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  • Bias supply and Rg1

    Hi all, I'm having a hard time finding a definitive answer to this - does the bias supply resistance itself factor into the maximum Rg1 rating of a power tube? Or is this only referencing the grid leak + stopper value?
    I'm working on something with KT120 which has a max Rg1 rating of 51K, but the PT I'm using has no bias tap. Deriving bias supply from the HT secondary will mean a big series resistance and I wonder if I'm better off adding a dedicated transformer for this.
    Thanks--

  • #2
    Originally posted by hylaphone View Post
    - does the bias supply resistance itself factor into the maximum Rg1 rating of a power tube? Or is this only referencing the grid leak + stopper value?
    The way I understand it, the limit applies to the total DCR between grid and cathode (or ground).
    So it is the sum of grid leak (aka bias feed resistor), grid stopper and bias supply shunt resistor.
    51k means a heavy load for the PI, lowering PI gain and drive capability.
    So make sure to use a strong PI (not a 12AX7 with a 100k plate resistor).
    Best would be CF drive.

    This said, most designs exceed the Rg1 limit of the power tubes.
    Making Rg1 too large bears the risk of thermal runaway caused by the grid building up a positive charge.

    - Own Opinions Only -

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    • #3
      Originally posted by Helmholtz View Post

      The way I understand it, the limit applies to the total DCR between grid and cathode (or ground).
      So it is the sum of grid leak (aka bias feed resistor), grid stopper and bias supply shunt resistor.
      51k means a heavy load for the PI, lowering PI gain and drive capability.
      So make sure to use a strong PI (not a 12AX7 with a 100k plate resistor).
      Best would be CF drive.

      This said, most designs exceed the Rg1 limit of the power tubes.
      Making Rg1 too large bears the risk of thermal runaway caused by the grid building up a positive charge.
      Great, I thought so. Any drawbacks, theoretical or practical, I should be aware of when getting bias voltage from HT secondary?

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      • #4
        Well if the bias supply shunt resistor (that's the resistor across the output of the bias supply) already needs to be around 50k, then you're having a problem.
        - Own Opinions Only -

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        • #5
          My understanding is that the g1-k resistance limit applies at full dissipation.
          As dissipation reduces, the g1-k grid circuit resistance can be increased.
          So provided your design isn't attempting to squeeze the KT120s hard, you may be ok to eg double the total effective grid circuit resistance.
          My band:- http://www.youtube.com/user/RedwingBand

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          • #6
            Originally posted by pdf64 View Post
            My understanding is that the g1-k resistance limit applies at full dissipation.
            As dissipation reduces, the g1-k grid circuit resistance can be increased.
            So provided your design isn't attempting to squeeze the KT120s hard, you may be ok to eg double the total effective grid circuit resistance.
            Yes, bias till max 70% power dissipation and it may double it. Some datasheets published the ratings for different power dissipation.
            "If it measures good and sounds bad, it is bad. If it measures bad and sounds good, you are measuring the wrong things."

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            • #7
              Originally posted by pdf64 View Post
              My understanding is that the g1-k resistance limit applies at full dissipation.
              As dissipation reduces, the g1-k grid circuit resistance can be increased.
              So provided your design isn't attempting to squeeze the KT120s hard, you may be ok to eg double the total effective grid circuit resistance.
              That makes sense as thermal runaway must depend on tube temperature.
              - Own Opinions Only -

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              • #8
                Originally posted by catalin gramada View Post

                Yes, bias till max 70% power dissipation and it may double it. Some datasheets published the ratings for different power dissipation.
                Can you post a datasheet example?
                - Own Opinions Only -

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                • #9
                  Originally posted by Helmholtz View Post

                  Can you post a datasheet example?
                  Here you go

                  Click image for larger version

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                  "If it measures good and sounds bad, it is bad. If it measures bad and sounds good, you are measuring the wrong things."

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