Need help with auto bias board malfunction

I just completed the install of the auto bias board in my ST-120. everything went well with the bias setting, holding steady at .500mv and such, so I hooked up the rest of the wires and put the tubes back in and hooked it back up to the system. After the delay kicked off, I got a fairly loud humming through both speakers with the volume all the way down, and turning up the volume didn't make a difference.
I shut it down immediately, and after a couple of minutes I swapped out the rectifier tube with a Weber copper cap and tried it again. Same hum. I shut it down and waited a few minutes for the tubes to cool so I could remove them. I noticed that the left front and right front tubes were both noticeably warmer than the rear tubes, leading me to believe I screwed something up affecting the front tube circuit. All heaters were on all tubes.
I'm pretty sure the problem lies in the driver board and not the auto bias board. Any ideas as what I may have done to cause this? I suspect I might have dislodged a pio cap or something similar, and was hoping someone could point me in the right direction on what to look for.
Merry Christmas to all of you on the forum, you are awesome!

best suggestion is to carry out a complete visual checkover, make sure no other wires have been disconnected, no wires have been incorrectly wired up, especially with the AB installation.
Check your complete AB installation and wiring against the documents.
Not seeing what has been done makes it near impossible to make any pointed suggestions via 'remote'. Perhaps take some detailed photo's and post them here as well.

OK, I went back over everything, and didn't find anything amiss. I added a ground wire from the auto bias board to chassis ground. The board biased perfectly again, but when I installed the tubes and hooked everything up, the hum is still there. I did notice a nice blue aura around the right front power tube before I shut it down.
Now here's something I forgot to mention earlier: I had a tube red plate (left rear power tube) before I underwent installing the bias board. I did notice the 10 watt resistor was toast when I removed it, but I didn't think to check for any other damage. Is there a possibility that I could have damaged something else that is causing the hum problem?
Also, I did disconnect the Time delay board per Bob's instructions with disconnecting the red wires and hooking them back up to their original places, but the other( green/yellow) wires are still connected. Is this the proper way to disconnect this or do I have to go back to the original set-up?
As you can probably tell, I'm pretty baffled by all of this and getting frustrated with myself for not being able to figure this out. I would appreciate any insight as to what might be going on.
The hum sounds like a 60hz hum, not affected by volume control.
Thanks,
Kevin

what did you set the bias reference to on the AB pcb, around 500mV/0.500V?
We also assume that you replaced the power tubes that red plated?
What type of driver tubes are you using. Although I doubt that it is a tube related issue, do you have a complete set of replacement tubes?
Those green/yellow wires are the center tap of the filament supplies and can be left connected to the timer pcb. Did you leave the ground connection to the timer pcb?

erhard-audio wrote:what did you set the bias reference to on the AB pcb, around 500mV/0.500V?
We also assume that you replaced the power tubes that re plated?
What type of driver tubes are you using. Although I doubt that it is a tube related issue, do you have a complete set of replacement tubes?
Those green/yellow wires are the center tap of the filament supplies and can be left connected to the delay pcb. Did you leave the ground connection to the timer pcb?

Yes, I set the bias at 500mv and stayed steady.
I replaced all 4 power tubes with a known good set.Gold Lion KT-88 re-issue. The tube that red plated was a vintage Tung-Sol 6550
I only changed up the red wires on the time delay board.
I have lots of driver tubes. Currently using vintage RCA clear tops on the ends with and Amperex Holland in the middle.
Thanks, Kevin

try this......short both inputs on the amp, power it up and see if you get hum....

kevinmi wrote:

erhard-audio wrote:what did you set the bias reference to on the AB pcb, around 500mV/0.500V?
We also assume that you replaced the power tubes that re plated?
What type of driver tubes are you using. Although I doubt that it is a tube related issue, do you have a complete set of replacement tubes?
Those green/yellow wires are the center tap of the filament supplies and can be left connected to the delay pcb. Did you leave the ground connection to the timer pcb?

