Need to replace resistor to get bias correct in 2203...

Discussion in 'Amps/Cabs Tech Corner: Amplifier, Cab & Speakers' started by JDW3, Feb 10, 2012.

  1. JDW3

    JDW3 Member

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    I built a Metro 2203 and it has the 400V tap on the PT. I was using the 490V but tried the 400V tonight.

    If my math is correct, I'm looking for 41 mA at 422V as opposed to 36mA at 490V.

    For some reason, I barely got the 36mA with the bias pot almost all the way up.

    Now, with 422V, I'm only getting 12mA with the pot fully cranked.

    I have a 47K resistor attached to the pot; make it a smaller value?

    The other two resistors in the circuit are 15K and 27K.
     
    Last edited: Feb 11, 2012
  2. ked

    ked Supporting Member

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    The 47k resistor is feeding voltage to the pot that goes to ground so go ahead and lower the value of that resistor. That should let you get more ma on your tubes.

    Ken
     
  3. JDW3

    JDW3 Member

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    Thanks! I just searched around for awhile and figured I'd be changing the bias range resistor.

    I wasn't sure if it would be larger or smaller.
     
  4. TweeDLX

    TweeDLX Member

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    Remember this:
    Increasing the value of the bias range resistor impedes the voltage to the bias circuit.
    LESS (-) VOLTAGE means MORE CURRENT.

    Decreasing the value of the bias range resistor allows more voltage into the bias circuit.
    MORE (-) VOLTAGE means LESS CURRENT.
     
  5. Fuchsaudio

    Fuchsaudio Member

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    Very common, do it all the time. Old tubes and new sometimes have different bias ranges, and the pots didn't have that much range either. Not unusual to have to do this sometimes.
     
  6. JDW3

    JDW3 Member

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    Thanks guys! Used a 33K and got it to 41mA.

    These are new Mullard RI el-34s. A couple of them bias 3 mA less than the others. This is fairly common?
     
  7. Fuchsaudio

    Fuchsaudio Member

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    Unless you bought a matched set, that's pretty close for non matched tubes. They will wander anyway. Check them in a month.
     

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