Biasing Red Knob "The Twin"

Discussion in 'Amps/Cabs Tech Corner: Amplifier, Cab & Speakers' started by John Coloccia, Jul 11, 2008.

  1. John Coloccia

    John Coloccia Cold Supporting Member

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    I don't have a manual for this. Is there any special procedure for biasing after changing out the 4 output tubes? Is it just replace the tubes and twist the knob until the numbers are right, or is there a particular setting I should have on the amp before doing this?

    Thanks much for any help!!
     
  2. PRNDL

    PRNDL Member

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    It's a little more complicated than that because there is a balance and bias knob.

    Begin by matching both pairs of tubes so that each pair has similar bias currents, then use the balance control to make both pairs equal. You set the bias based on plate voltage and how hot you like to run them.

    It also helps to get a matched quad, or two matched pairs.

    Does that make sense?
     
    Last edited: Jul 11, 2008
  3. John Coloccia

    John Coloccia Cold Supporting Member

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    Sort of makes sense. On The Twin there's one bias pot. I'm not sure I can set anything individually. From what I can tell, you're supposed to replace all 4 tubes and then bias all 4 at the same time from the back panel probe point. That's kind of what I envisioned doing but it seemed a little strange to me.

    My plan was to install a matched quartet of tubes, set the balance and then set the bias. It almost sounds like you're saying I should adjust something with just 2 tubes in there at one time (the outer pair, and then the inner pair) but I don't know how I could adjust the two pairs individually with one control.

    I'm just having a blond moment, that's all :D
     
  4. stoo

    stoo Member

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    http://www.amp-fix.com/fender_twin_amp.htm

    The test points are across a 1 ohm resistor to ground on the cathodes. One for each pair of 6l6s. So you set one for the voltage you want then measure from one test point to the other and set it for zero. This only means that the 2 sets of tubes are biased the same. I looked at the schematic and Fender recommends 40mv which is 40ma for 2 tubes which is really cold.
    Check out page 5 here http://www.21frets.com/manuals/Twin_Amp.pdf
    Stew
     
  5. RedRock

    RedRock Member

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    The recommended bias for this amp is a bit cold compared to other amps,
    but for this circuit and series speaker connection, IMO it works well. It extends
    tube life, especially if you use the amp in the low wattage mode.
     

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