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Fender Super Reverb AA763 Bias

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  • Fender Super Reverb AA763 Bias

    Hello - I have a fender super reverb original AA763 that I just finished re-capping all paper caps. It has JJ 6L6GC power tubes, I'm trying to bias but not sure if this is correct:

    (I have a bias probe with 1 ohm on it)
    Plate Voltage: 460

    I calculate out 46mA for 70%... (that is per tube right?) I was going to start around 40mA, but I see that I can't reach that even with the bias pot full, I can only get about 32ma. It has a 22k to ground instead of the factory 27k. Do I need to lower or raise that resistor to get closer? And why does Fender schematic suggest -52v, that only gets about 11mA per tube when I measure with the probe I have... or is that the sign of another issue?

    Thanks

    -kdawg

  • #2
    There's a bias calculator on the webervst website (Weber Bias Calculator). When you scroll down theres even a table (for common tubes)
    with current values at different voltages.
    Fender used to bias the amps quite on the cold side. I'd say a good starting point would be 35ma (per tube with your present voltage) and turning up from there. I wouldn't go too high just to reach this magical (overhyped?) 70% mark. Believe in your ears and save your tubes. You don't have to go up to 70% dissipation when the amp sounds good at 60%. BTW many Fender amps do sound great, right with those quite cool settings.
    Just my 2C

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    • #3
      Originally posted by kdawg View Post
      ...It has a 22k to ground instead of the factory 27k. Do I need to lower or raise that resistor to get closer?
      try a 15K....lowering bias resistor value increases bias current

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