Auto-bias Upgrade Decision

I certainly agree that auto-bias for my M125/KT-120 amps is a very nice upgrade. Manno van der Veen, Tentlabs, etc. have been pushing this for many years in Europe.
See: http://www.tentlabs.com/Components/Tubeamp/page24/page24.html

But before I go ahead with a purchase I would like/hope to see a number of things included.

1. I would like to see Bob's/Pavel's 6MAR18 interconnect drawing upgraded with socket numbers (more like a true wire list) for the M125, so there is no chance for misinterpretation.

2. I would like to see the Tubes4hifi site include a description of the board and what comes with it and what I must purchase separately (varistors, etc.?) to complete the installation of the AB module. I cannot find much on the Tubes4hifi website.

3. Step-by-step instructions for demoding the M125 add-on time delay module and installing the new AB module for best results.

4. What options one has for purchasing adapter hardware in order to facilitate a clean install. When I think about my experience with adding the time delay module I wished at the time that the M125 chassis was a half inch thicker. The cost would have been worth it to me for the mod flexibility offered.

5. A better picture of the module installed in a working M125 that allows one to benefit from those with more experience. I was one of the first to purchase and build the M125 amps and wished I'd had a good picture of the chassis wiring done by an expert either included or downloadable to help guide my efforts to replicate the best wiring job possible.

any M125 owners with auto-bias installed please post some photos here !

This is one of the things that I want to see before pulling the trigger on a pair of 125's. When I build them, I want them fully loaded!

I like doing my own bias on the m125's, it's part of what makes tubes fun.
I set my bias once after about 30minutes of play time.
One and done.
What's the big deal?
Why do people want to change something that works fine as it is.
If it's not broke..don't fix it!
MO...they are your amplifiers.

bluemeanies wrote:I like doing my own bias on the m125's, it's part of what makes tubes fun.
I set my bias once after about 30minutes of play time.
One and done.
What's the big deal?
Why do people want to change something that works fine as it is.
If it's not broke..don't fix it!
MO...they are your amplifiers.

Though convenient, the need not to have to constantly adjust bias is not “the big deal here”, the other benefits are far more important.

Tube longevity is one of them and I can vouch for that. Variations of these modules have been around for over 10 years and I installed  one in my amp in 2011 and the same set of tubes are still cruising along at 38mA 7 years later!

Another important benefit is that the module restricts the flow of DC current through the output transformer. (DC can saturate the output transformer which results in increased distortion of low-frequency sounds) I’m getting great base performance.

Pavel’s website lists many other benefits:

https://www.audioamp.eu/en-detail-901899168-modules-for-to-optimize-bias-with-an-supply-of-bias-ab-q-st70-st120.html

mijohn wrote:

bluemeanies wrote:I like doing my own bias on the m125's, it's part of what makes tubes fun.
I set my bias once after about 30minutes of play time.
One and done.
What's the big deal?
Why do people want to change something that works fine as it is.
If it's not broke..don't fix it!
MO...they are your amplifiers.

Though convenient, the need not to have to constantly adjust bias is not “the big deal here”, the other benefits are far more important.

Tube longevity is one of them and I can vouch for that. Variations of these modules have been around for over 10 years and I installed  one in my amp in 2011 and the same set of tubes are still cruising along at 38mA 7 years later!

Another important benefit is that the module restricts the flow of DC current through the output transformer. (DC can saturate the output transformer which results in increased distortion of low-frequency sounds) I’m getting great base performance.

Pavel’s website lists many other benefits:

https://www.audioamp.eu/en-detail-901899168-modules-for-to-optimize-bias-with-an-supply-of-bias-ab-q-st70-st120.html

the AB-Q also functions as anode voltage source protection as well as OT protection:
https://dynacotubeaudio.forumotion.com/t3627p175-auto-bias-board-on-roy-s-website

Every other Saturday I take pride waking up early, polishing my stainless chassis, warming up the bottles with some cool Bossa-Nova for a couple hours. Then, muting the inputs and out comes the DMM and screwdriver to touch-up the bias adjustment (all of 3-minutes).  My amp is beautiful, simple by design and works flawlessly, I would hate to pull it apart to install the auto-bias mod. That's the reasoning behind me not investing in one already.

