How to measure current draw on a cathode biased amp?

Discussion in 'Amps/Cabs Tech Corner: Amplifier, Cab & Speakers' started by burner, Jan 20, 2008.


  1. burner

    burner Member

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    I have a 5E3 and I'm wondering how I measure the tubes current draw for biasing since I presume there is no 1 ohm resistor to ground.
    This is the first cathode biased amplifier I've owned.
    Do I measue the voltage across the cathode resistor and divide in half?
    thanks
     
  2. hasserl

    hasserl Member

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    1. Measure the voltage drop across the resistor and divide by the value of the resistor. The result is the cathode current of both tubes (not the same as the plate current, but we can work with it).
    2. Measure the voltage between the plate and the cathode.
    3. Multiply the cathode current by the Plate to Cathode voltage. The result is the total watts for both tubes.
    4. Divide by 2 to get the total watts per tube.
     
  3. burner

    burner Member

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    Great!

    Question though:
    Step 2:
    I put one lead on the plate pin and one on the cathode pin?

    Also to build an adjustable bias, all I need to do is substitue a pot for the Cathode resistor...correct?

    Wait...I don't want the total WATTS do I?
    I just want the total current in milliamps.
    So I would only do step one?
     
  4. Blue Strat

    Blue Strat Member

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    In reality, you DO want to know idle power (watts) per tube to be sure that you're not running above 12 watts (the maximum rating, which is often violated in 5E3s, for 6V6s). Idle current alone doesn't tell you very much.

    No, you cannot install a pot where the cathode resistor is because of the power requirement for that component (10 watts or so). There is a way to do what you want. I'll see if I can find a schematic.
     
  5. burner

    burner Member

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    Thanks

    Yes please do because I'd be interested in adding an adjustable bias.

    BTW, I am running 6L6s in this circuit with a 25watt OT
    So, that would change the wattage requirements.

    I should be able to use 6V6 or 6L6 in the same circuit with very little adjustment
     
  6. Blue Strat

    Blue Strat Member

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    Be sure that the power transformer is able to supply double the heater current for 6L6s. Standard 5E3 power transformers are unable to do this safely.
     
  7. Swarty

    Swarty Member

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    Actually EL34/6V6 are very close (typically swapable)(EL34s require much more heater current than 6L6s). 6L6s will require a larger value resistor.

    My suggestion is a rotary switch with various cathode resistor values.
     
  8. burner

    burner Member

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    Yes it has a 25watt Hammond OT and will run with either 6V6 or 6L6
    (Li'l Dawg D'luxe)
    I guess I should have specified but I was mainly interested in knowing how to measure how many milliamps the tubes are drawing so that I can alter it to suit if need be.
     

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