Biasing voltage drop. Check my numbers

Discussion in 'The Workbench' started by Joshabr1, Oct 23, 2021.

  1. Joshabr1

    Joshabr1 Well-Known Member

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    Plate voltage 481

    v4/5 17.2ohms //// .520 voltage drop

    v6/7 15.5ohms//// .550 voltage drop

    so 17.2/.520 and 15.5/.550
     
  2. Joshabr1

    Joshabr1 Well-Known Member

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    So I get v4/5 = 33.07

    v6/7=28.18
     
  3. Joshabr1

    Joshabr1 Well-Known Member

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    If my math is correct I’m happy with these numbers. Right? Been a while several years actually since I done this method. So thanks for the help
     
  4. Joshabr1

    Joshabr1 Well-Known Member

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    So for 65% dissipation I’m looking for 33ma

    so .033* 17.2 = .562 so I keep my voltage drop under .562 correct???
     
  5. Joshabr1

    Joshabr1 Well-Known Member

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    Anyone in the good ole USA awake tonight??
     
  6. thetragichero

    thetragichero Well-Known Member

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  7. Joshabr1

    Joshabr1 Well-Known Member

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    Well. That is
    So can you look at the numbers above and answer my question ? Am I doing it right??
     
  8. thetragichero

    thetragichero Well-Known Member

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    voltage drop of 520mV across a 17.2 ohm resistance is 30.2mA, but if this is for a pair of tubes that's 15.2mA per tube so assuming a 25w max plate dissipation that's 29.1% of max plate dissipation at idle
     
  9. Joshabr1

    Joshabr1 Well-Known Member

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    So what is the math for my superbass. That’s what I am asking
     
  10. Seanxk

    Seanxk Well-Known Member

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    If it's a 4 cylinder SB you need 2 x .562 = 1.12

    My SB, 14.9 ohm and 16.3 ohm, & 490DCV plate divide 17.5 dis = .035 x 2 = .070
    .070 x 14.9 ohm = 1.14 pin 3 to C tap.
     
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  11. mickeydg5

    mickeydg5 Well-Known Member

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    Sorry I did not see your message until now.

    Your math is fine. Always do it twice, double check.

    From above this is a Super Bass, model 1992 100W output amplifier.
    So that is four power tubes, two tubes on each side of the push-pull.

    .520 / 17.2 = 30.2mA, basically 15.1mA per tube >That is low
    .550 / 15.5 = 35.4mA, basically 17.7mA per tube >That is low

    Now with about 480VDC on the power tube plates then this amplifier should have more like average bias of 72mA and maximum bias current of 84mA.

    When your up the bias the plate voltage may drop so recalculate as necessary.

    I know this may have all been hashed through already but I will reiterate. :)
     
    Last edited: Oct 30, 2021
  12. neikeel

    neikeel Well-Known Member

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    We’ve done all this by pm some days ago:(
     
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  13. Matthews Guitars

    Matthews Guitars Well-Known Member

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    I just install precision 1.00 ohm resistors between pins 1,8 and ground and direct read the bias current off them. The mV reading across them equals the mA current through them. No messing with calculations.
     
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  14. mickeydg5

    mickeydg5 Well-Known Member

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    If I do not have the 1 ohm bias resistors then I use the OT calculation method.
     
  15. Matthews Guitars

    Matthews Guitars Well-Known Member

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    I ordered up a bag of 100 of those resistors. I don't expect to run out this year or next year either.
     

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