Yes, I set the bias at 500mv and stayed steady.
I replaced all 4 power tubes with a known good set.Gold Lion KT-88 re-issue. The tube that red plated was a vintage Tung-Sol 6550
I only changed up the red wires on the time delay board.
I have lots of driver tubes. Currently using vintage RCA clear tops on the ends with and Amperex Holland in the middle.
 Thanks, Kevin

Ok, I tried starting it up again to see if I could hear a difference by turning up the volume on the preamp, and the right front power tube started red plating. I've definitely got an issue somewhere!
I'm going to take it back apart and have a look-see.

yes, that is the only way right now. You seem to have some wiring issues.
Sometimes it is best to 'start over again'.
Take your time, also take note of the wiring as you check it and take it apart and once again, triple and quadruple all wiring to the AB pcb!

Remind me never to put an auto-bias board in my vintage 70 - nor any other tube amp that I own.

A little maintenance on a regular basis keeps you in tune with your amp. Trusting in automation, when it comes to tube amps, is much like trusting that scorpion when swimming across the lake.

Peter W. wrote:Remind me never to put an auto-bias board in my vintage 70 - nor any other tube amp that I own.

A little maintenance on a regular basis keeps you in tune with your amp. Trusting in automation, when it comes to tube amps, is much like trusting that scorpion when swimming across the lake.

this is quite a misleading statement, as it infers that there is an inherent problem with the AB pcb. THERE IS NOT!...PERIOD
Installing one could be somewhat confusing for someone not very familiar with electronics, but that comes down to the individual and not the AB pcb.
I use them in all my power amp builds, never an issue, they work great and take the guesswork out of having to keep BIAS at the correct levels.
Again, for those of us who have a better understanding of electronics, manually adjusting boas is not an issue or a 'scary' thing.
But for those who just want to listen to a tube amp and not be bothered with bias etc., the AB pcb is priceless!
So lets not make posts or statements that could throw a shadow on the excellent AB pcb!

erhard-audio wrote:

Peter W. wrote:Remind me never to put an auto-bias board in my vintage 70 - nor any other tube amp that I own.

A little maintenance on a regular basis keeps you in tune with your amp. Trusting in automation, when it comes to tube amps, is much like trusting that scorpion when swimming across the lake.

this is quite a misleading statement, as it infers that there is an inherent problem with the AB pcb. THERE IS NOT!...PERIOD
Installing one could be somewhat confusing for someone not very familiar with electronics, but that comes down to the individual and not the AB pcb.
I use them in all my power amp builds, never an issue, they work great and take the guesswork out of having to keep BIAS at the correct levels.
Again, for those of us who have a better understanding of electronics, manually adjusting boas is not an issue or a 'scary' thing.
But for those who just want to listen to a tube amp and not be bothered with bias etc., the AB pcb is priceless!
So lets not make posts or statements that could throw a shadow on the excellent AB pcb!

All true - save that we have had a string of failures recently. And, from all evidence, not due to the board(s) themselves as much as failures in installation. Which leads to the inevitable conclusion that the directions that come with said boards is not up to the standards of the original amp instructions.

Does anyone remember how Heathkit wrote their manuals? They would recruit walk-bys at supermarkets, drug stores, wherever, mostly women, mostly middle-aged. They would give them a kit and observe every mistake, every question, and then include preventive, prescriptive, and explicit directions against such things. THAT is how manuals should be written. And, again, clearly not the case with this board.

Not one more of these boards should be shipped or sold without extensive and complete editing and correcting of the directions. For the entire, complete and full fault of these failures lies there. If the Original assembler of the amp could build that amp, then their skills and ability to follow (good) directions is demonstrated. And to be a bit strident and repetitive - this demonstrates the quality of the directions, and therefore the quality of the results - which comes to the same thing. Take nothing for granted. These are your customers, and it is entirely unfair that they should suffer.

Peter W. wrote:

erhard-audio wrote:

Peter W. wrote:Remind me never to put an auto-bias board in my vintage 70 - nor any other tube amp that I own.

