Anyone Modify M125 for Individual Bias?

I understand if you use a matched set of output tubes in the M125 the current bias setup should be fine but what happens if you have a tube failure and you have to replace one tube. I was wondering if anyone has modified their M125 for individual bias. If so, do you have a schematic and wiring diagram?

Thanks, Ed

I haven't done what you're requesting. However, if replacing just one tube, disregarding for the moment the amount of hours on the remaining tubes, you can supply the tube vendor with the test measurements from the original tube so they can attempt to choose a tube that matches as close as possible. There should've been test measurements on the box the original tube came in, like this:


I'd be interested to see how the wiring would look for individual bias as well.

I've given up completely on matched quads - I just use four good tubes and set the bias accordingly. If I lose I tube - I just replace it and reset the bias. Been doing it this way a long time.

Also when you use only two output tubes - then you can bias each tube - in fact - I've had different tube brands - sounded just fine - ex gold lion mixed with Sovtek (both KT88s)

Yea with 8 tubes it is almost impossible to keep a matched quad and a fat wallet. One will invariably blow first and need to be replaced, for whatever reason. Honestly, if you pull tubes to work on the amp or whatever reason, things can get mixed up!

Im with rebellovw here, although I dont mix tube vendors and a "try" to get close with the replacement. Just re bias. Never an issue.

The unit ships with a schematic, after a quick look, Im guessing it's more than just adding 2 pots per side. I'd guess that for the M125 this would add more than just two bias pots, and more opportunity to be concerned about "mismatch" (eg probably a 1/2 12AU7/coupling cap per tube). All this happens on the driver board. The 120 has individual tube bias.  *Maybe* you can just use a 120 driver board with a few resistor tweaks to hit M125 B+ voltages and run the input stage in parallel?

I wouldn't overthink it. Just watch your bias every. As I type I am enjoying my (biased) M125 and I have no clue as to whether any tube matches.

Juice is not worth the squeeze in my opinion, and would likely introduce complexity in the preamp stage that could negatively affect the sound.

Bob

Adding auto biasing boards will convert the amps to individual bias control. Pretested modern tubes may start out matched, but I dont believe they stay that way for long. In first 2 years I accumulated enough orphan tubes I figure to see me through next decade, now that I can use them up. Or maybe even longer, enjoying significantly increased tube life now as well.

Rec wrote:

…All this happens on the driver board…

Bob

After looking at it a little more closely I realized I would probably have to modify the driver board or install another board to move the existing bias pots and add two more. Could be done but would be a pain to do. I just rebuilt a Sansui 1000A that had a single bias pot and made a new board to convert it to 4 bias pots.

LeGrace wrote:Adding auto biasing boards will convert the amps to individual bias control. Pretested modern tubes may start out matched, but I dont believe they stay that way for long. In first 2 years I accumulated enough orphan tubes I figure to see me through next decade, now that I can use them up. Or maybe even longer, enjoying significantly increased tube life now as well.

I’m new to this forum but looking over the history it seems like the AB mod has fallen out of favor.

98indypacer wrote:

LeGrace wrote:Adding auto biasing boards will convert the amps to individual bias control. Pretested modern tubes may start out matched, but I dont believe they stay that way for long. In first 2 years I accumulated enough orphan tubes I figure to see me through next decade, now that I can use them up. Or maybe even longer, enjoying significantly increased tube life now as well.

I’m new to this forum but looking over the history it seems like the AB mod has fallen out of favor.

I can only share my personal experience which has been stellar since installing them about 2.5 years ago.  Important to note I installed newer rev 2 boards, not the original problematic rev 1.

LeGrace wrote:

I can only share my personal experience which has been stellar since installing them about 2.5 years ago.  Important to note I installed newer rev 2 boards, not the original problematic rev 1.

Are you running the AB board with a tube rectifier or using solid state?

LeGrace wrote:

98indypacer wrote:

LeGrace wrote:Adding auto biasing boards will convert the amps to individual bias control. Pretested modern tubes may start out matched, but I dont believe they stay that way for long. In first 2 years I accumulated enough orphan tubes I figure to see me through next decade, now that I can use them up. Or maybe even longer, enjoying significantly increased tube life now as well.

I’m new to this forum but looking over the history it seems like the AB mod has fallen out of favor.

I can only share my personal experience which has been stellar since installing them about 2.5 years ago.  Important to note I installed newer rev 2 boards, not the original problematic rev 1.

I have had auto biasing boards in my VTA-120 and M-125s for nearly 2 years and they have been very reliable. I run tube rectifiers in both systems.
I am of the understanding that constant adjustment of the pot on the AB board is detrimental to it's survival.
Both systems are on bucking transformers at 115V. Before the auto biasing boards, I was amazed at the fluctuations in bias voltage due to inconsistant service voltage from the local utility.
Also, this is my experience with the AB boards.

98indypacer wrote:

LeGrace wrote:

I can only share my personal experience which has been stellar since installing them about 2.5 years ago.  Important to note I installed newer rev 2 boards, not the original problematic rev 1.

Are you running the AB board with a tube rectifier or using solid state?

Tube rectifier type GZ37. My experience mirrors that of @JohnC, pre AB bias settings required frequent attention. Post AB zero attention. Oh I'll double check once in awhile but the readings never deviate from original setup.

My bias never needs adjustment. But I use a Variac so the voltage at startup is always the same. Months go by with no need of adjustment.