Help biasing a jubilee

Discussion in 'The Workbench' started by marco_giampa23, Mar 26, 2020 at 5:31 PM.

  1. marco_giampa23

    marco_giampa23 Active Member

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    so just got my new jubilee, its my first tube amp in a while and didnt do any maintenance on my past amps as I resold them quickly.
    can someone also help me figure out which pin is which, I only have this grainy photo I took when I pulled the chassis yesterday. Are pins 1 and 8 the two that seem to be tied togethor to ground on the left socket?

    I found this information below on some old forum so wondering if this is any good

    IMG_20200326_162358 (2).jpg
    1) If not using a bias probe, you need to have a 1 ohm resistor on each power tube socket (one end of resistor to ground; the other end tied to pins 1 & 8 ) Use a 1% tolerance resistor.

    2) Turn power on and let tubes warm up. Switch standby to "on".

    3) *Be very careful on this step (high voltages)*
    Set your meter to the highest DC volts (600 or higher) and put the common (ground) lead in one of the chassis mounting nuts or alligator clip it. With one hand behind your back for safety, touch the positive lead to pin 3 (plate) of one of the power tubes. This is your plate or "B+" voltage. Write it down.

    4) EL34's are 25 watts max and you want to set the bias to be between 60% and 70% maximum dissipation watts. (60% = 15 watts; 70% = 17.5 watts) Divide your desired wattage by your plate voltage from step 3. (example: 15/450 = .033 or 33mV) This is your desired bias voltage (probably between 30 and 40 mV).

    5) Set your meter to lowest DCV setting (200mV or less) and touch the ground lead to the ground side of the 1 ohm resistor and the positive lead to the pin 1 side. Do this on all power tubes and adjust the tube with the highest value to your desired bias voltage from step 4.

    Plate voltage will change a little when adjusting the bias so you may need to take another reading from pin 3 and adjust your equation accordingly and repeat steps 4 & 5.
     
  2. South Park

    South Park Well-Known Member

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    The plate voltage does not change with the bias. You want to take the amp reading on the grids . It controls plate current . You can pull the center tap on the OT and read the amps of both power tubes and divide by two . Hope that helps
     
  3. Chris-in-LA

    Chris-in-LA Well-Known Member

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    Yes, the two pins tied together are also marked 1 and 8.
     
  4. South Park

    South Park Well-Known Member

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    One of the pins is ground the ether is a control screen when you make it negative of the plate you get more power out of it . I hope that confuses you
     
  5. Lance Chambers

    Lance Chambers Well-Known Member

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  6. CROWEMAG

    CROWEMAG Active Member

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    They are numbered.
     

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