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DRRI bias setting

Discussion in 'Amp Central Station' started by owb3, Apr 27, 2004.

  1. owb3

    owb3 TDPRI Member

    Jun 5, 2003
    I finally purchased a gently used DRRI in great condition and am about to retube with JJ's all around. The biasing discussions I have heard are based on using a Bias-Rite.

    But I will be using the method of dropping the chassis, placing the probe on the bias test point (TP36, which is R57), taking a reading and turning the bias adjust pot to set the bias. The Fender schematic says the bias adjustment using this method is -37 VDC.

    I am used to biasing my HRDlx with this method; the Fender schematic says +60mV at TP30, but HRDlx guys seem to prefer slightly hotter at +68mV.

    What would a comparable setting (or an acceptable range) be for the DRRI? Is -37 VDC cold? Hot? Thanks for any suggestions.
  2. racehorse

    racehorse Tele-Meister

    Apr 28, 2003
    it s a matter of...............

    watts of static plate dissipation.

    6v6s you would want no more than 12 watts .

    watts = volts X amps or plate voltes X current in milliamps through the tube.

    If you don`t know how to measure these , you should stop here and not go any further, until you know how.

    KCA tubes has links to several sites that explain biasing.

    Lord Valve /NBS electronics has some good info .

    DR if the plate voltage is 420v and 12watts is the max. plate dissipation for a 6v6 . 12 watts / 420v= app .028 amps or 28ma (milliamps)

    voltage and current (ma) are inversely proportional , one goes up the other goes down. so as you adjust the bias take ALOT of measurments.

    425v X .025(25ma) = 10.625 watts this is a good safe range of operation.

    again if you don`t understand this , or don`t know how to make these measurments DO NOT GO ANY FURTHER UNTIL EDUCATE YOURSELF on this subject.

    after that be carefull and have fun.

    this is not opinion it is basic electronics and math

    hope this helps ya
  3. Teleologist

    Teleologist Tele-Meister

    Nov 8, 2003
    That's just a test point for diagnosis and like all voltages on the schematics there's a variance listed in the notes(5 or 10%). I.E. - with the tubes properly biased using one of the other methods, the voltage at that point should fall within that range. You need not bother with it unless there's something wrong with the amp(runaway power tubes, certain types of distortion, or after a power tube self-destructs in a cloud of smoke).
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