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    Auto-Bias board on Roy's website?

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    Re: Auto-Bias board on Roy's website?

    Post by tubes4hifi on Tue Apr 24, 2018 5:50 pm

    oh, forgot to show all the wiring connected, so here's a photo showing how the board is wired (except I don't have any output tubes in the bare chassis ! Very Happy

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    Re: Auto-Bias board on Roy's website?

    Post by tubes4hifi on Tue Apr 24, 2018 7:18 pm

    next up is the M125 version, a little more involved for install since you need to add two capacitors and two resistors.
    On a stock M125, the output tubes are connected in pairs.   With this board EACH tube will be individually controlled to EXACT bias and balance.  
    I'll supply the resistors, you supply matching caps depending on what you currently have (or let me know and I can order duplicates for you).
    Pictured here is the M125 board under test, and showing the big K40Y caps which are slightly larger than the current standard K42Y (green) caps.
    One photo shows the mini red LED during warmup.  This one came preset for 58ma per tube, I readjusted down to 50ma as per Bob's current recommendation,
    although most KT88 amps are fine with 55-65ma so you could just leave it as is.   That was about 1.5 turns CCW. Same nice slow 40 second ramp up.



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    Re: Auto-Bias board on Roy's website?

    Post by tubes4hifi on Wed Apr 25, 2018 12:19 pm

    for the M125s which are running two pairs of output tubes, you need to add two capacitors and two resistors, so that EACH tube can be controlled individually
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    Re: Auto-Bias board on Roy's website?

    Post by tubes4hifi on Wed Apr 25, 2018 12:26 pm

    yes, the ten ohm resistors on the auto-bias board replace the ones that were in the amp originally, in the same location, so they still have the same function.
    It's nearly impossible to blow one of these out, as they are 1w resistors, and if you have 0.5v of bias running thru ten ohms, that is 50ma x .5v = 25mw.
    To blow out a 1w resistor would require more than 300ma of tube current (300ma x 3v= 0.9 watts).
    Of course with a functioning auto-bias circuit, this would NEVER happen. If the tube goes bad and tries to run-away with current, the auto-bias circuit will set the current thru the tube at zero immediately.
    This is one of the two huge benefits of auto-bias, you should never ever lose a tube due to red-plating. The other benefits of course are never having to set bias, and always having PERFECT bias and balance.
    Perfect output symmetry, minimal distortion.
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    Re: Auto-Bias board on Roy's website?

    Post by jfine on Mon Apr 30, 2018 7:05 pm

    So for the M125's, once the boards are completely installed, what is the best way to check the bias? Power it up, do I need any tubes installed, I noticed the positive test pad next to the bias adjustment, but do I just ground the other lead to chassis, etc., is that it?
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    Re: Auto-Bias board on Roy's website?

    Post by tubes4hifi on Tue May 01, 2018 12:01 pm

    jfine
    connect the filament voltage (6.3vac on AB board to V2 output tube pins 2 & 7, then power up amp (no tubes in amp needed).
    After 40 seconds AB red LED will go out, and bias voltage on test point will ramp up to either 0.40vdc (on ST70 boards) or to 0.58vdc (on M125 boards).
    Test point is round dot right next to C13.  Red meter lead to test point, black meter lead to chassis ground, or ground hole on AB board.


    Last edited by tubes4hifi on Tue May 01, 2018 12:37 pm; edited 1 time in total
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    Re: Auto-Bias board on Roy's website?

    Post by tubes4hifi on Tue May 01, 2018 12:05 pm

    OK, so I don't have an amp here, but here's a drawing that should make this a simple install in the ST70 or ST120 amps.
    Download the photo from here to see it full size.  http://www.tubes4hifi.com/AB-ST70install3.jpg
    It's color coded, so a B&W print won't help much!
    Connections to R29, 30, 31, 32 are to the opposite end of outputs on VTA board (the ends that used to connect to the bias pots !!)

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    Re: Auto-Bias board on Roy's website?

    Post by pichacker on Wed May 02, 2018 6:13 am

    How robust is the auto bias board to tube flashovers?

    If a tube goes short, or draws high current for any reason, then there is a possibility of a high voltage being presented to the cathode path. On a stock version this fuses the 10 ohm resistor. On the auto bias board this would of course make its way to the sensing circuit that would normally only be looking for 0.6V. With a defective tube we would have little control using the grid bias voltage.
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    Re: Auto-Bias board on Roy's website?

    Post by mazeeff on Thu May 03, 2018 7:27 am

    pichacker wrote:How robust is the auto bias board to tube flashovers?

    If a tube goes short, or draws high current for any reason, then there is a possibility of a high voltage being presented to the cathode path. On a stock version this fuses the 10 ohm resistor. On the auto bias board this would of course make its way to the sensing circuit that would normally only be looking for 0.6V. With a defective tube we would have little control using the grid bias voltage.

