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    Problem with a MK IV



    Posts : 7
    Join date : 2012-11-13
    Age : 69
    Location : Tampa, Florida

    Problem with a MK IV

    Post by Austuriano on Sun Jan 17, 2016 12:47 am

    I am having a problem with one of my MKIV's. It's intermittent which makes it tough to troubleshoot. When I power up the amp, one of the power tubes has a pulsing blue glow around the interior metal parts. The power transformer develops a very loud hum. I have a switch which allows for the amp to be run in ultra-linear to triode mode. Only one of the power tubes is reacting this way, and when I switch from triode to UL the problem switches to the other power tube. So as you look at the amp from the crossover side; in triode the right tube has the problem, in UL it is the left tube. I am not sure how to troubleshoot the problem. I am using Roy Mottram's high-gain 12AT7 boards which are excellent. I have checked the power tubes which are 6P3S-E, and they are fine.
    I would appreciate any thoughts, or advice you might have on this issue



    Posts : 944
    Join date : 2012-12-25
    Location : gothenburg, sweden

    Re: Problem with a MK IV

    Post by peterh on Sun Jan 17, 2016 4:07 am

    It seems to be oscillating ( hint, power transformer get's loaded).
    Confirm with a scope. Then make sure you have grid-stoppers ( 100ohm ) on g2,
    and grid-stoppers ( 1k ) on the grids. All carbon resistors.

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