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Thread: Hiwatt DR103 need more bias voltage to new tubes

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    Hiwatt DR103 need more bias voltage to new tubes

    Dear,
    I have a hiwatt Custom 100W DR103 70 .
    I add a new power tubes and the bias current is 50mA. I change the bias resistor 47K (-36Vdc) to 220k (-44Vdc). But the bias current
    it is still elevated. I tried to put 330K bias resistor bit the voltage no changed.
    How can I down the current of polarization?
    I am doing something wrong?
    thanks, regards

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    the solution is to create a voltage doubler circuit,so instead of -38V you get around 70,and then you can adjust it with a voltage divider with a trimmer.
    https://hiwatt.org/Schematics/BiasCkts2.gif

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    You could also try reducing the 1K that feeds the diode. It may help some, but the doubler that alexradium mentioned is the best solution.

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    alexrarium has the common solution. Just to add clarity to your trouble though...

    Notice that the series resistor for the bias voltage divider circuit is only 1k while the load is 47k. The division is only 1/47th of the voltage. This is why adding resistance to the load portion of the divider made so little difference. There simply isn't much voltage from the source to increase.

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    Actually -44V bias should be enough to tame any real EL34. I run my 100W Marshall at -38V for 36mA (@Vp= 470V) plate idle current (Svetlanas). What tubes do you use?

    Just to make sure that you don't have leaky coupling caps: What is the bias voltage measured directly at the EL34 sockets (pin 5)?

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    Last edited by Helmholtz; 04-13-2019 at 10:02 PM.
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    Quote Originally Posted by Helmholtz View Post
    Actually -44V bias should be enough to tame any real EL34. I run my 100W Marshall at -38V for 36mA (@Vp= 470V) plate idle current (Svetlanas).
    Tie the suppressor grids to -Vbias and you should be able to get away with even less.

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