VTA ST70 Auto Bias Hum

At this point I have to agree with Peter W, auto bias can work, does work (most of the time) but can also fail to work, for unknown reasons.
At this point, if your amp works, don't fix it. I'm also with DeePee on this, I like to check my bias every couple weeks anyway.
Been doing it for 30+ years now with no failures.

erhard-audio wrote:

mijohn wrote:

erhard-audio wrote:
One interesting point to consider and perhaps Pavel can chime in here. I use a separate mini 9VAC transformer to power the auto bias modules as my 6.3VAC winding are referenced to ground via a pair of 100R resistors to eliminate hum, that is, one 100R resistor from each end of the 6.3VAC winding to ground, since my custom power transformers do not have a center tap in their 6.3VAC windings.
Now, the Dynaco PA060 does have center taps on the 6.3VAC winding and these are tied to ground via a 0.022uF cap each. Does this in ANY way cause an issue with the auto bias module.

Pavel states quite clearly on his website that the ABS modules are "Not suitable where the 6.3VAC filament supply is referenced to ground" and instead indicates the use of another of his ABS modules with build-in 6.3 volt transformer, or by inference as you have used, a seperate transformer.

BTW: I have been using different versions of these modules for 7 years now and have had no issues at all and great performance from them.

https://www.audioamp.eu/en-detail-901899156-modules-for-to-optimize-bias-with-an-supply-of-bias-ab-q.html

yes, exactly, and that is why I use the separate mini power transformer for my builds.
So this is now where Pavel and Roy need to step in. Is using the PA060 6.3VAC winding, with the 0.022uF capacitor tied to ground, the right way for the model AB-Q, or is a separate small power transformer required, or use the model ABF-Q.
I believe this needs to be dealt with quite urgently.

Hi all,

I am sorry to hear about the issues some have been experiencing with the AB-Q auto bias module.
Let me start by saying that I stand by my products of which many have been sold around the world with any ill effects or technical issues.
With regards to the ST-70, ST-120 and M125 amps. Having the center tap of the filament windings connected to ground via the capacitors is quite different to having the center tap connected directly to ground. So, this does not have any negative effect on the operation of the AB-Q module.
One important thing that has come to my attention over many months points to the quality of power tubes, especially for example some examples coming out of China. Due to some being manufactured using lower quality materials, this can lead to 'leakage' between the cathode and filaments and hence the filament windings on a power transformer. In this scenario, this short circuit is through the AB-Q module which will destroy the module.
This scenario can also happen with NOS tubes which have a lot of hours on them.
The only way to overcome this is to use a mini 1:1 isolation transformer on the AB-Q module.
In order to solve any future issues with the ST-70, ST-120 and M125 amps, I have re designed the special Tubes4HiFi pcb's to accommodate a small isolation transformer. This way, the AB-Q can still be powered from the filament winding of the power transformer.
Again, I am sorry to hear about the issues some have had with the AB-Q module.

thank you Pavel for the explanation and update, it is good to know that at least you stand behind the products you market and sell!

pavlikkkk wrote:

erhard-audio wrote:

mijohn wrote:

erhard-audio wrote:
One interesting point to consider and perhaps Pavel can chime in here. I use a separate mini 9VAC transformer to power the auto bias modules as my 6.3VAC winding are referenced to ground via a pair of 100R resistors to eliminate hum, that is, one 100R resistor from each end of the 6.3VAC winding to ground, since my custom power transformers do not have a center tap in their 6.3VAC windings.
Now, the Dynaco PA060 does have center taps on the 6.3VAC winding and these are tied to ground via a 0.022uF cap each. Does this in ANY way cause an issue with the auto bias module.

Pavel states quite clearly on his website that the ABS modules are "Not suitable where the 6.3VAC filament supply is referenced to ground" and instead indicates the use of another of his ABS modules with build-in 6.3 volt transformer, or by inference as you have used, a seperate transformer.