We've missed ya, Ctetus Areetus!
I think like you. But we (you and I) have ears and eyes possibly more attuned to change and malfunction than most - no smear intended to anyone. And the additional benefits of this certain upgrade may be worth considering - except that we add a LOT of components to our circuits, and that might just prove counter-productive.

I'm good without the auto bias. I had 6 years on my last set of output tubes. My new Gold Lions are doing great. I tested the bias this weekend and it only required minor adjustment.

It would be silly to hold up a purchase on this very new feature. Love my M125s

Sound benefits alone should be the thing,I would think.....what am I missing?

Got this info on the PrimaLuna site......Are thay true?

" There are sensors that constantly monitor the performance of each individual power tube in real-time. That data is supplied to active circuitry that adapts instantly and continuously to keep the tube in its linear operating position, regardless of the age or condition of the tube.    
This means the tubes always perform where they are most effective, resulting in the lowest possible distortion, and dramatically improving sound. It also means that you get every last minute of usable life from the tubes, and that saves you money for years to come."

"As far as we know, there are only three companies that have a real “Auto Bias”. The first was PrimaLuna, which introduced it in 2003. The other is Mystere, a sister company to PrimaLuna. The third is VAC (Valve Amplification Company), who offers it only on their Statement 450 IQ model, selling for $112,000 a pair."

Thanks for any an all info on getting better sound out of Dynaco tube Amps.


Thanks for the site Bob...……..

zx wrote:Sound benefits alone should be the thing,I would think.....what am I missing?

Got this info on the PrimaLuna site......Are thay true?

" There are sensors that constantly monitor the performance of each individual power tube in real-time. That data is supplied to active circuitry that adapts instantly and continuously to keep the tube in its linear operating position, regardless of the age or condition of the tube.    
This means the tubes always perform where they are most effective, resulting in the lowest possible distortion, and dramatically improving sound. It also means that you get every last minute of usable life from the tubes, and that saves you money for years to come."

"As far as we know, there are only three companies that have a real “Auto Bias”. The first was PrimaLuna, which introduced it in 2003. The other is Mystere, a sister company to PrimaLuna. The third is VAC (Valve Amplification Company), who offers it only on their Statement 450 IQ model, selling for $112,000 a pair."

Thanks for any an all info on getting better sound out of Dynaco tube Amps.


Thanks for the site Bob...……..

Not only are there sound benefits from using these modules there are sound technical reasons as well.
That PrimaLuna quote could also be describing what the VTA Auto-Bias module does.

I have had my KT88's now for 3years with no auto bias. I am not saying that there is no benefit to auto-bias but not for me. Even as the technology has been here for ten years why has it not been a part of all of BL's VTA amplifiers. Realizing Bob likes to keep things simple and keep the cost down why not offer it as an option?
Sorry I might be thinking old school...we have remotes for everything to keep our A_ _ _ S planted on the sofa.
Just something else in the mix that could and will go wrong...eventually.
Keep it PURE as BL meant it to be!
Auto-bias=less distortion? If what I have is distortion then give me more.
None the less everyone has an opinion and are going to do as they please and there is nothing wrong auto-biasing in itself.
Again it's not for me...I love you just the way you are...m125

To all discussing the auto bias situation, I say, go for it.  I installed one in my VTA-120, the ABQ, and one in my Marantz 8B, the ABS-Q+, which has its own supply of bias like the tentlabs board.  As far as installation, its easy.  However, if you want assistance go to audioamp.eu and email Pavel.  If you email him a schematic of your amp, he'll redo it to show you exactly how to work it into the circuit.  

In my VTA-120 I used a small toroidal transformer to provide the 6.3vAC to power the board because I did not want to use the 6.3vAC from the amp's transformer.  In my Marantz 8B I used an even smaller toroidal to power the board because, since the 6.3 volt winding in the the Marantz is center tapped and grounded, and I did not want to lift the ground, I had to do it that way.

