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The Dynaco Tube Audio Forum

Dedicated to the restoration and preservation of all original Dynaco tube audio equipment - Customer support for Tubes4hifi VTA tube amp and preamp kits and all Dynakitparts.com products


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Steve G
Bob Latino
6 posters

    M125 AutoBias Goes Out

    avatar
    IntarsiaGuy

    Posts : 20
    Join date : 2021-04-07

    M125 AutoBias Goes Out Empty M125 AutoBias Goes Out

    Post by IntarsiaGuy Fri Jul 09, 2021 3:17 pm

    Recently built 2 monoblocks. Built them first to make sure they worked, then added an autobias board (erhard audio). I wired leds so I can see the lights with the system closed up. On one of the monoblocks, after some amount of time - usually 45 minutes to 90 minutes, the 2nd light goes out. Only once have I measured the bias voltage when that happened and it was up around .8 instead of the .5 I set on the board.
    - using kt88 tubes
    - sprayed deoxit in the 2nd tube socket
    - swapping tubes - problem stayed with socket 2.

    When things start up all voltages on the sockets are consistent. Resoldered all connections to socket 2, and all connections related to installing the autobias board. Any advice on diagnosing this? Thanks.
    Bob Latino
    Bob Latino
    Admin

    Posts : 3080
    Join date : 2008-11-26
    Location : Massachusetts

    M125 AutoBias Goes Out Empty Re: M125 AutoBias Goes Out

    Post by Bob Latino Fri Jul 09, 2021 5:05 pm

    IntarsiaGuy wrote:Recently built 2 monoblocks.  Built them first to make sure they worked, then added an autobias board (erhard audio).  I wired leds so I can see the lights with the system closed up.  On one of the monoblocks, after some amount of time - usually 45 minutes to 90 minutes, the 2nd light goes out.  Only once have I measured the bias voltage when that happened and it was up around .8 instead of the .5 I set on the board.
    - using kt88 tubes
    - sprayed deoxit in the 2nd tube socket
    - swapping tubes - problem stayed with socket 2.

    When things start up all voltages on the sockets are consistent.  Resoldered all connections to socket 2, and all connections related to installing the autobias board.  Any advice on diagnosing this?  Thanks.

    This is main reason why we do not recommend the use of the AutoBias boards for any of the VTA amps anymore. These boards do install fine and do work but long term durability has been an issue on SOME of them. After a period of time SOME of them just fail to keep the bias steady anymore on one or more tube sockets ? My recommendation is to remove the AutoBias boards from both amps and convert the amps back to manual bias. Read at the end of post # 1 at the link below.

    AutoBias board post from 6/16/20

    Bob
    avatar
    Steve G

    Posts : 18
    Join date : 2017-03-17
    Location : Carson City, NV

    M125 AutoBias Goes Out Empty Re: M125 AutoBias Goes Out

    Post by Steve G Fri Jul 09, 2021 5:33 pm

    I've had my M125's for quite a few years I used to fret over the bias in the beginning, now I rarely check it and when I do if their within a few tenths I leave it alone. Don't chase this setting.

    mijohn, 10-E-C and Tubes4ever like this post

    LeGrace
    LeGrace

    Posts : 355
    Join date : 2016-08-07
    Location : Ontario, Canada

    M125 AutoBias Goes Out Empty Re: M125 AutoBias Goes Out

    Post by LeGrace Sun Jul 11, 2021 4:14 pm

    Rev 2 AB boards with the isolation transformer are a major improvement. Best upgrade I ever applied to my M125s. No more red plates since the install.

