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    Permanent Bias Meters


    Posts : 13
    Join date : 2011-07-31

    Permanent Bias Meters

    Post by SoundBound on Sun Jul 31, 2011 10:31 pm


    New to the forum, first posting.

    I am currently refurbishing a set of Mark IIIs and about to install the VTA driver board for these lovely puppies. On my wish list, possibly, is the installation of a permanent meter(s) for monitoring the tube bias.

    There are two ways to do this;
    1. A volt meter wired across the bias measurement resistor(s). Scaled 0-2V at best.
    2. A milliameter wired in series with the bias measurement resistor(s). Scaled 0-100 or so mA.

    Question, is this an ok thing to do and which method is preferable to monitor the bias, voltage or current. I know that you can set the bias by either method but will either approach have negative effects on the Mark IIIs ?

    If anyone has any comments in regards, glad to hear them.
    Bob Latino

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    Re: Permanent Bias Meters

    Post by Bob Latino on Mon Aug 01, 2011 5:51 am


    Your choice # 1 is the best method. It is basically the same way as bias is now measured on a Mark III except you want a permanent meter to constantly monitor bias.

    Since a VTA driver board on a Mark III has TWO bias measuring points, you will need either two meters for each Mark III or some type of switching device to switch the meter between the two output tubes on a Mark III.



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    Re: Permanent Bias Meters

    Post by GP49 on Tue Aug 02, 2011 11:37 am

    #1 is the practical choice, unless you want to replace the 0-100mA meter in series with the cathode circuit, if ever a tube runs away.

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    Join date : 2008-11-30

    Re: Permanent Bias Meters

    Post by tubes4hifi on Tue Aug 02, 2011 2:01 pm

    I agree, #1 is the way to go, a volt meter won't likely burn out, a ammeter will burn out if the tube runs away.

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