None the less, subsequently I installed the 115vAC ABS-Q+ in the Marantz so I don't need to power the board with 6.3vAC.  It is powered from the mains voltage, 115vAC +/- about 5 volts, so it is for North American use.  There is a 230vAC version available also.  The ABS-Q+, as I noted, has its own bias supply, like Tentlabs, so I don't need to use the amp's bias either. Also, both boards are fused on the primary side of their transformers.  Installing these boards has improved the sound, and makes the amps that much more pleasurable to listen to and live with.

In the VTA 120 I further installed digital bias meters at the front circular cutouts of the amp.  I used a double throw/double pole switch put in place where the cutout on the front of the amp for the stereo/mono switch, which is not used on the VTA-120, would go.  I also installed a separate on/off switch on the back of the amp for the meter's power supply so that I can turn the meters off while the amp is still on after I've checked the bias.  With the ABQ board it is always spot on; and with the dpdt switch I can check the two back tubes, switch in up position, independently, and the two front tube, switch in down position, independently.   In the middle position the meters read zero, but, as noted, when I am not checking the bias I can turn them off.

So, putting the board in is relatively simple.  Pavel provided me with redone schematics for both amps.  However, the actual logistic of it was a bit more complex for the VTA-120 then it was for the Marantz.  The Marantz is all point-to-point wiring, so removing the bias pots from the circuit was easy.  Of course I left the bias pots in place for aesthetic reasons, but they are not necessary with the board.  With the VTA when I  built it, I installed the bias trimpots, all four of them, on the driver board, again for aesthetic reasons, but it was necessary to not install certain resistors, small ones, on the board to remove them from the circuit.  This was absolutely not a problem since I can read a schematic.  There were other slight modification I had to do to the driver board, but the amp's original schematic, and Pavel's redrawn schematic helped there too.

Anyway, when I last posted here I said I was going to upload some pictures.  When I tried, I was told I was to new to the forum to have the privilege of posting pictures.  Hopefully I have been on the forum long enough now, though not posting much, to now have that privilege; so very soon I will post some pictures to show the VTA with all the features I have add to it, including the DC heater board I add to it.  But that is another story.

gguareri wrote:To all discussing the auto bias situation, I say, go for it.  I installed one in my VTA-120, the ABQ, and one in my Marantz 8B, the ABS-Q+, which has its own supply of bias like the tentlabs board.  As far as installation, its easy.  However, if you want assistance go to audioamp.eu and email Pavel.  If you email him a schematic of your amp, he'll redo it to show you exactly how to work it into the circuit.

Appreciate very much the input. Look forward to seeing your pictures. I, too, believe sonic performance and reliability (i.e. medium and long term costs of operation) will benefit from auto bias. Although I already utilize a Circuit Specialists TDGC2-2 with 20 Amp Max Output that Bob recommended years ago to protect against the nominal 122 VRMS we see where I live (set to 117 VRMS). I believe auto-bias adds additional protection plus sonic quality improvements especially at lower listening levels when one considers speakers driven by amps using OPTs.

Here's a quote from one of several Menno van der Veen papers on the subject;

van der Veen wrote:"It proves that small signal levels get weakened and it shows that the calculated and measured starts of weakening lay at different output levels (they differ almost a factor of 10 which equals 20 dB). What is the cause of the difference? Closer research showed that the quiescent currents of the power valves were not exactly equal during this measurement, they differed about 0.3 mA, which is little. This small difference current was able to DC-magnetize the core of the OPT and consequently the magnetic domains became extra bound to their position. This created the extra weakening in the measured situation. If we apply the new auto-bias unit, then the quiescent currents are equal within 0.05 mA (or better) and the dotted line shifts left towards the red line. In other words: without any DC-magnetization of the core we can hear almost 20 dB deeper into the sound before any extra weakening of the micro details occur. Now it is clear why we hear the micro details much better with the auto-bias unit."

SEE: https://mennovanderveen.nl/eng/autobias-improves.html
Especially going from higher to lower audio level after a sufficient period can cause sonic details to recede due to DC magnetization of OPT core.

