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The Dynaco Tube Audio Forum

Dedicated to the restoration and preservation of all original Dynaco tube audio equipment - Customer support for Tubes4hifi VTA tube amp and preamp kits and all Dynakitparts.com products


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CletusB
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    Pavel's Auto-Bias Board

    HarryY
    HarryY


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    Post by HarryY Wed Aug 14, 2019 12:08 pm

    erhard-audio wrote:
    BryceJ wrote:The manual for the AB module states the following:

    Should any of these blue LED’s go out or not go on after power up, means that here is an issue with that
    given output tube and that the tube in question needs to be immediately replaced.


    I seems that the blue LEDs should be always be visible to take advantage of this feature and to get full value out of the AB module.

    please see previous message in this thread about just this topic and why it is not practical to sell the AB module with 'external' LED's.


    A plate that would mount in place of one of the front octal
    sockets would be a good option so they could be displayed.

    If someone wanted to be really slick they could push out the
    pins of an octal socket and then drill it so LEDs could be slipped
    in from the rear of the socket, maintaining the stock look.
    (In that case orange or amber LEDs would probably look nice.)





    pavlikkkk
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    Post by pavlikkkk Wed Aug 14, 2019 1:02 pm

    HarryY wrote:
    erhard-audio wrote:
    BryceJ wrote:The manual for the AB module states the following:

    Should any of these blue LED’s go out or not go on after power up, means that here is an issue with that
    given output tube and that the tube in question needs to be immediately replaced.


    I seems that the blue LEDs should be always be visible to take advantage of this feature and to get full value out of the AB module.

    please see previous message in this thread about just this topic and why it is not practical to sell the AB module with 'external' LED's.


    A plate that would mount in place of one of the front octal
    sockets would be a good option so they could be displayed.

    If someone wanted to be really slick they could push out the
    pins of an octal socket and then drill it so LEDs could be slipped
    in from the rear of the socket, maintaining the stock look.
    (In that case orange or amber LEDs would probably look nice.)


    This is of course possible, LEDs can have any color and luminosity..
    avatar
    BryceJ


    Posts : 22
    Join date : 2019-04-25

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    Post by BryceJ Wed Aug 14, 2019 1:19 pm

    pavlikkkk wrote:
    BryceJ wrote:The manual for the AB module states the following:

    Should any of these blue LED’s go out or not go on after power up, means that here is an issue with that
    given output tube and that the tube in question needs to be immediately replaced.


    I seems that the blue LEDs should be always be visible to take advantage of this feature and to get full value out of the AB module.
    you can desoldered them off AB. There are cable holes. Then you can mount them where you want.

    I had seen that mentioned. Does this apply to the M125 boards as well? It would be nice to see what people have done, if they have made this change.
    pavlikkkk
    pavlikkkk


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    Post by pavlikkkk Wed Aug 14, 2019 2:15 pm

    BryceJ wrote:
    pavlikkkk wrote:
    BryceJ wrote:The manual for the AB module states the following:

    Should any of these blue LED’s go out or not go on after power up, means that here is an issue with that
    given output tube and that the tube in question needs to be immediately replaced.


    I seems that the blue LEDs should be always be visible to take advantage of this feature and to get full value out of the AB module.
    you can desoldered them off AB. There are cable holes. Then you can mount them where you want.

    I had seen that mentioned. Does this apply to the M125 boards as well? It would be nice to see what people have done, if they have made this change.

    yes, the same for AB M125 boards.
    deepee99
    deepee99


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    Pavel's Auto-Bias Board - Page 3 Empty Confessions of a reformed AB Board denier