    Good luck finding the issue.


    avatar
    IntarsiaGuy

    Posts : 20
    Join date : 2021-04-07

    M125 AutoBias Goes Out Empty Re: M125 AutoBias Goes Out

    Post by IntarsiaGuy Sun Jul 11, 2021 6:03 pm

    I'm not sure if it was a cold solder joint or a break in a single-strand wire, but when I replaced all wires (and thus re-resoldered) the problem appears to be resolved. Ran for 12 hours straight today without a recurrence, which is about 5x longer than it took to occur in the past. Since it was intermittent, I'll feel better after a few weeks and not just a long day, but am optimistic the board is fine and it was my fault. Will update later for those interested.
    LeGrace
    LeGrace

    Posts : 355
    Join date : 2016-08-07
    Location : Ontario, Canada

    M125 AutoBias Goes Out Empty Re: M125 AutoBias Goes Out

    Post by LeGrace Fri Jul 16, 2021 4:33 pm

    Still hanging in there?
    avatar
    IntarsiaGuy

    Posts : 20
    Join date : 2021-04-07

    M125 AutoBias Goes Out Empty Re: M125 AutoBias Goes Out

    Post by IntarsiaGuy Fri Jul 16, 2021 4:38 pm

    Yes, have been playing the monoblocks every day and the leds indicating the autobias has locked in have been rock-steady.

    LeGrace likes this post

    avatar
    vinylhead

    Posts : 1
    Join date : 2017-10-23

    M125 AutoBias Goes Out Empty Autobias

    Post by vinylhead Sat Jul 24, 2021 9:06 pm

    I just would never fit autobias. It is just more to go wrong. My ST120 has manual bias no probs, easy to set, check it every once in a while.
    In fact I want manual bias so I CAN KEEP an eye on the general state of each channel.
    D.W
    New Zealand.

    tubes4hifi, wgallupe, Dale Stevens, Tubes4ever and Rabindu like this post

    pavlikkkk
    pavlikkkk

    Posts : 55
    Join date : 2018-03-02
    Age : 74
    Location : Czech Republic

    M125 AutoBias Goes Out Empty Re: M125 AutoBias Goes Out

    Post by pavlikkkk Sun Sep 12, 2021 4:12 pm

    Bob Latino wrote:
    IntarsiaGuy wrote:Recently built 2 monoblocks.  Built them first to make sure they worked, then added an autobias board (erhard audio).  I wired leds so I can see the lights with the system closed up.  On one of the monoblocks, after some amount of time - usually 45 minutes to 90 minutes, the 2nd light goes out.  Only once have I measured the bias voltage when that happened and it was up around .8 instead of the .5 I set on the board.
    - using kt88 tubes
    - sprayed deoxit in the 2nd tube socket
    - swapping tubes - problem stayed with socket 2.

    When things start up all voltages on the sockets are consistent.  Resoldered all connections to socket 2, and all connections related to installing the autobias board.  Any advice on diagnosing this?  Thanks.

    This is main reason why we do not recommend the use of the AutoBias boards for any of the VTA amps anymore. These boards do install fine and do work but long term durability has been an issue on SOME of them. After a period of time SOME of them just fail to keep the bias steady anymore on one or more tube sockets ? My recommendation is to remove the AutoBias boards from both amps and convert the amps back to manual bias. Read at the end of post # 1 at the link below.

    AutoBias board post from 6/16/20

    Bob

    I fixed several amplifiers, some of which had this problem with ABautobias.
    The resistance paths of the adjusting trimmers for the tubes preload have ALWAYS been destroyed. I write it everywhere, but as you can see, this wrong practice keeps repeating itself. People just keep twisting it. They confuse the trimmer with a potentiometer. Mad
    The fact is that throwing out autobios will solve the problem.... Laughing
    pavlikkkk
    pavlikkkk

    Posts : 55
    Join date : 2018-03-02
    Age : 74
    Location : Czech Republic

    M125 AutoBias Goes Out Empty Re: M125 AutoBias Goes Out

    Post by pavlikkkk Wed Sep 15, 2021 12:59 pm

    IntarsiaGuy wrote:I'm not sure if it was a cold solder joint or a break in a single-strand wire, but when I replaced all wires (and thus re-resoldered) the problem appears to be resolved.  Ran for 12 hours straight today without a recurrence, which is about 5x longer than it took to occur in the past.  Since it was intermittent, I'll feel better after a few weeks and not just a long day, but am optimistic the board is fine and it was my fault.  Will update later for those interested.

    So in the end, not autobias Laughing

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    M125 AutoBias Goes Out Empty Re: M125 AutoBias Goes Out

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