For my situation I have one M125/KT120 unit that continues to exhibit some kind of discharge noise through my speaker when I turn the power off. Seems to be getting more pronounced. I have mentioned this to Bob but couldn't get certain feedback on the issue. I may go ahead and buy AB board from Roy and then ship my one M125/KT120 unit with issues to Sal Brisindi (Salb203@aol.com) that Bob recommends to get some analysis(?), rewiring, removal of Time Delay unit and install AB board in unit then try to reproduce what Sal accomplished in my second unit, if that makes sense and I can arrange with VTA powers that be. I also like your idea of using a separate small toroidal transformer for the AB board as my M125 pwr trans already gets very warm but may not be possible for M125s.

costerdock wrote:I'm good without the auto bias.  I had 6 years on my last set of output tubes.   My new Gold Lions are doing great.   I tested the bias this weekend and it only required minor adjustment.  

It would be silly to hold up a purchase on this very new feature.    Love my M125s

I'm with you "costerdock"!
I love my m125's just the way they are.
Since self bias has been around for a decade I would think that even left as an option BOB LATINO thought about keeping the m125's as well as the ST70 and ST120 simple for the the DIY'er. Although I have not posed the question to Bob about him supporting the auto bias I am thinking that people wanting to re-invent the wheel will be on their own.  
If I were in Bob's position I would wash my hands of it entirely.

I do wonder what the negatives are to auto bias since nothing is perfect.

Can't think of too many negatives except for the added complexity.

I've been following this discussion closely. Something in the back of my mind has been bothering me, and it has finally flashed into consciousness.
From my limited understanding, output tube bias is set at idle, to properly present the tubes with a "point of departure", and that the actual bias during operation varies according to the musical signal being amplified. How, then, can a constantly regulated bias current be anything other than a distortion? Also, there must in fact be some small time-delay involved in the auto-correct of this current, which to my {limited} understanding should wreak some kind of havoc on the quality of the musical signal. As in phase irregularities, etc.
Add the sheer complexity of this add-on which is in the signal path, and I have serious reservations. Someone enlighten me, please. Like a big ol' candle in the wilderness.

The auto bias is not any more in the signal path than the original fixed bias pots would be. So this should not alter the sound quality.

In relation to controlling the bias point you need to consider the damping of the control loop. For the auto bias circuit this is in the order of 10's of seconds so a transient audio signal will not affect the bias voltage to any great degree.

If the control loop was too quick to correct then you would be correct in your assumption as the bias would be attempting to "correct" the cathode current changes as the tube drove the output transformer. A sort of negative feedback. But of course this is not the case.

Fear not, from my measurements, the circuit works very well and will track long term changes in line voltage to maintain a good standing bias. Of course short term transients will still make a change if they are significantly shorter than the control loop damping factor.

Ah-hah! You've perfectly resolved my dilemma, sir. My hat goes off to you.
So the auto-bias board is designed to maintain an overall constant rather than micro-managing the bias current. Now it makes sense.

Reading through the Tentlabs article it does make mention of a switch connected to one version of their board that reduces the bias current set point. Great for standby operation or where complete fidelity is not required.

This basically reduces the common reference voltage to the comparitors and decreases the bias set point on all 4 tubes at the same time to a common lower vaule. This in thoery could extend tube life and reduce standby current consumption / heat. Makes for a lot happier 5AR4. 6550's

Of course this would not be so easy using the standard fixed bias controls.

I have to admit that I srtuggle to hear a diffence in SQ between 40mA and 20mA standing current but of course I do not have any instruments to measure the effects on power output, distortion etc. Only my calibrated ears.

If you set too low a current then the sound does become brittle, cold and generally not nice.

Our "calibrated ears" are the best instruments we have when it comes to measuring SQ!

The rationale behind the switch on that version of the module is discussed in this link written by the designer, Menno van der Veen.

https://mennovanderveen.nl/eng/autobias_2011.html

bluemeanies wrote:I like doing my own bias on the m125's, it's part of what makes tubes fun.
I set my bias once after about 30minutes of play time.
One and done.
What's the big deal?
Why do people want to change something that works fine as it is.
If it's not broke..don't fix it!
MO...they are your amplifiers.

Reason I will put in a AB board is for future use by my wife when I am gone. I'm 70 and don't have much time left.
I would like her to use it as long as possible without relying on a son in law that "might" be up to putting in a new tube
if he could identify with one was bad.