    Post by deepee99 Wed Aug 14, 2019 5:33 pm

    Due to a bad experience with what turned out to be wrongly-grounded (by the installer(s) AB boards, I'd given them a wide berth and said some pretty nasty things about Pavel's little board. I stand before you to-day to recant my earlier misgivings and to suggest that while they won't bring the world to Jesus or win the war for the Allies, they're a pretty good thing to have.
    Had Holger at Erhard Audio not dragged me screaming and kicking into this revelation, I might well still be stamping my feet, looking in.
    My three-odd months' experience with the AB, installed by Holger during his construction of my ST-120 (upgraded with Lundahl OPTs, which are definitely worth the extra bucks) informs me, firstly, that the things can live longer than 60 days and do no harm.
    Which is damnation with faint praise, the perfection of which phrase I leave to PeterW.
    The AB board definitely keeps its bias mud. Suspicious as I was, I had holger leave me bias probe points for each tube on the back of the amp, and a Fluke confirms this as rock-steady at 0.375/tube, which is rather an ideal bias capable of serving both KT-88 and El-34 tubes in their "happy range."
    While I, as I expect many others, don't consider manual biasing a chore but rather a pleasurable duty, the precision the AB board maintains cannot be accomplished with the tiny bias pots on the driver board. Even the pressure of a wooden screwdriver can deflect the meter a needle's-width or more so ideal accuracy is not attainable this way.
    I am not a fan of "post-blight" tubes and use some fairly expensive NOS bottles in the big holes. I do like knowing that they're not going to endure even a second's over-voltage on the grid, at least. Tube longevity is one of the Pavel board's selling points and while I haven't run it through a full 10,000-hour cycle, I'm certain that an upset bias won't be the cause of their ultimate deaths -- just old age.
    There's no change in the sound that I can detect. I just consider the AB board as a cheap insurance policy and a more precise way of doing things.
    A lot of misinformation about Pavel's board has been posted here. There are only two people one should direct questions to, and that's Holger and Pavel. Trust me, disregard anything anyone else, no matter how high up in the VTA hierarchy (except for Bob) says about them.
    The after-the-fact installation of the board seems to be the most problematic, even with the PC board changes Pavel and Holger have made to them. So let me cut the baby in half: If you're yet to build one of the kits, put the AB board into it as you're building it. If you already own a fully-built and operating amp, don't bother with it. But DO check your bias regularly if you want your tubes, rectifiers, and power trannies to breathe a bit easier.
    pavlikkkk
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    Post by pavlikkkk Thu Aug 15, 2019 1:51 am

    deepee99 wrote:Due to a bad experience with what turned out to be wrongly-grounded (by the installer(s) AB boards, I'd given them a wide berth and said some pretty nasty things about Pavel's little board. I stand before you to-day to recant my earlier misgivings and to suggest that while they won't bring the world to Jesus or win the war for the Allies, they're a pretty good thing to have.
    Had Holger at Erhard Audio not dragged me screaming and kicking into this revelation, I might well still be stamping my feet, looking in.
    My three-odd months' experience with the AB, installed by Holger during his construction of my ST-120 (upgraded with Lundahl OPTs, which are definitely worth the extra bucks) informs me, firstly, that the things can live longer than 60 days and do no harm.
    Which is damnation with faint praise, the perfection of which phrase I leave to PeterW.
    The AB board definitely keeps its bias mud. Suspicious as I was, I had holger leave me bias probe points for each tube on the back of the amp, and a Fluke confirms this as rock-steady at 0.375/tube, which is rather an ideal bias capable of serving both KT-88 and El-34 tubes in their "happy range."
    While I, as I expect many others, don't consider manual biasing a chore but rather a pleasurable duty, the precision the AB board maintains cannot be accomplished with the tiny bias pots on the driver board. Even the pressure of a wooden screwdriver can deflect the meter a needle's-width or more so ideal accuracy is not attainable this way.
    I am not a fan of "post-blight" tubes and use some fairly expensive NOS bottles in the big holes. I do like knowing that they're not going to endure even a second's over-voltage on the grid, at least. Tube longevity is one of the Pavel board's selling points and while I haven't run it through a full 10,000-hour cycle, I'm certain that an upset bias won't be the cause of their ultimate deaths -- just old age.
    There's no change in the sound that I can detect. I just consider the AB board as a cheap insurance policy and a more precise way of doing things.
    A lot of misinformation about Pavel's board has been posted here. There are only two people one should direct questions to, and that's Holger and Pavel. Trust me, disregard anything anyone else, no matter how high up in the VTA hierarchy (except for Bob) says about them.
    The after-the-fact installation of the board seems to be the most problematic, even with the PC board changes Pavel and Holger have made to them. So let me cut the baby in half: If you're yet to build one of the kits, put the AB board into it as you're building it. If you already own a fully-built and operating amp, don't bother with it. But DO check your bias regularly if you want your tubes, rectifiers, and power trannies to breathe a bit easier.
    Thanks for your appreciation. Whenever I like to help again, and certainly and Holger will.
    Pavel
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    Dale Stevens


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    Post by Dale Stevens Thu Aug 15, 2019 10:45 am

    Dee, You're bias @ 0.375 on the KT88 tubes? Thought that was a tad low. Bob??
    Bob Latino
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    Post by Bob Latino Thu Aug 15, 2019 10:52 am

    Dale Stevens wrote:Dee, You're bias @ 0.375 on the KT88 tubes? Thought that was a tad low.  Bob??

