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    Mk III Bias issues

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    stevenally

    Posts : 6
    Join date : 2013-12-07

    Mk III Bias issues

    Post by stevenally on Mon Nov 27, 2017 9:07 am

    Have a pair of Mk III amps that I recently restored with all NOS carbon comp resistors , mil spec teflon coated wire , teflon sockets , new quad cap ,ClarityCap capacitors. Tubea are Gold Lion KT88 , Sovtek 5AR4 , & RCA 6AN8A.
    Everything is run through a Variac at 117V.
    For the first 3 months all went well. This morning one of the amps showed a low bias reading. Both amps were set to 1.56V and ended up near the middle of the pot adjustment. Now one of the amps will only go up to .88/.90 with the pot maxed out.
    Can anyone point me in the right direction to troubleshoot this amp ?
    I have already switched the tubes from amp to amp and the problem remains with the original amp so I do not believe the problem is tube related.
    Thanks
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    peterh

    Posts : 892
    Join date : 2012-12-25
    Location : gothenburg, sweden

    Re: Mk III Bias issues

    Post by peterh on Mon Nov 27, 2017 9:20 am

    stevenally wrote:Have a pair of Mk III amps that I recently restored with all NOS carbon comp resistors , mil spec teflon coated wire , teflon sockets , new quad cap ,ClarityCap capacitors. Tubea are Gold Lion KT88 , Sovtek 5AR4 , & RCA 6AN8A.
    Everything is run through a Variac at 117V.
    For the first 3 months all went well. This morning one of the amps showed a low bias reading. Both amps were set to 1.56V and ended up near the middle of the pot adjustment. Now one of the amps will only go up to .88/.90 with the pot maxed out.
    Can anyone point me in the right direction to troubleshoot this amp ?
    I have already switched the tubes from amp to amp and the problem remains with the original amp so I do not believe the problem is tube related.
    Thanks
    Start with checking voltages as per the dynaco manual ( p.14) Also check the voltage
    around the bias adjustment pot.
    In addition , double-check all soldering where components are changed.
    Are the selenium rectifier replaced ?
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    Peter W.

    Posts : 577
    Join date : 2016-08-07
    Location : Melrose Park, PA

    Re: Mk III Bias issues

    Post by Peter W. on Mon Nov 27, 2017 11:03 am

    As the 'other Peter' suggests, wandering bias is the first sign of a selenium rectifier on its way out. When you replace it with a silicon device, you will need to add resistance. Dynakit parts sells a package if you do not want to hunt the parts down for yourself.

    http://www.dynakitparts.com/dynakit-products/bias-kit-parts/MARK-III-BIAS-CIRCUIT-KIT
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    j beede

    Posts : 425
    Join date : 2011-02-07
    Location : California

    Re: Mk III Bias issues

    Post by j beede on Tue Nov 28, 2017 12:49 am

    Hard to imagine rebuilding a MkIII and leaving the Selenium stack in place. Use the same type of 1n4007 for the bias circuit as you used to do the "yellow sheet" mod. You did do the yellow sheet mod... right?
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    pichacker

    Posts : 29
    Join date : 2016-08-01
    Age : 53
    Location : Near to London - UK

    Re: Mk III Bias issues

    Post by pichacker on Wed Nov 29, 2017 3:57 am

    Also wouldn't an aging selinium rectifier reduce the -ve voltage and cause the valves to conduct harder creating a higher bias voltage?

    The OP has a low standing current which would indicate a larger -ve voltage on the grids, or a low HT. Could it be that only one of the OP valves is conducting? Assuming there is no red plating.
    Best get the DMM out and compare voltages especially the HT and Grids between amps.

    stevenally

    Posts : 6
    Join date : 2013-12-07

    Re: Mk III Bias issues

    Post by stevenally on Wed Nov 29, 2017 8:22 am

    Have replaced selenium stack with kit from Dynakit parts. Also included "yellow sheet" mod at time of rebuild. Put on a huge pot of coffee and will start checking voltages this morning.

    stevenally

    Posts : 6
    Join date : 2013-12-07

    Thanks to all who replied

    Post by stevenally on Wed Nov 29, 2017 9:39 am

    Installed cap board and checked all connections. Amps now working fine.

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