A little maintenance on a regular basis keeps you in tune with your amp. Trusting in automation, when it comes to tube amps, is much like trusting that scorpion when swimming across the lake.

this is quite a misleading statement, as it infers that there is an inherent problem with the AB pcb. THERE IS NOT!...PERIOD
Installing one could be somewhat confusing for someone not very familiar with electronics, but that comes down to the individual and not the AB pcb.
I use them in all my power amp builds, never an issue, they work great and take the guesswork out of having to keep BIAS at the correct levels.
Again, for those of us who have a better understanding of electronics, manually adjusting boas is not an issue or a 'scary' thing.
But for those who just want to listen to a tube amp and not be bothered with bias etc., the AB pcb is priceless!
So lets not make posts or statements that could throw a shadow on the excellent AB pcb!

All true - save that we have had a string of failures recently. And, from all evidence, not due to the board(s) themselves as much as failures in installation. Which leads to the inevitable conclusion that the directions that come with said boards is not up to the standards of the original amp instructions.

Does anyone remember how Heathkit wrote their manuals? They would recruit walk-bys at supermarkets, drug stores, wherever, mostly women, mostly middle-aged. They would give them a kit and observe every mistake, every question, and then include preventive, prescriptive, and explicit directions against such things. THAT is how manuals should be written. And, again, clearly not the case with this board.

Not one more of these boards should be shipped or sold without extensive and complete editing and correcting of the directions. For the entire, complete and full fault of these failures lies there. If the Original assembler of the amp could build that amp, then their skills and ability to follow (good) directions is demonstrated. And to be a bit strident and repetitive - this demonstrates the quality of the directions, and therefore the quality of the results - which comes to the same thing. Take nothing for granted. These are your customers, and it is entirely unfair that they should suffer.

I have to agree here. Well written instructions accompanied by some good diagrams/sketches/photo's will make sure that even the hobbyist will understand them.
The AB takes care of a part of a tube amp that is VITAL for its correct operation and if incorrectly installed, can, and does, as has been covered in this forum, cause some pretty nasty and potentially expensive surprises.

I am sure that Roy will see these posts and will make the necessary updates/changes to the installation instructions for the AB-Q series of pcb's.

I've got an older version driver board, the one that uses 12at7 tubes, and I'm thinking about doing a swap since I cannot find anything wrong elsewhere. But I'm afraid if something else got damaged when the tube red plated I might fry that board also. Any suggestions as to what I should check beyond the driver board that might have failed?
I guess in hindsight I should have made sure the amp was in working condition before I installed the auto bias board. Hopefully someone will learn from my goof ups!

Update: I pulled the right side power tubes and turned on the amp. Dead quiet. I put in a pair of good tubes in the right side and turned it on again. Soon as the time delay went off, snap crackle, and pop! So it looks like I've narrowed it down to the right front tube wiring.(right front tube from another set started red plating yesterday) Wish me luck, I'm going in...

I was just thinking and assuming that the amp was working OK before you installed the AB pcb.
Yes, a good lesson learned there

I've been trying to get VTA-like instructions for the AB board for a while. My ST120 sounds so good now that I'm afraid to mess with it.

Monthly or so biasing is no more than a 3 minute job for me. It's not about being out of touch with what my amp's doing. I just want to keep my tube cage on as it will have a wired fan installed in it (posted in another thread). Once I do do the board, I'll also put in the Weber WS-1t that's been waiting to join the party. I even got wiring to match the colors on the wiring diagram. I've had back and forth with Pavel and Roy. Also, my wife wants the table back...

Real instructions would be helpful and they don't have to be to the point of Peter's Heathkit reference. I'm sure more of these boards would be sold as well. You could tack on another $5 or so to cover the time it takes to do this.