BTW: I have been using different versions of these modules for 7 years now and have had no issues at all and great performance from them.

https://www.audioamp.eu/en-detail-901899156-modules-for-to-optimize-bias-with-an-supply-of-bias-ab-q.html

yes, exactly, and that is why I use the separate mini power transformer for my builds.
So this is now where Pavel and Roy need to step in. Is using the PA060 6.3VAC winding, with the 0.022uF capacitor tied to ground, the right way for the model AB-Q, or is a separate small power transformer required, or use the model ABF-Q.
I believe this needs to be dealt with quite urgently.

Hi all,

I am sorry to hear about the issues some have been experiencing with the AB-Q auto bias module.
Let me start by saying that I stand by my products of which many have been sold around the world with any ill effects or technical issues.
With regards to the ST-70, ST-120 and M125 amps. Having the center tap of the filament windings connected to ground via the capacitors is quite different to having the center tap connected directly to ground. So, this does not have any negative effect on the operation of the AB-Q module.
One important thing that has come to my attention over many months points to the quality of power tubes, especially for example some examples coming out of China. Due to some being manufactured using lower quality materials, this can lead to 'leakage' between the cathode and filaments and hence the filament windings on a power transformer. In this scenario, this short circuit is through the AB-Q module which will destroy the module.
This scenario can also happen with NOS tubes which have a lot of hours on them.
The only way to overcome this is to use a mini 1:1 isolation transformer on the AB-Q module.
In order to solve any future issues with the ST-70, ST-120 and M125 amps, I have re designed the special Tubes4HiFi pcb's to accommodate a small isolation transformer. This way, the AB-Q can still be powered from the filament winding of the power transformer.
Again, I am sorry to hear about the issues some have had with the AB-Q module.

Hi, having just invested a significant amount of money in this topic, I am sorry to hear there are some in the VTA user-base who do not fully embrace this for other than philosophical reasons.

My ship has sailed and I've not experienced any issues, but have probably less than 100 hours usage so far since the upgrade, so won't declare an anxiety-free zone around my VTA until a full year of use is in (it takes a full year of use before I ever declare anything analog truly failure free).

That said, I fully appreciate the added functionality and convenience this upgrade has provided.

One piece of information that might put that topic into more perspective, and may have influenced my decision had I had it in hand prior to the upgrade, is the known failure rate of these boards or at least the most recent version plus the prior 2 revs back (i.e., true failure rates of properly installed boards, not ones that failed due to poor installation instructions, improper installation, etc...).

My hope given what I've got $$ into my VTA-120 is that this number is very small--otherwise I may need 2 years....

Thanks

jsl1234 wrote:

pavlikkkk wrote:

erhard-audio wrote:

mijohn wrote:

erhard-audio wrote:
One interesting point to consider and perhaps Pavel can chime in here. I use a separate mini 9VAC transformer to power the auto bias modules as my 6.3VAC winding are referenced to ground via a pair of 100R resistors to eliminate hum, that is, one 100R resistor from each end of the 6.3VAC winding to ground, since my custom power transformers do not have a center tap in their 6.3VAC windings.
Now, the Dynaco PA060 does have center taps on the 6.3VAC winding and these are tied to ground via a 0.022uF cap each. Does this in ANY way cause an issue with the auto bias module.

Pavel states quite clearly on his website that the ABS modules are "Not suitable where the 6.3VAC filament supply is referenced to ground" and instead indicates the use of another of his ABS modules with build-in 6.3 volt transformer, or by inference as you have used, a seperate transformer.

BTW: I have been using different versions of these modules for 7 years now and have had no issues at all and great performance from them.

https://www.audioamp.eu/en-detail-901899156-modules-for-to-optimize-bias-with-an-supply-of-bias-ab-q.html

yes, exactly, and that is why I use the separate mini power transformer for my builds.
So this is now where Pavel and Roy need to step in. Is using the PA060 6.3VAC winding, with the 0.022uF capacitor tied to ground, the right way for the model AB-Q, or is a separate small power transformer required, or use the model ABF-Q.
I believe this needs to be dealt with quite urgently.