    I share the same concern. Perhaps someone with the board can describe the part used to control the cathode current. That part would need to be able to withstand 500v on it, given a tube failure. A part number or schematic would be helpful in determining the max voltage of the part. I would also like to convince myself that the circuit prevents blowing the 10 ohm resistor, as Roy pointed out earlier. When these 10 ohm resistors blow, they generate an immense amount of heat. That heat may jeopardize the circuit board, if the 10 ohm resistors continue to act as fuses, in the event of tube failure.
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    Re: Auto-Bias board on Roy's website?

    Post by corndog71 on Thu May 03, 2018 9:19 am

    You can always add fuses between the cathodes and their respective auto-bias connection.  But you can't just throw in any ordinary fuse.  You need a high breaking ceramic fuse like this https://www.digikey.com/products/en?keywords=%20F4603-ND   This will work for each power tube of an ST70, ST120, and each paralleled pair of tubes of the M125.

    I know what some are thinking.  Why use a $1.50 fuse when you can just let a 0.10 cent resistor burn up?  Well, if you've ever smelled a resistor go up smoke then you know it's not a pleasant experience.  Then you have to unsolder it and resolder a new one.  It's a convenience to simply replace a fuse.  Granted there's not a lot of room in the classic dynaco chassis.  This is another reason why I prefer a custom layout.
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    Re: Auto-Bias board on Roy's website?

    Post by Bennyhaha812 on Thu May 03, 2018 6:40 pm

    Pretty sure I read earlier that if a tube dies power is immediately clamped off preventing the blowing of the resistor by this bias board so no more blown resistors! I wish I could remember where I read this and link it......
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    Re: Auto-Bias board on Roy's website?

    Post by mazeeff on Fri May 04, 2018 7:09 am

    The problem of what happens when the auto-bias circuit encounters a tube with a plate to cathode short can be easily tested by Roy and/or Pavel. With the power off, remove a output tube, and short the plate to cathode, with a large jumper. This would emulate a common problem associated with redplating, where the excess heat of redplating results in metal fatigue, and quite often a short. This type of failure would put 400-500 volts on the cathode, and would normally vaporize the 10 ohm resistor. With eye protection, and a fire suit, flip on the power and see what happens. If the silicon based current limiter and/or the 10 ohm resistor fails, we have a problem. If the circuit works as Roy described in post #79, and limits the current, then we are fine. The alternative, is for Roy or Pavel to disclose part numbers associated with the current limiting circuit. With that information, we could quickly determine if the circuit can withstand a 500v short. In the past 10 years, I have had three tubes fail with a internal short like this, and two times it took out the 10 ohm resistor. Based on reading the forum, a number of folks have blown a 10 ohm resistor, like myself. It is not a rare event. I have wondered, if Roy purposely designed the VTA with the 10 ohm resistor external, because it is easier to replace when it is soldered to a socket and ground. Without an answer to this, we will just have to wait until one of the early adopters encounters a tube failure resulting in a short!

    PS: Please do not take my posts as a criticism, in any way. I am a old retired EE, who was taught the value of being a "devil's advocate" early on. We used to hold VERY tough design reviews, where my EE peers were encouraged to find faults in my designs. Saved my Bacon more than once!
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    Re: Auto-Bias board on Roy's website?

    Post by pavlikkkk on Fri May 04, 2018 8:18 am

    mazeeff wrote:The problem of what happens when the auto-bias circuit encounters a tube with a plate to cathode short can be easily tested by Roy and/or Pavel. With the power off, remove a output tube, and short the plate to cathode, with a large jumper. This would emulate a common problem associated with redplating, where the excess heat of redplating results in metal fatigue, and quite often a short. This type of failure would put 400-500 volts on the cathode, and would normally vaporize the 10 ohm resistor. With eye protection, and a fire suit, flip on the power and see what happens. If the silicon based current limiter and/or the 10 ohm resistor fails, we have a problem. If the circuit works as Roy described in post #79, and limits the current, then we are fine. The alternative, is for Roy or Pavel to disclose part numbers associated with the current limiting circuit. With that information, we could quickly determine if the circuit can withstand a 500v short. In the past 10 years, I have had three tubes fail with a internal short like this, and two times it took out the 10 ohm resistor. Based on reading the forum, a number of folks have blown a 10 ohm resistor, like myself. It is not a rare event. I have wondered, if Roy purposely designed the VTA with the 10 ohm resistor external, because it is easier to replace when it is soldered to a socket and ground. Without an answer to this, we will just have to wait until one of the early adopters encounters a tube failure resulting in a short!

    PS: Please do not take my posts as a criticism, in any way. I am a old retired EE, who was taught the value of being a "devil's advocate" early on. We used to hold VERY tough design reviews, where my EE peers were encouraged to find faults in my designs. Saved my Bacon more than once!

