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  • #1

    bias adjustment ?

    Hey guys I have a deluxe reverb reissue and i want to bias the tubes. I have my meter set to 200ma and i'm using a single bias checker which plugs directly into the meter. I can only get a reading of 1ma and when adjusting the bias pot the reading doesn't move.is there something i should check in the bias circuit that may be causing the amp not to give me a bias reading?

    Chuck.
    Tags: None
  • #2
    Not sure what variety bias check you have. Most that I've seen will put out a mV (not mA) signal to the meter. if the tube still functions without the meter present (plugged into the bias checker), then that's the kind you have.
    If it still won't get loud enough, it's probably broken. - Steve Conner
    If the thing works, stop fixing it. - Enzo
    We need more chaos in music, in art... I'm here to make it. - Justin Thomas
    MANY things in human experience can be easily differentiated, yet *impossible* to express as a measurement. - Juan Fahey

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    • #3
      hey eschertron, yes i just set my multimeter to dc voltage and got a reading of 11v, so i adjusted the voltage to 24v. i'm assuming that the bias adjustment for a 6v6 is between 18v to 30v. i'm using 6v6 JJ's.

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      • #4
        Do you mean mV rather than v ?
        Originally posted by Enzo
        I have a sign in my shop that says, "Never think up reasons not to check something."


        ​

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        • #5
          Originally posted by chuckb View Post
          hey eschertron, yes i just set my multimeter to dc voltage and got a reading of 11v, so i adjusted the voltage to 24v. i'm assuming that the bias adjustment for a 6v6 is between 18v to 30v. i'm using 6v6 JJ's.
          The ideal bias point will depend on the plate and screen voltages. Also, Fender marks up their schematics with expected voltages at test points. a good ballpark starting point would be whatever bias voltage they list, and then see what the bias current is at that point.

          Why the bias adjustment? New tubes, or bad crossover distortion? The specific reason may inform your decision in how radically you want to change the bias. And as long as you are not over-dissipating the tubes, your ears are your best test instrument.
          If it still won't get loud enough, it's probably broken. - Steve Conner
          If the thing works, stop fixing it. - Enzo
          We need more chaos in music, in art... I'm here to make it. - Justin Thomas
          MANY things in human experience can be easily differentiated, yet *impossible* to express as a measurement. - Juan Fahey

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          • #6
            yes, sorry that's 24Mv and i replaced the power tubes so i needed to re-bias.

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            • #7
              Schematic shows 390V for plate voltage, so at 24mV you are idling about 9.4 watts, 6V6 is rated for 14W so you are approx. 67%.
              For longest tube life and reliability I recommend biasing as cold as you can without an objectionable effect on tone.
              Originally posted by Enzo
              I have a sign in my shop that says, "Never think up reasons not to check something."


              ​

              Comment

              • #8
                I think with g-one's above math, you are right where you want to be. For JJ's maybe even a bit higher if you like that tone, hotter and a bit faster to load up and overdrive.
                It's weird, because it WAS working fine.....

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                • #9
                  ya my plate voltage is 410 v so i'm running them at about 70%

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