Hi all,

I am sorry to hear about the issues some have been experiencing with the AB-Q auto bias module.
Let me start by saying that I stand by my products of which many have been sold around the world with any ill effects or technical issues.
With regards to the ST-70, ST-120 and M125 amps. Having the center tap of the filament windings connected to ground via the capacitors is quite different to having the center tap connected directly to ground. So, this does not have any negative effect on the operation of the AB-Q module.
One important thing that has come to my attention over many months points to the quality of power tubes, especially for example some examples coming out of China. Due to some being manufactured using lower quality materials, this can lead to 'leakage' between the cathode and filaments and hence the filament windings on a power transformer. In this scenario, this short circuit is through the AB-Q module which will destroy the module.
This scenario can also happen with NOS tubes which have a lot of hours on them.
The only way to overcome this is to use a mini 1:1 isolation transformer on the AB-Q module.
In order to solve any future issues with the ST-70, ST-120 and M125 amps, I have re designed the special Tubes4HiFi pcb's to accommodate a small isolation transformer. This way, the AB-Q can still be powered from the filament winding of the power transformer.
Again, I am sorry to hear about the issues some have had with the AB-Q module.

Hi, having just invested a significant amount of money in this topic, I am sorry to hear there are some in the VTA user-base who do not fully embrace this for other than philosophical reasons.

My ship has sailed and I've not experienced any issues, but have probably less than 100 hours usage so far since the upgrade, so won't declare an anxiety-free zone around my VTA until a full year of use is in (it takes a full year of use before I ever declare anything analog truly failure free).

That said, I fully appreciate the added functionality and convenience this upgrade has provided.

One piece of information that might put that topic into more perspective, and may have influenced my decision had I had it in hand prior to the upgrade, is the known failure rate of these boards or at least the most recent version plus the prior 2 revs back (i.e., true failure rates of properly installed boards, not ones that failed due to poor installation instructions, improper installation, etc...).

My hope given what I've got $$ into my VTA-120 is that this number is very small--otherwise I may need 2 years....

Thanks

1.0 bias will blow up any "post blight" and brought my old Mullards (cost: $1500) just short of there. Not putting that stuff at risk, no way, no how.

1.0 bias will blow up any "post blight" and brought my old Mullards (cost: $1500) just short of there. Not putting that stuff at risk, no way, no how.

Dave, have you tested your Mullards since the meltdown of your modules? Pavel is saying above that it can be a failure in the tube that could take out the AB-Q, not necessarily the other way round.

Not a chance. They didn't red-plate even at 1.0, still making plenty of music.

This is one weird thread. Still can’t tell if Dave is blaming the AB-Q board for the failures or not. My personal conclusion would be that if there’s been three failures in the same system and virtually no failures elsewhere - it’s obviously the system and not the board. Just my two cents.

At least I hope I’m right - I just got my ST-120 back a week ago from $300+ in repairs and installation of the AB-Q board. Little unnerving seeing all this and especially that a new design is coming - trusting that’s why the site now has “email Roy” instead of being able to directly buy the board.

Pavil is coming out next week with a new board with power supply integrated onto the board. Don't have a need for one, of course, but I could be interested after running straight pipes for a year or two down the road. But it's straight pipes for the ST-120 so nothing for 1 year or two. Then I might try it out with "post-blight" (Peter's term) tubes to see if it blows them up.

[QUOTE] 1.0 bias will blow up any "post blight" and brought my old Mullards (cost: $1500) just short of there. Not putting that stuff at risk, no way, no how.[QUOTE]

Great, that's good to hear, especially when there's $1500 at stake!

Is that amp working normally again with the AB-Q removed? As someone else has written above, all the the other issues have been to do with faulty installation of the module and yours is the first failure on this forum that I know of.

deepee99 wrote:Pavil is coming out next week with a new board with power supply integrated onto the board. Don't have a need for one, of course, but I could be interested after running straight pipes for a year or two down the road. But it's straight pipes for the ST-120 so nothing for 1 year or two. Then I might try it out with "post-blight" (Peter's term) tubes to see if it blows them up.

