VTA ST70 Auto Bias Hum

Just a quick question regarding my VTA ST70. Since installing the Tubes4HiFI auto
bias board I have noticed I get a low speaker hum during warmup of the
amp. At the point where the auto bias board clicks in (after 40
seconds) the hum disappears. Is this a usual situation, or something I
should be concerned about? If a concern, where would one start
troubleshooting?

Thanks for any suggestions.

Bruce

If all is silent after warm up i'd not worry too much.

As an off side pull the rectifier and see if the speaker hum is present. If so this could be magnetic coupling from power transformer to output transformers. I appreciate that they are positioned such that the magnetic couping is as small as possible but you cannot be 100% perfect.

If this is the case then the reason you hear it is that during the "warm up" the ABQ holds the output tubes at cut off and no anode current is flowing. There is no standing current loading on the output transformers and of course no electrical damping.

I'm sick of those AB boards. Have had three go up in flames.

deepee99 wrote:I'm sick of those AB boards. Have had three go up in flames.

I never took the chance on one, been over 4 years on my M125s with NO problems.

deepee99 wrote:I'm sick of those AB boards. Have had three go up in flames.

interesting, I've not had one issue with them and I have installed quite a few by now. What happens/are the symptoms? Do they just go all by themselves during power up or when they've been running for a while?
One of the main difference I guess with my installations is that I use a small separate 9VAC transformer just for the AB module power, as my AC filaments in my builds are referenced to ground via 100R resistors to eliminate hum, as my custom power transformers do not have a center tap on the filament windings. I doubt though that this could be an issue/cause

deepee99 wrote:I'm sick of those AB boards. Have had three go up in flames.

Curious about this too. First I’ve heard of any issues with them.

deepee99 wrote:I'm sick of those AB boards. Have had three go up in flames.

I'm sorry you're having this problem but could you please elaborate?

My ST120 will undergo the knife in a month or so. Maybe I shouldn't do it..?

New2Tubez wrote:

deepee99 wrote:I'm sick of those AB boards. Have had three go up in flames.

I'm sorry you're having this problem but could you please elaborate?

My ST120 will undergo the knife in a month or so. Maybe I shouldn't do it..?

I won't go any further with details in a public forum. All I can add is that I didn't install them myself and that they failed miserably, the bias running amok to 1.0 which brought a $1500 rack of original British Gold Lions near to red-plate and would have destroyed any of the Russian or Chinese knock-offs. These failures all occurred within a month's time. Perhaps Holger is right and it needs to be powered by its own source. It was never made clear to me whether it was the board itself or something upstream.
I am even giving myself fantasies of a zero-maintenance sand amp. Everything downstream will remain tubular, of course.
Short answer to your question: Don't do it.
I am hoping the fellow who bought my Bill Gratix-built M-125s might be willing to part with them at a reasonable price. Besides, I miss controlling the bias manually in the VTA products. I never went the auto-bias route out of sloth. I bought them to experience their other benefits they purportedly brought. The amp has been on the bench far longer than I've been able to listen it.
Auto-bias isn't an option on Bob's amps. Maybe that tells you something. I don't know what M-125 kits cost these days; they used to be included in Roy's homepage, but I can't find them now.

perhaps we need to be careful here in just blaming the auto bias per se.
Yes, it is unusual that you've had such failures, but we need to hear from others as well to see if they've had any issues, as well as from Roy if he's had any negative feedback.
Also, Pavel, have you heard any such issues?
Again, I've had zero failures in the ones that I have installed in my builds.
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.

Holger, I do believe something downstream is causing the problem - not the AB board. Logic would suggest that, anyways. Point is, I'm done messing with somebody else's problem. On Thursday I will be ordering myself a brand-knew VTA ST 120. It will give me something to fiddle with, probably swap out some caps for really spendy Mundorfs, upgrade the PTP with Dueland wire, pimp it out a little bit, mebbe spray the chassis chartreuse . . . anything to make it stand out from the madding crowd.
My trusted Fluke is still in the top drawer of the hi-fi cabinet (Used it more with auto-bias than I did when I had manual bias.)
I see why Bob L. is so conservative: his amps don't break.

deepee99 wrote:Holger, I do believe something downstream is causing the problem - not the AB board. Logic would suggest that, anyways. Point is, I'm done messing with somebody else's problem. On Thursday I will be ordering myself a brand-knew VTA ST 120. It will give me something to fiddle with, probably swap out some caps for really spendy Mundorfs,  upgrade the PTP with Dueland wire, pimp it out a little bit, mebbe spray the chassis chartreuse . . . anything to make it stand out from the madding crowd.
My trusted Fluke is still in the top drawer of the hi-fi cabinet (Used it more with auto-bias than I did when I had manual bias.)
I see why Bob L. is so conservative: his amps don't break.

