Catastrophic failure of auto bias board in M-125

I have been noticing some wandering bias voltage. The measured voltage would creep up. At first I thought that I was dealing with failing tubes. But a fuse blew when I was out of the room today. In an effort to track it down I put the rectifier and the tubes from the good amp after a fuse blew. When I turned the amp on there was a bright flash from inside. The pictures show the damage. Thoughts on a cause? This is the first generation of the auto bias that had some install issues. I’m guessing for the repair my best bet is a complete new board. Thanks in advance for any insight.

The AB design has been improved, agree installing latest version is in order.

The later edition of AB boards have an isolation transformer installed on the board. I cannot tell if this transformer would have prevented the problem in your case, but it might have. Heck, you need to replace this board anyways. I love my AB boards and have had not a single problem in the year that I have had my M-125s. Below is the website to order another. If it were me I would get a pair so both amps are exact matches. If you want a more technical explanation of the improvements, I would suggest that you send Holger an email from the Erhard Audio website. It also lists his phone number Mon-Thurs, if you prefer a conversation over the phone.

Good luck,
Mike

http://www.erhard-audio.com/AutoBias.html

I have the isolation transformer upgrade sitting in a box and am running the first Gen AB board on my ST-120... guess I better break out the soldering gun this weekend!

I have the gen 2 boards and one thing I do lately every time is pay close attention to the LED`s status during power on. I watch them for the first minute to make sure they transition from red to blue. That way I know the amp has biased up properly.

FearOfMusic wrote:I have been noticing some wandering bias voltage. The measured voltage would creep up. At first I thought that I was dealing with failing tubes. But a fuse blew when I was out of the room today. In an effort to track it down I put the rectifier and the tubes from the good amp after a fuse blew. When I turned the amp on there was a bright flash from inside. The pictures show the damage. Thoughts on a cause? This is the first generation of the auto bias that had some install issues. I’m guessing for the repair my best bet is a complete new board. Thanks in advance for any insight.

Hi,
what caused the disaster? The picture shows that the cathode resistor has burned out.
Simple answer - short circuit in a controlled power tube.
The reason for the complete destruction of the module is clear. CATH input protection of the AB varistor module with fuses was not used:
https://www.audioamp.eu - Section "Protective elements for modules".
I strongly recommend this protection everywhere. Protects not only AB but also OT.
An isolating transformer would not save anything here.
You have a sad experience, but don't despair. I will send you a new module with fuses and varistors for free, if you send me the address.

Pavel
audioamp

Ouch - I think I'll stick with manual bias. No issues and almost 10 years.

I also recommend one additional mod that hasn't given me any trouble: https://dynacotubeaudio.forumotion.com/t3627p75-auto-bias-board-on-roy-s-website

In fact, I had a power tube go blue and blow the fuse. The tube was bad and it may have saved my auto bias board. I replaced the pair of tubes and the fuse and all was good.

I contacted Erhard Audio and their response was if it makes you feel better do it, but that they dont bother with any of their builds due to extreme rarity of such occurrence.

costerdock wrote:Ouch - I think I'll stick with manual bias.  No issues and almost 10 years.

If an internal short in an output tube has occurred as Pavel has suggested, a manual bias amp may not have escaped damage either. The cause of the problem is the tube, not the bias system in this scenario.

markeby wrote:I also recommend one additional mod that hasn't given me any trouble: https://dynacotubeaudio.forumotion.com/t3627p75-auto-bias-board-on-roy-s-website

In fact, I had a power tube go blue and blow the fuse.  The tube was bad and it may have saved my auto bias board.  I replaced the pair of tubes and the fuse and all was good.

Don Sachs shared with me this solution that he uses on his amps. It’s saved me a couple of times already.  

Aguaazul.