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    Mark III bias question

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    snav

    Posts : 18
    Join date : 2011-05-18

    Mark III bias question

    Post by snav on Tue Aug 02, 2011 7:44 pm


    Is there a circuit around for providing individual bias adjustments and a balance adjustment for the stock MK3?

    Does anyone have the experience of having a bias resistor open? What is/are the unbiased voltages and or indications of a loss of bias?


    Thanks

    GP49

    Posts : 735
    Join date : 2009-04-30
    Location : East of the sun and west of the moon

    Re: Mark III bias question

    Post by GP49 on Wed Aug 03, 2011 10:31 am

    snav wrote:
    Is there a circuit around for providing individual bias adjustments and a balance adjustment for the stock MK3?

    Does anyone have the experience of having a bias resistor open? What is/are the unbiased voltages and or indications of a loss of bias?
    Thanks

    If you mean by "a bias resistor open", you mean the 1000Ω resistor prior to the bias adjust pot...if it (or the rectifier) goes open you will have no negative
    voltage on the grids of the output tubes. They will run away with excess current, the plates will glow red, and they will fail in very short order,
    meanwhile the output transformers will heat up and may be damaged.

    This is, unless the cathode resistor overheats and fails first.

    The result of this resistor going open is so catastrophic, I would not experiment to find out what the actual voltages would be. The result of
    the wiper circuit of the bias adjust pot going open would be the same.

    If the 18,000Ω resistor in the bias circuit goes open, the voltage on the grids of the output tubes would go TOO negative and the idle (bias)
    current in the output tubes might be too low for you to attain the 1.56V measurement across the cathode resistor.

    Before you change to dual bias adjustments, read the prior thread about individual bias adjustment:

    http://dynacotubeaudio.forumotion.com/t917-questions-about-the-dual-bias-pots#5512

      Current date/time is Thu Dec 08, 2016 11:58 am