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    Mk III won't bias properly

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    aliveatfive

    Posts : 7
    Join date : 2009-03-21

    Mk III won't bias properly

    Post by aliveatfive on Wed Aug 26, 2009 2:36 pm

    I'm hoping one of you electro-techs can help me figure this one out.
    The problem:
    New construction Mk III built from parts-bin stuff and new Triode-El transformers. Amp went together easily. Surprised at the simplicity of this amp. Plugged it in and got a case of run-away bias. Checked polarity of diodes and electrolytics. Measured resistor and cap values. Everything checks out. Rechecked all connections re schematic and everything seems to be correct. When bias was measured, I pulled the plug as soon as voltage got over 2 v. Bias pot is not able to control the high voltage. Any suggeations greatly appreciated. Thanks!
    PS - tried 2 new rectifier tubes and 2 sets of KT 88s. Same behavior with all tubes.

    Bob Latino
    Admin

    Posts : 2411
    Join date : 2008-11-26
    Location : Massachusetts

    Re: Mk III won't bias properly

    Post by Bob Latino on Wed Aug 26, 2009 5:13 pm

    aliveatfive wrote:I'm hoping one of you electro-techs can help me figure this one out.
    The problem:
    New construction Mk III built from parts-bin stuff and new Triode-El transformers. Amp went together easily. Surprised at the simplicity of this amp. Plugged it in and got a case of run-away bias. Checked polarity of diodes and electrolytics. Measured resistor and cap values. Everything checks out. Rechecked all connections re schematic and everything seems to be correct. When bias was measured, I pulled the plug as soon as voltage got over 2 v. Bias pot is not able to control the high voltage. Any suggeations greatly appreciated. Thanks!
    PS - tried 2 new rectifier tubes and 2 sets of KT 88s. Same behavior with all tubes.

    Hi,

    Remove all the tubes and turn the amp on > Check the negative DC voltage at pin 5 or pin 6 on each output tube. As you rotate the bias pot the negative DC voltage on a Mark III should vary from roughly -40 to -65 volts DC. If you don't get any bias voltage on pins 5 and 6 then the bias system is not working and there is no control voltage and you will get run-away bias as you mentioned. Let us know if you can measure any voltage on pins 5 and 6?

    Bob

    aliveatfive

    Posts : 7
    Join date : 2009-03-21

    Re: Mk III won't bias properly

    Post by aliveatfive on Wed Aug 26, 2009 6:01 pm

    Bob -
    Thanks much for your prompt reply. I just checked the voltage on pins 5 and 6 of both output tubes and got a value of between -48 and -74 v depending on where the pot is set. Can we figure anything from this?

    Bob Latino
    Admin

    Posts : 2411
    Join date : 2008-11-26
    Location : Massachusetts

    Re: Mk III won't bias properly

    Post by Bob Latino on Wed Aug 26, 2009 7:37 pm

    Hi,

    With those voltages that you mention we can figure that the bias system is working as it should. Check for a leaking coupling capacitor. Lift one side of each coupling cap and see if there is continuity across the cap. There shouldn't be ... Maybe you have a dead short across one of the two main coupling capcitors. Check that the main bias resistor has the proper resistance also. Do this by measuring (with the amp OFF) the resistance at the bias measuring point instead of the DC voltage. Stock Mark III's should have a 11.2 ohm (or pretty close to that) resistance reading.

    Bob

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