Yes, I tend to agree re the auto bias. So we really need to establish WHY you've had such issues and so we NEED to hear from Roy and Pavel, thanks guys!

erhard-audio wrote: 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.

What's the function of that 0.022 capacitor? Reduction of Hum?.

deepee99 wrote:Holger, I do believe something downstream is causing the problem - not the AB board. Logic would suggest that, anyways. Point is, I'm done messing with somebody else's problem. On Thursday I will be ordering myself a brand-knew VTA ST 120. It will give me something to fiddle with, probably swap out some caps for really spendy Mundorfs,  upgrade the PTP with Dueland wire, pimp it out a little bit, mebbe spray the chassis chartreuse . . . anything to make it stand out from the madding crowd.
My trusted Fluke is still in the top drawer of the hi-fi cabinet (Used it more with auto-bias than I did when I had manual bias.)
I see why Bob L. is so conservative: his amps don't break.

All due respect - that’s not at all how I took your original post, or your follow up. It seemed pretty clear that you were suspicious of the AB board. Thanks for clearing it up and I sincerely look forward to seeing what you do with the ST-120! Favorite amp I’ve ever owned.

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 built-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

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

That makes a hellova lotta sense. Maybe after running straight pipes I'll try again with Pavel's invention.

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.

Yep. How do you stuff that power supply into a 120?

Easy to stuff a lot more into an ST120 chassis.... Take a look at the underside of mine. If one of the chokes is moved to the other side you have plenty of room. I have a toroidal transformer mounted under the control logic which could also have fed the AB.

For my AB board I chose to use one of the original heater windings. I made sure that I used the one that only fed the phase inverter. That way the more sensitve center tube can still have the "floating" heater supply.

https://photos.app.goo.gl/MKkn56f4P89HKKo97

Zero issues with the AB board. Works a treat.

My take on "auto bias" is pretty simple. If, once per month or so, I have to get up-close and personal with those two of my vintage amps that require such attention - how could that be a bad thing? I get to check the bias, the connections, check for any funny smells, and so forth.

And, without casting aspersions - an auto-bias board is an act of faith that I am not willing to make.

pichacker wrote:Easy to stuff a lot more into an ST120 chassis.... Take a look at the underside of mine. If one of the chokes is moved to the other side you have plenty of room. I have a toroidal transformer mounted under the control logic which could also have fed the AB.

For my AB board I chose to use one of the original heater windings. I made sure that I used the one that only fed the phase inverter. That way the more sensitve center tube can still have the "floating" heater supply.

https://photos.app.goo.gl/MKkn56f4P89HKKo97

Zero issues with the AB board. Works a treat.

Hi and hello to Aulde Blighty. Thank-you for the picture. Helps to visualize and another handy bit of advice about powering it up. Pavel's power-supply board is almost as big as the AB board. Again, I'm going to run the 120 without any glitter for a few months first. Thrice bitten, thrice shy.
Cheerio,
Dave

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.

Believe that I'm working on it..
Pavel

Autobias is complicated. Who knew?

Peter W. wrote:Autobias is complicated. Who knew?

this kind of statement REALLY DOES NOT HELP!!
Stop spreading fear and unease about the Auto Bias unit, which is an excellent product!
It is best for ALL to wait and get ALL the facts before making negative comments!

With respect, there are now, essentially three (3) full threads with dozens of posts - and no clear resolution of anything, much. We do know:

a) Autobias can work.
b) Autobias often does work.
c) Autobias can fail.
d) Not as often as it works in some cases, but often enough. Why?
e) Solutions are pending.
f) Further solutions are under discussions and/or being researched.

That, for the record, is complicated, at least to me. We also know that Bob's amps work just fine without it. William of Occam stated that one should not needlessly multiply entities. About 675 years ago. When will we ever learn?

Sorry for attempting a bit of humor here, but if driven to the heavy hand, mine ready, willing and able.

Peter W. wrote:With respect, there are now, essentially three (3) full threads with dozens of posts - and no clear resolution of anything, much.  We do know:

a) Autobias can work.
b) Autobias often does work.
c) Autobias can fail.
d) Not as often as it works in some cases, but often enough. Why?
e) Solutions are pending.
f) Further solutions are under discussions and/or being researched.

That, for the record, is complicated, at least to me. We also know that Bob's amps work just fine without it. William of Occam stated that one should not needlessly multiply entities. About 675 years ago. When will we ever learn?

Sorry for attempting a bit of humor here, but if driven to the heavy hand, mine ready, willing and able.  

then it is even more important to wait until Pavel has had a chance to set things straight. And I don't mean with regards to the design and reliability of the Auto Bias module/s. Again, I have dozens out there, never ever has one failed, never!