Issues with M125 after Auto Bias install

OK all M125 users, thanks to our friend Holger for showing me my own "blind spot".
When I wire these amps up myself, I never use step-by-step, I just look at the diagram and do it right.
But my thoughts didn't translate to the written directions correctly, so now I have updated them,  here's the correction in RED for the V1 & V2 outputs on the AB board.
My deepest apologies !!!!   (this is all so much simpler on an ST70 amp)
and here is the PDF document to go with the photo below . . . www.tubes4hifi.com/ABC-M125-install.pdf

this might be a better view of where those V3 & V4 grid wires go (where the center pin of the old bias pot and R30/R32 are) and this shows the trace connection on the driver board

Great, Thanks Roy, I will correct tonight and post my results. Thanks to Holger as well.

That fixes it, it is again loud and clear. Sounds great. When I purchased I did not recive instruction for the first parting setting the unit. One amp doesn't bias well, Voltage on C13 lower than voltages on tubes pin 1. Could the issue be not setting to a reference? How can I do that now that hey are installed?

ericoto wrote:That fixes it, it is again loud and clear. Sounds great. When I purchased I did not recive instruction for the first parting setting the unit. One amp doesn't bias well, Voltage on C13 lower than voltages on tubes pin 1. Could the issue be not setting to a reference? How can I do that now that hey are installed?

The following is to set the bias for the output tubes once the AB pcb has been installed and wired up to an amp. It may not be the solution to your particular issue, but it is still worth while to go through this process just to make sure both amps are set up the same!

power down your amp and let the power supply caps discharge. Remove all tubes, including the rectifier tube.
Power the amp back up and you should see the small RED LED on the AB pcb light up. It will go off after about 10-15 seconds. Now, set your voltmeter to read no more than 2V or similar, put the black lead on a GND pad on the AB pcb and the red meter lead on the test pad next to the electro capacitor which itself is next to the blue trimmer, see below.
Now adjust that blue trimmer to read say 0.500V or 500mV, which is a good setting for the KT120. You can go as high as 600mV, but that will shorten the tube life. I found 500 to 525mV is a good value.
Once adjusted, power down your amp. Re install all the tubes, power it back up, that small RED LED will go on first then go off after about 10s or so and after about 40-50s all four small BLUE LED's should go on indicating that bias is good for all four output tubes. You're done!
I would do the same process with both amps, set them both to the same 500 or 525mV.

After all these years on this forum I still trying to figure out why pics posted by Roy or Bob never fit my computer screen

pedrocols wrote:After all these years on this forum I still trying to figure out why pics posted by Roy or Bob never fit my computer screen

Lessen the 'zoom' on your browser.  I can see most of the images with the zoom down to 75%.  Hope that helps. It's real tough on a mobile device.

Cheers,

Bluewater

aguaazul wrote:

pedrocols wrote:After all these years on this forum I still trying to figure out why pics posted by Roy or Bob never fit my computer screen

Lessen the 'zoom' on your browser.  I can see most of the images with the zoom down to 75%.  Hope that helps. It's real tough on a mobile device.

Cheers,

Bluewater

Or just hold down your CTRL button and spin your mouse wheel fore or aft.

deepee99 wrote:

aguaazul wrote:

pedrocols wrote:After all these years on this forum I still trying to figure out why pics posted by Roy or Bob never fit my computer screen

Lessen the 'zoom' on your browser.  I can see most of the images with the zoom down to 75%.  Hope that helps. It's real tough on a mobile device.

Cheers,

Bluewater

Or just hold down your CTRL button and spin your mouse wheel fore or aft.

Yep, easy way to zoom in & out!

aguaazul wrote:

pedrocols wrote:After all these years on this forum I still trying to figure out why pics posted by Roy or Bob never fit my computer screen

Lessen the 'zoom' on your browser.  I can see most of the images with the zoom down to 75%.  Hope that helps. It's real tough on a mobile device.

Cheers,

Bluewater

That's exactly my point.  I shouldn't have to be doing that. I am sure there is a way in which they can post so that isn't necessary.

deepee99 wrote:

aguaazul wrote:

pedrocols wrote:After all these years on this forum I still trying to figure out why pics posted by Roy or Bob never fit my computer screen

Lessen the 'zoom' on your browser.  I can see most of the images with the zoom down to 75%.  Hope that helps. It's real tough on a mobile device.

Cheers,

Bluewater

Or just hold down your CTRL button and spin your mouse wheel fore or aft.

Yes but if I do that then I can't read the content because then the text will be way too small to read.

I was thinking about you after reading this and hoping you would read this too.

[quote="Skydogcable"]

tubes4hifi wrote:OK all M125 users, thanks to our friend Holger for showing me my own "blind spot".
When I wire these amps up myself, I never use step-by-step, I just look at the diagram and do it right.
But my thoughts didn't translate to the written directions correctly, so now I have updated them,  here's the correction in RED for the V1 & V2 outputs on the AB board.
My deepest apologies !!!!   (this is all so much simpler on an ST70 amp)
and here is the PDF document to go with the photo below . . . www.tubes4hifi.com/ABC-M125-install.pdf

To avoid further doubts and disputes, I am creating a very detailed installation guide for AB-Q ST70 and AB-Q MK125.
I will publish it here on the forum.
Pavel

Well, corrected the Ab board wiring, one M125 works great, other initially seemed to work, but bias was off and different on all 4 tubes, some much higher than at the c13 pad. Initially all 4 blue lights lit up, now only 2 do, the ones on v2 and v4, wiring looks the same, swapped out output tubes no changes. Any thoughts? I did remove and reinstall these several times when they were not working, could have I have damaged something else? Any thoughts/assistance appreciated.

Please all note, I just read a post from Pavel in the other thread, that he is working on a completely new and revised AB-Q-M125 module installation document.
It may be prudent to wait until this is released?
It may take a few days but is probably worth while to down tools until the revised instructions are released.

I did remove the jumper, I did also here from Pavel who wants to check the unit out. Will ship back to him, but another month w/o the M125's, will also look forward to his instructions, I have an older m125, was planning on updating driver board anyway. Thanks for comments.

follow up - both problems were solved by both Skydog and Ericoto

also - completely new install directions fully updated are here . ..

new detailed assembly manuals AB-Q ST70-ST120 and AB-Q MK125.

AB-Q M125

AB-Q ST70-ST120

Ref M125, is AB board compatible with 6550 tubes? PDF only mention KT88 or KT120. I am using 6550.

LeGrace wrote:Ref M125, is AB board compatible with 6550 tubes? PDF only mention KT88 or KT120. I am using 6550.

Yes, the AB's are NOT 'tube dependent'. What you will need to check is the recommended bias level for the 6550 or any other tube you may be using. I would say just under the KT88, so around the 400mV mark or so.

My post was deleted. Is there a reason exactly?

jfine,
a couple of older posts were deleted as I cleared out some older conflicting info and updated and clarified the thread, so as not to confuse anyone coming into this thread as a newbie.

I'll try and make it clearer, sorry about that:

It's possible that the AB board for an M125, without the correct installation instructions, can cause issues, (my speakers were lucky not to get damaged). It's prudent to be aware. It could be an expensive lesson.

It also helps to have a history of this issue, as that can greatly help in troubleshooting if ever another problem were to creep up.

definitely correct, which is why the M125 (and also the ST70-120 version) install instructions have been completely re-written, all is OK now,
unfortunately the instructions weren't bullet-proof as originally written