    Experiences:
    from the beginning of the sale of auto bias modules, there were four cases of burned 10 Ohm resistors in a tube failure (internal short circuit).
    Always burned only a carbon cathode resistor 10 Ohm and nothing else not happened. I just changed the resistor and everything was OK.
    In one module, the IC (integrated circuit) in the stabilizer were destroyed. When a tube failure (650V B+) Burned arc on the module and destroyed IC. After washing PCB and replacing IC, the module was ok.
    Pavel
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    Re: Auto-Bias board on Roy's website?

    Post by pavlikkkk on Fri May 04, 2018 11:10 am

    mazeeff wrote:Pavel. Thank you for a solid answer. Four 10 ohm failures, seems to justify our concerns. Back to Corndog71's idea of a fuse. If the fuse has a consistent 1.24 ohms, why not use this value as the bias resistance? You could easily mount PCB based fuse holders to make replacement easier. If I understand it, the value of 10 ohms was chosen to simply make the math easier! Would 1.24 ohms work just as well? Of course, these fuses would need to be 1.24 +/- 1% or so (same tolerance as the resistor). Pavel's experiences seem to run counter to Roy's post #79.

    you understand it correctly. The conversion is 10. When you add the fuse, it will be 11.24.
    For Icath 50 mA x 11.24 = 562 mV
    But beware ... The fuse is not a resistor. It does not have a constant resistance. But with the 10 Ohm resistor it is still a small error and it can be used..
    Pavel
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    Re: Auto-Bias board on Roy's website?

    Post by jfine on Fri May 04, 2018 10:13 pm

    Anyone got some detail pics of this installed in the M125 yet? Smile
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    Re: Auto-Bias board on Roy's website?

    Post by Bob Latino on Fri May 04, 2018 10:50 pm

    jfine wrote:Anyone got some detail pics of this installed in the M125 yet? Smile

    See photo below of the Tubes4hifi AutoBias board installed into a VTA M-125 working great with all 4 blue lights indicating that the board has the proper control of the bias on each of the 4 output tubes. The front of the amp is on the right side of the photo.

    Bob




    Last edited by Bob Latino on Sat May 05, 2018 6:46 am; edited 1 time in total
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    Re: Auto-Bias board on Roy's website?

    Post by jfine on Fri May 04, 2018 11:41 pm

    Great shot! it'd be nice to see more of an overview top view of the wiring, maybe under the board as well, if possible.
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    Re: Auto-Bias board on Roy's website?

    Post by mazeeff on Sat May 05, 2018 5:29 am

    First of all, let me state that I have the utmost respect for Bob, Roy & Pavel. Your products have brought a great amount of joy to my life. I have three VTA amps, and my St120 runs 12 hours a day every day in my shop. Recently, my sister stopped over to listen to the ST120 and my Klipsch Cornwalls. She wants a ST120 real bad, but has no technical skills, and refuses to consider even biasing a amp. I live too far away to make repairs, so she needs a amp that is easy to maintain. Pavel's auto-bias board opened the door for her to own one of these. I only offer the following suggestions, to make it a little easier for someone like my sister to enjoy these amps.

    1. Make the 10 ohm resistors as easy to replace as possible. Perhaps using a multi-screw terminal strip or similar. No soldering/unsoldering would be great for those like my sister! If those resistors could be up top, then they could be easily replaced without too much skill, and no need to flip and open the amp.

    2. Move the auto-bias led's to the top, so that they can be easily seen. We are going to want to show off those multi-color led's to our friends, and they provide feedback that everything is ok.

    3. Consider combining the VTA and Auto-Bias boards into one big board. This eliminates wiring, and cleans things up a bit. It would also provide a easy way to get the led's on top, and fix the 10 ohm mounting.

    4. Move the bias adjuster to the top as well! This will aid in more easily changing the bias target for KT88's (50ma) and KT120's (55-60ma) in a ST120. Helps the tube swappers!

    Personally, I am fine with adjusting my own bias, and soldering/unsoldering 10 ohm resistors. If the product is made easier (like with Pavel's auto-bias), then it opens up the market for non technical folks like my sister. I would bet that 99% of VTA owners are male with at least some level of technical and/or mechanical skills. Might be nice to get some females involved, and get them on the forum! Of course, Skizoid and deepee99 would have to behave themselves, if my sister joined the forum!

    Roy has surprised us before, in how quickly he can design and produce new boards!!!!!

    Also, my sister will be curious as to whether Bob will offer the auto-bias option in a fully assembled amp?
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    Re: Auto-Bias board on Roy's website?

    Post by jwb474 on Sun May 06, 2018 2:18 am

    Here are a couple of pictures of the final form for the Auto-Bias installation on my old ST70. I set the current to 50ma.

