tubes from one side to the other to see if the problem follows the tubes, or if it is localized to one side?
     
  11. Oscar Pauly

    Oscar Pauly New Member

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    Yes I have, and the problem still occurs in that same side. I mean, swapping the tubes does not change that the bias in one side does not go beyond 30mV and under 120mV in the other one. And a little smoke is still present.
     
  12. m1989jmp

    m1989jmp Active Member

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    Just to be clear, the bias test points are for inserting ammeter and measuring bias current directly?

    If you want to do it the right way, you should check all of the variables BEFORE inserting tubes and cooking them.

    What are the voltages on the pins 5 of all power tubes without tubes in; also voltages on the points Bias1 and Bias2? If the tubes are fine, you most likely have leaking coupling cap(s) coming off PI.

    At first you said 90mA, then later 120mA, which value is it?

    Do not plug tubes in without verifying everything is set up properly beforehand.
     
    Last edited: Jun 13, 2018 at 6:03 PM
  13. ampmadscientist

    ampmadscientist Well-Known Member

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    "And a little smoke is still present...."

    I think you should stop working on that and take it to a technician, before it's damaged more than it already is...
     
  14. mickeydg5

    mickeydg5 Well-Known Member VIP Member

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    And yet he does not indicate where the source of smoke might be located.

    Take it to a decent technician as previously stated.
     

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