Bias isn't high enough, but the pot won't go higher

Discussion in 'Amps/Cabs Tech Corner: Amplifier, Cab & Speakers' started by MikeNiteRail, Sep 5, 2008.

  1. MikeNiteRail

    MikeNiteRail Member

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    I borrowed some tubes and am trying my Allen Old Flame with TAD 6L6 tubes and a 5R4 recitifer to brind down the volume. When doing so, I can only turn the bias to 26mA before the bias pot stops.

    Is there a way to get up to 30mA or so?
     
  2. GearHeadFred

    GearHeadFred Member

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    You'll have to change a resistor in the circuit to give you more range on the pot. I don't know the Old Flame circuit, but someone will chime in here I'm sure.
     
  3. Tonefish

    Tonefish Member

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    Yep, gonna take a resistor swap. They should help you get the right value at Allen.
     
  4. Blue Strat

    Blue Strat Member

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    What they said. In a Fender amp (which is probably similar or the same as yours) you would decrease the value of the resistor connected between the bias pot and ground. One standard resistor value will probably do it.
     
  5. MikeNiteRail

    MikeNiteRail Member

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    Thanks for the info guys! I am still deciding if I want to mess with the rectifier or try 6v6 tubes. If I go with 6v6s, I'd use a 5u4gb. In other words, I can't use the 5R4 with the 6v6s?
     
  6. jbltwin1

    jbltwin1 Member

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    As Mike says, and if you want a comparison to go FROM, pull the power tubes out and measure the negative voltage on pin five and write it down. If you have -40v, you want LESS control voltage which will alow MORE cathode current. You would want to get the voltage down to go up.
     
  7. Blue Strat

    Blue Strat Member

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    5R4 is very close to a 5U4 in terms of plate voltage. A 5AR4 isn't and may not be safe for 6V6s in some amps.
     
  8. Structo

    Structo Member

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    OK, Mike this is something that has been bugging me for a long time.

    You always hear about hot and cold bias.

    What exactly do these terms mean in relation to the bias.

    I understand how to bias my amp, just wanted to clear up the confusion caused by the use of these terms.

    When you raise the bias current this lowers the cathode current right?

    And when you lower the bias current this raises the cathode current?

    Kind of like the water faucet analogy.

    So a hot bias would be a lower bias number?

    And a cold bias would be a higher bias number?

    That's what confuses me, a hot bias sounds like more current is flowing through the tube.

    And a cold bias sounds like less.

    Can you clear up the confusion please?

    Thanks
     
  9. Blue Strat

    Blue Strat Member

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    Cathode current IS the bias current (well, for purists, the bias current is plate current and cathode current is plate current + screen current, but screen current is so small that we ignore it).

    Bias VOLTAGE or grid voltage to pin 5 of the standard octal power tubes, runs inversey proportionally to bias current. As one increases the other decreases and vice versa.

    Cold bias is when you have low bias current. There's not text book definition, but let's say that anything that gives you less than 25% dissipation would be considered "cold".
     
  10. booj

    booj Member

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    more negative voltage, less current
    less negative voltage, more current
     

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