Let my clarify a couple of things here. I decided to install the auto bias board because after 9 years of trouble free performance, I had a power tube red plate on me. I decided to install the ab board since I would have to open the hood anyway to replace the grounding resistor. Since I knew the resistors would be eliminated anyway, I went ahead and installed the board.
Well, without any tubes installed, I followed the instructions on setting the initial bias at .5vdc. Everything worked as is should, with the board holding a constant bias. After I installed the tubes, I had problems with hum, and another tube (different socket, different tube) red plated.
This is my fault because I obviously had an underlying problem somewhere that I wasn't aware of before the installation of the ab board. I do not hold the new board or installation instructions accountable for this malfunction. The ab board continues to hold the bias at exactly .5vdc as it was set up.
I'll have to remove the ab board before it destroy it and try to figure out what I have got going on causing the problem. I certainly don't want anyone to get discouraged and not trust the board.
I did have to use a magnifying glass to read the tiny print on the instructions.
Thanks,
Kevin

I sat on this reply overnight. And as I am given to be a bit strident - I am in full Strident Mode:

Any object is a system that includes:

Purpose/Need
Suitability
Functionality
Installation
Operation

What is the need: Managing the output tube bias on a tube amp. The AB Board is offered for this purpose.

It represents itself as suitable. By offering functional examples.

Installation has been both successful and unsuccessful. Where it has been unsuccessful, one (1) defective board is given, and multiple examples of poor instructions are given.

Leading to multiple Operational failures.

The board does not necessarily deserve a bad rap. But the system surely does.

Now, what does Ford, Mercedes, Nissan, Toyota, Creative Playthings, Aldi, Steiff and any of thousands of companies and manufacturers big and small do when they have a faulty product or system? They issue a recall, they make repairs, they do whatever it takes to make their Customers whole. As they are 100% responsible, without reservation, for the conditions created by the defect.

If I were building a new kit, I wouldn't think twice of ordering the AB boards...no brainer. My amps sound and work so nice, for me, the risk of this elective surgery outweighs the reward. Been using GZ33s for a while now, bias is pretty much the same month to month. In fact I went and purchased a couple of spare GX33s since they work so well in the M-125s.

I guess I might sound like a complete newbie by some of the remarks I've read about what might be wrong.

Yes, I re tension my tube sockets every time I switch tubes

Yes, I switch tubes out if they red plated.

Yes, I check for loose connections.

Yes, I know what a cold solder joint is.

Yes, I followed the instructions.

I have 2 tube testers and know how to use them

I have a few multitesters and know how to read them.

I'm not trying to sound snarky, although it does seem to come to me naturally. I'm just trying to point out that something may be amiss that even someone with experience might be missing, besides the obvious stuff.
On another note, I read on another forum that someone had a red plate issue with an amp that has an auto bias board installed that has been working properly. That makes 3 people including me that I have read about so far. I'm not saying that the board is at fault, I'm only saying that we need to find out what we are doing wrong so we can correct this.
Please don't ask me if I plugged it in the correct receptacle (OK, that was snarky)! Happy New Year to all!

nothing is infallible, even the AB
One way to perhaps protect tubes is to use the MOV/fuse installation for each power tube with the AB, details of which can be found here on the forum or on Pavel's web site.
Short of beating a dead horse so to say, the installation instructions do need to be looked at and revamped.

GOOD NEWS! I got the amp fixed!  
 Here's what I found- apparently when I tried to flatten out the Russian PIO caps so they would fit between the boards,I dislodged one of the leads enough to break the trace on the driver board. After running a jumper and checking everything out, presto, we have music. The amp is dead quiet at idle and seems to be working perfectly. It only took me a few weeks of my time and 3 power tubes! I was looking for an excuse to buy more tubes anyway.
 Hopefully my mistakes will help someone out in the future. And like I had said before, I didn't think it was the AB board, but rather something I did wrong. And once again, I was right. Thanks to everyone who offered advice along the way, you are all awesome!

Way cool!
I would keep a daily check on the bias. It'll reach its peak after about 20 minutes. If it trends beyond what you've set it for while no music is playing, the board has a problem. Hope that won't be the case: you've suffered enough! Anyways, congratulations.

excellent to hear, glad its all working for you now! Enjoy tube music!!