Need help with auto bias board malfunction

deepee - "I would keep a daily check on the bias. It'll reach its peak after about 20 minutes. "

If the AB is working correctly then measuring the cathode voltages under no signal conditions will show no difference once the system is up to working temp and the AB is in closed loop. Mine stabilises within 1 minute of coming to life.

Where you may see changes would be in the grid voltages on the respective output valves. This is perfectly normal and will depend upon may factors including line voltage, rectifier efficiency, and the OP valve physical charateristics.

For example here are some readings I took on my amp when I installed the AB board.

Line voltage 244V

5AR4 - VHT 509V
Vg 1 -59V
Vg 2 -59V
Vg 3 -60V
Vg 4 -63V

SS Rectifier - VHT 540V
Vg 1 -64V
Vg 2 -64V
Vg 3 -65V
Vg 4 -68V

In all the above cases the cathode voltage was a constant 0.5V and the raw unregulated bias supply read -73.4V after the rectifier.

I have installed about 8 of these boards in amplifiers for myself and customers alike and have never had an issue with any of them.  There are instructions and a wiring diagram sent with the boards which are IMHO very simple to follow.  If a person is having difficulty with these instructions perhaps they should ask someone with more experience for assistance.  Not only are they simple to install, but the overall effect they have on the sound of the amplifiers cannot be denied.  Less distortion as the tubes run much more class A.  They also provide the benefit of extended tube  life.  I have plugged tubes that were creating bias run away into an amp with the auto bias board installed and the board stopped the run away!  No lie.  I have also had a tube fail in my amp with the auto bias board and in normal circumstances this results in blown bias resistors, not the case with the auto bias board.  I really feel they are a wonderful option with much benefit.  If someone is concerned about installation or having issues feel free to PM me and I will help.

Glad you found the problem! Good work!

deepee, don't get me wrong. I'm not saying that the amp will not change sound etc as it warms up. This is an undenable characteristic of a mechanical system and heat. What I am referring to is the measuring of the bias voltages and the fact that the AB will compensate for changes during the warm up period. So in reality you'll not see a great change in the measured bias voltage at the cathodes once the system is in closed loop control.

If the AB is under control then the only items that will affect the readings to any great degree will be the stability of the reference voltage generator circuit and any thermal drift in the 10R bias measuring resistors.

If Google Images will let me share my sketches you'll see a representation of the AB circuit. MODS - If not appropriate please remove links... Ta. Steve.

Grid Drive Comparitor.
https://photos.app.goo.gl/VeMrFxg897N55zAW7

Reference Voltage Generator
https://photos.app.goo.gl/Txg6vS4X18Pxp4Ai9

I like to sketch the circuits of anything I build and work upon for reference. A sketch makes things a lot clearer at a later stage when you've forgotten what you've done and why......

Give Roy a break - without him. there would be no SP14, no VTA driver boards, no VTA site or this forum.

And skip the risk and complication of the autobias board unless you're willing to accept FULL responsibility for your handiwork.  

I'd never even consider this "mod" as necessary.  Get a meter and screwdriver and man up.

"Get a meter and screwdriver and man up."
HA!

hello all,
I've been gone on vacation the past ten days and now I come back and find all this mud-slinging and back-stabbiing going on in the forum.
Please let me kindly remind everyone that the purpose of this forum is for friendly support and sharing of information, not for character or product assassination!
I do apologize (again!) to those who have had trouble with installing the AB board.   Now I will ask you, since no one seems to have been SPECIFIC,
rather than just complaining, would you kindly point out to me what you find in the instructions that is not clear or is confusing?
And please, this is addressed to those who have actually purchased the product, not to the others who have to SPECULATE on things they do have have
personal knowledge of !!  Here is the primary drawing and instructions of the very simple manual, which I would gladly upgrade if someone could stop complaining
and make some useful comments and suggestions on !!   That is the purpose of this forum !!   Also I will say that both Pavel and I are in the process of
creating new instruction manuals for those who need ten pages rather than two pages.
Roy

audiobill wrote:Give Roy a break - without him. there would be no SP14, no VTA driver boards, no VTA site or this forum.

And skip the risk and complication of the autobias board unless you're willing to accept FULL responsibility for your handiwork.  

I'd never even consider this "mod" as necessary.  Get a meter and screwdriver and man up.

Amen to that!

The last sentence, that is.

Peter W. wrote:

audiobill wrote:Give Roy a break - without him. there would be no SP14, no VTA driver boards, no VTA site or this forum.

And skip the risk and complication of the autobias board unless you're willing to accept FULL responsibility for your handiwork.  

I'd never even consider this "mod" as necessary.  Get a meter and screwdriver and man up.

Amen to that!

The last sentence, that is.

Peter and Bill-

There also wouldn't be an AB board here. I don't think Roy would be selling it if he didn't think is was useful.

I accept my handiness and appreciate constructive help from this forum and it's members.

I didn't get the AB as a necessity as I've explained elsewhere in this forum. I know how and do bias my amp- it's a piece of cake.

I did "man up" but CHOOSE to try the AB for the sound and reliability. Not having to remove my tube cage every one or two months is a side affect. I'll install it soon. People coming from building a VTA amp got great, thorough instructions so maybe this is why the AB is a source of contention. Pavel and Roy have answered my questions maybe not on the first go around but they have been helpful enough to where I feel I can get it done. If I hit a snag, consider either helping out or not posting.

Your condescension is less than helpful.

here's the updated manuals as promised . . .

AB-Q M125

AB-Q ST70-ST120

Excellent instructions Roy/Pavel...

After reading just a few helpful comments:-

For the older ST120/70 driver board, like mine, you'll need to lie the two bias filter caps (C17 & C18) down to allow clearance for the AB-Q.

Can you recheck the bias voltages on page 8 as these should match the original figures quoted...
    Page 4 - For the ST70 or ST120 which KT88 power tubes, we recommend a bias level of 0.5V or 500mV. For the ST70 with EL34 power tubes, we recommend a bias level of 0.425V or 425mV
    Page 8 - The meter should read close to 0.425V/425mV if KT88’s are used or 0.380V/380mV if EL34’s are used.

I know it's just for guidance but the schematic diagram shown on page 6 has the power feed to the output transformer shown as coming from the wrong side of the 2k2 resistor. (I'm sure this is a 4K7 on my ST120?? and the M125 PSU looks more like the upgraded ST120.)