It's not a power supply integrated on the board, it's an isolation transformer that will protect the board from tube failure as I understand it. The AB-Q will still get is 6.3V from the filament winding.

mijohn wrote:

deepee99 wrote:Pavil is coming out next week with a new board with power supply integrated onto the board. Don't have a need for one, of course, but I could be interested after running straight pipes for a year or two down the road. But it's straight pipes for the ST-120 so nothing for 1 year or two. Then I might try it out with "post-blight" (Peter's term) tubes to see if it blows them up.

It's not a power supply integrated on the board, it's an isolation transformer that will protect the board from tube failure as I understand it. The AB-Q will still get is 6.3V from the filament winding.

That is my impression, too.

Dave, have you sent those 3 failed AB-Q modules to Pavel so he can establish the cause of failure. Although, to have 3 failures to 3 replacement modules in the same amp when other failures have been few and far between does suggest an external issue. Your situation is a bit of a mystery.

mijohn wrote:

deepee99 wrote:Pavil is coming out next week with a new board with power supply integrated onto the board. Don't have a need for one, of course, but I could be interested after running straight pipes for a year or two down the road. But it's straight pipes for the ST-120 so nothing for 1 year or two. Then I might try it out with "post-blight" (Peter's term) tubes to see if it blows them up.

It's not a power supply integrated on the board, it's an isolation transformer that will protect the board from tube failure as I understand it. The AB-Q will still get is 6.3V from the filament winding.

Hi, this is the new AB protection.
If varistor and fuse protection is used, AB and OT they are safe.
Pavel

Thanks Pavel, that provides good protection for the AB-Q from a shorted tube.

here's a photo of the new ST70 auto-bias board, same as before but with the isolation transformer in the corner
no longer shown on my web page as I only order them as needed, I have one in stock

How difficult would this be to retrofit when an older style AB-Q is already in place?

it's exactly the same board except the little transformer, exact same connections

slate1 wrote:How difficult would this be to retrofit when an older style AB-Q is already in place?

It is just a 1: 1 isolation transformer for low AC voltage. It may not be on AB and will be connected to the AB 6.3V AC module input according to the diagram n°39.
The mini pcb together with isolation transformer will sell
http://www.erhard-audio.com/AutoBias.html

pavlikkkk wrote:

slate1 wrote:How difficult would this be to retrofit when an older style AB-Q is already in place?

It is just a 1: 1 isolation transformer for low AC voltage. It may not be on AB and will be connected to the AB 6.3V AC module input according to the diagram n°39.
The mini pcb together with isolation transformer will sell
http://www.erhard-audio.com/AutoBias.html

Exactly the information I was looking for - ordered one up. Thanks!

slate1 wrote:

pavlikkkk wrote:

slate1 wrote:How difficult would this be to retrofit when an older style AB-Q is already in place?

It is just a 1: 1 isolation transformer for low AC voltage. It may not be on AB and will be connected to the AB 6.3V AC module input according to the diagram n°39.
The mini pcb together with isolation transformer will sell
http://www.erhard-audio.com/AutoBias.html

Exactly the information I was looking for - ordered one up.  Thanks!  

Great Solution. Better than replacing the board. Good looking out Holger!

I just checked my AB Bias and it is where it is ti be expected.

Aguaazul

I designed and 3D printed a custom bracket for the vta amps to hold the small transformer board in place vertically between the two power tube sockets. It uses the screws on the outside edge of each socket and the trans board is narrow enough so it fits perfectly. I used one in my amp and it made the retrofit very easy. I used Petg to print it so it has a high melting point and should be just fine with the temps inside the amp.

I forgot to take pics of the actual installation but I printed another and took some photos of that. I will open the amp back up when I have a chance and post some more photos if people are curious. I can print a few more and send them to people if anyone wants to give it a try.