M125 AutoBias Goes Out

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.

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.
The fact is that throwing out autobios will solve the problem....

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.