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Thread: Biasing pots and where/how to measure

  1. #11
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  2. #12
    Join Date: Apr 2015

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    I'm martin.

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    Finally got round to measuring plate voltage and the bias - it seems to have been waaaaaay off!
    I just hope that the resistor is indeed 10ohms! The amp is AB.

    Plate voltage 470V
    I re-biased to 31.6mv

    The values were:

    1 was 41mv
    2 was 45mv
    3 was 44mv
    4 was 35mv
    Last edited by martinswimmer; 13-03-2018 at 15:00.

  3. #13
    Join Date: Oct 2016

    Location: Bolton, England

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    Quote Originally Posted by martinswimmer View Post
    I just hope that the resistor is indeed 10ohms! The amp is AB.

    Plate voltage 470V
    I re-biased to 31.6mv
    31.6mV across a 10 ohm resistor means there is 3.16mA flowing through it. I have no idea what the circuit is but 3mA quiescent current sounds a bit lower that what is typical.

    This bit makes no sense to me:
    Quote Originally Posted by martinswimmer View Post
    The values were:

    1 plate voltage: was 41mv
    2 plate voltage: was 45mv
    3 was 44mv
    4 was 35mv
    41mV plate voltage??

  4. #14
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    Quote Originally Posted by RothwellAudio View Post
    31.6mV across a 10 ohm resistor means there is 3.16mA flowing through it. I have no idea what the circuit is but 3mA quiescent current sounds a bit lower that what is typical.

    This bit makes no sense to me:

    41mV plate voltage??
    No, I took off the plate voltage because it was 470V and forgot to delete "plate voltage" bit.
    CORRECTED!

    I used a calculator for biasing - so picked 3.16mA - would you say that's too low?

  5. #15
    Join Date: Oct 2016

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    Quote Originally Posted by CornishPasty View Post
    I used to have one of Lim's original AVC EL34 amps and the output sections looks pretty much like the OP's Diva. Bias was stated as 39mA but I used to set mine at 36mA. If I can find my original circuit diagram I may be able to confirm the anode voltage.
    Yes, round about 30mA sounds like a typical value. 3mA? Sounds very low.
    I suspect the OP is measuring something else entirely, though I could be wrong. Without a circuit diagram, or some brave soul with a lot of spare time to trace out the circuit, there's too much guesswork here.

  6. #16
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    Quote Originally Posted by RothwellAudio View Post
    Yes, round about 30mA sounds like a typical value. 3mA? Sounds very low.
    I suspect the OP is measuring something else entirely, though I could be wrong. Without a circuit diagram, or some brave soul with a lot of spare time to trace out the circuit, there's too much guesswork here.
    I divided the value by 10 - so 316mv

  7. #17
    Join Date: Oct 2016

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    Quote Originally Posted by martinswimmer View Post
    I used a calculator for biasing - so picked 3.16mA
    I'm not sure what that means.
    Quote Originally Posted by martinswimmer View Post
    I divided the value by 10 - so 316mv
    I'm not sure what that means either.

    What resistor are you measuring the voltage across and what is it doing? Are those voltages measured with respect to 0V or are you simply measuring across the resistor?

  8. #18
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    Common ground.

  9. #19
    Join Date: Oct 2016

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    Quote Originally Posted by martinswimmer View Post
    I divided the value by 10 - so 316mv
    31.6 / 10 = 316

  10. #20
    Join Date: Apr 2015

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    Quote Originally Posted by RothwellAudio View Post
    31.6 / 10 = 316
    316 / 10 = 31.6

    In any case, sounds much better now.

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