    I had mentioned before that with the delay relay and solid state rectification, the caps will charge to the peak of the secondary ac. Another quirk I noticed that the
    Auto-Bias soft-start does not completely reset with a quick power interruption. This coupled with 17 second delay of the relay, no voltage is present on the quad cap, this
    causes the Auto-Bias to "think" the tube bias is to low and for 17 seconds the charge on the 22uf caps increases. Now when the relay closes there was about 150ma of tube current
    until the Auto-Bias corrects it.

    All in all I am happy with the way it works. I do have schematics and only if Roy says it is OK I will share them, otherwise if anyone hear has a question on the operation I will try to answer
    them.



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    Re: Auto-Bias board on Roy's website?

    Post by mazeeff on Sun May 06, 2018 4:29 am

    jwb474; From your pictures, it looks like you have the auto-bias board installed topside, instead of inside the case. Is that correct? I see the the blue led's through the cage!

    Regarding the "quick power interruption issue". How long does it take for the auto-bias to reduce the 150ma to 50ma? I use one of these to prevent the issue. If it loses power, it remains off, when power returns. Glad you tested that! I always cringe a bit, when short cycling power like that!

    https://www.amazon.com/gp/product/B001Q9EFUK/ref=oh_aui_detailpage_o08_s00?ie=UTF8&psc=1
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    Re: Auto-Bias board on Roy's website?

    Post by mazeeff on Sun May 06, 2018 5:32 am

    150ma on a el34 would far exceed its power specification. As many times as Bob as warned us about keeping the bias current low to improve reliability, 150ma would make him twitch! Does the problem go away, if you remove the TDR board? Is 150ma the max current that the auto-bias circuit can sink? Why 150ma?
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    Re: Auto-Bias board on Roy's website?

    Post by jwb474 on Sun May 06, 2018 6:55 am

    mazeeff wrote:150ma on a el34 would far exceed its power specification. As many times as Bob as warned us about keeping the bias current low to improve reliability, 150ma would make him twitch! Does the problem go away, if you remove the TDR board? Is 150ma the max current that the auto-bias circuit can sink? Why 150ma?

    As I stated above, using a tube rectifier basically eliminates the problem since the HV is immediately restored. The Auto-Bias then does it's thing. I noticed hardly any increase is bias current when restored from a dropout using a tube rectifier, but you don't get the improvements that solid state rectifier gives you. Maybe I should test using solid state rectification without delay. I do like having a "St80" which improved base transient response.

    Look, this is a great hobby playing with all the various trade offs in design circuits. For me not having to fool with constant bias re-adjustments is well worth some of the compromises. I have my on bias regulators designs since the days of Audio Amateur and Glass Audio. I have never lost a tube due to the design issues of a Auto-Bias circuit.

    I look forward with playing which the MK3.
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    Re: Auto-Bias board on Roy's website?

    Post by jfine on Sun May 06, 2018 11:27 am

    Questions,

    1) Is this autobias system basically the same as what Primaluna does? (What they call "adaptive")

    http://audiofast.pl/main.asp?idm=37&ids=155&wersja=1

    Heck they even claim distortion is reduced by 40% to 50%!


    2) I have read that a cathode bias system (using a large resistor) can drop power by up to 10%. "The down side of cathode bias is that at full power the cathode voltage rises, reducing the current in the tube, reducing power and increasing distortion."

    Looking at this AB board here for the M125, it looks as though there are resistors, and electrolytics, similar to what a cathode bias system contains. Isn't there some power loss, even if minimal?

    3) The signal now goes thru the existing driver board coupling caps, but now it will also pass thru the 2 PIO's on the AB board? This must affect sonics, i.e., both of these positions are now candidates for cap rolling?
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    Re: Auto-Bias board on Roy's website?

    Post by jfine on Sun May 06, 2018 1:00 pm

    Also an interesting read about VAC amps:

    http://www.vac-amps.com/press/factsheets/iQsystem.pdf
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    Re: Auto-Bias board on Roy's website?

    Post by tubes4hifi on Sun May 06, 2018 2:11 pm

    J FIne,
    almost all hi-end modern tube amplifiers are now using auto-bias. Since we don't have the schematics for either the Pavel design we (Tubes4HiFi) are using, and we don't have the schematics for the PrimaLuna amps,
    there's no way to compare for sure. This version could certainly be called "adaptive", because if you substitute one tube for another then it's still going to automatically adjust bias to the preset value.
    This is still a "fixed bias" method, which is the best for highest power and lowest distortion. It is NOT cathode bias.
    We are still using the 10 ohm resistor to measure and compare bias, same as before. A "cathode bias" method would use an appropriate resistor for the tube type (say maybe 200 ohm 10 watts)
    for an EL34, along with a large 220-1000uF capacitor in parallel with it. An amp like that has lower power and higher distortion.

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