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voltage at power tube cathode?

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  • #16
    Originally posted by dmartn149 View Post
    ok, I disconnected the coupling cap at pin 5 of the power tube. still getting an extra 1.5v at cathode AND still getting 3vdc at pin 5. So, is that a bad tube?
    If the increased grid voltage stays with the tube when you exchange power tubes, it should be the tube. If it stays with the socket, this could be the culprit.


    BTW, it just crossed my mind that cathode biasing with a common cathode resistor generally tends to emphasize asymmetry: If one of the tubes draws more idle current, cathode voltage increases. Increased cathode voltage in turn makes the "colder" tube run even colder. So using matched tubes seems much more desirable than with fixed bias (where also individual tube biasing is easy).
    Using individual cathode resistors could be a benefit.
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    • #17
      Yes, a bad mismatch could result in the hot tube over dissipating, which increases grid current, which increases plate current and so on.
      With shared cathode bias (which Merlin advises can add even harmonics), a 1 ohm resistor in series with each cathode is handy to monitor the current in each tube.
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      • #18
        which increases grid current
        Why grid current?

        Generally a cathode resistor counteracts thermal runaway. This is a benefit compared to fixed biasing.

        What I meant was that the hotter tube indirectly "steals" idle current from the cooler tube, thus increasing asymmetry.


        Additional individual cathode resistors are a good and logical proposal to allow for measuring individual cathode currents.
        Last edited by Helmholtz; 10-13-2019, 03:32 PM.
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        • #19
          I replaced the tube that had dc at the grid. now both tubes have 0vdc at pin 5, but I'm still getting higher than expected voltage at the cathodes (about 1v more than what's on the schematic) If it's just mismatched tubes I'm ok with that. "it's just a guitar amp"
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          • #20
            I replaced the tube that had dc at the grid. now both tubes have 0vdc at pin 5
            Seems you had a gassy tube that produces reverse (positive) grid current.
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            • #21
              Originally posted by Helmholtz View Post
              Seems you had a gassy tube that produces reverse (positive) grid current.
              yes, it would seem so. Thanks for all the help, guys.
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