    Yes - In my opinion .375 VDC bias voltage per each KT88 or 6550 or KT120 tube in a VTA ST-120 or VTA M-125 is a bit low.

    Bob
    deepee99
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    Post by deepee99 Thu Aug 15, 2019 11:44 am

    Bob Latino wrote:
    Dale Stevens wrote:Dee, You're bias @ 0.375 on the KT88 tubes? Thought that was a tad low.  Bob??

    Yes - In my opinion .375 VDC bias voltage per each KT88 or 6550 or KT120 tube in a VTA ST-120 or VTA M-125 is a bit low.

    Bob
    Bob, that's what I thought, too, but it's what Holger recommended and sounds every bit as good (IMHO) as our old bias settings of 0.6VDC/tube. It does make swapping out 6550s for El-34s effortless.
    So I dunno. But I'm not fiddling with anything until Pavel or Holger tell me to crank it up a bit on the master pot -- which they make it a royal PITA to get to, on purpose.
    That may have been what cratered my previous AB boards; I was setting the master at 0.55VDC/tube. That, and I know an un-floated ground to derive power from the filament tap will gum one up. The latter is no longer an issue because the new gadgets come with an isolation transformer on board.
    Maybe one of those guys will weigh in. But for me right now, it's working just fine as-is and I'm not going to invade the innards and risk breaking something till Pavel or Holger weighs in on this.
    pavlikkkk
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    Post by pavlikkkk Sat Aug 17, 2019 2:13 pm

    One more experience..
    quite often I encounter wiring errors.
    The fault is that the all blue LED lights up and some after a while turn off. Some of the tubes they begin lit red anode.
    This is due to poor connection of the wires to the AB.
    I attach a picture of the correct connection of GRD and CATH points to the amp.
    Pavel's Auto-Bias Board - Page 3 Contro12
    avatar
    Guest
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    Post by Guest Mon Aug 19, 2019 10:01 am

    deepee99 wrote:
    Bob Latino wrote:
    Dale Stevens wrote:Dee, You're bias @ 0.375 on the KT88 tubes? Thought that was a tad low.  Bob??

    Yes - In my opinion .375 VDC bias voltage per each KT88 or 6550 or KT120 tube in a VTA ST-120 or VTA M-125 is a bit low.

    Bob
    Bob, that's what I thought, too, but it's what Holger recommended and sounds every bit as good (IMHO) as our old bias settings of 0.6VDC/tube. It does make swapping out 6550s for El-34s effortless.
    So I dunno. But I'm not fiddling with anything until Pavel or Holger tell me to crank it up a bit on the master pot -- which they make it a royal PITA to get to, on purpose.
    That may have been what cratered my previous AB boards; I was setting the master at 0.55VDC/tube. That, and I know an un-floated ground to derive power from the filament tap will gum one up. The latter is no longer an issue because the new gadgets come with an isolation transformer on board.
    Maybe one of those guys will weigh in. But for me right now, it's working just fine as-is and I'm not going to invade the innards and risk breaking something till Pavel or Holger weighs in on this.

    Actually, it is .450V, not .375V, that I set your KT88's to. Not sure where you got the .375V from.
    deepee99
    deepee99


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    Post by deepee99 Mon Aug 19, 2019 10:12 am

    erhard-audio wrote:
    deepee99 wrote:
    Bob Latino wrote:
    Dale Stevens wrote:Dee, You're bias @ 0.375 on the KT88 tubes? Thought that was a tad low.  Bob??

    Yes - In my opinion .375 VDC bias voltage per each KT88 or 6550 or KT120 tube in a VTA ST-120 or VTA M-125 is a bit low.

    Bob
    Bob, that's what I thought, too, but it's what Holger recommended and sounds every bit as good (IMHO) as our old bias settings of 0.6VDC/tube. It does make swapping out 6550s for El-34s effortless.
    So I dunno. But I'm not fiddling with anything until Pavel or Holger tell me to crank it up a bit on the master pot -- which they make it a royal PITA to get to, on purpose.
    That may have been what cratered my previous AB boards; I was setting the master at 0.55VDC/tube. That, and I know an un-floated ground to derive power from the filament tap will gum one up. The latter is no longer an issue because the new gadgets come with an isolation transformer on board.
    Maybe one of those guys will weigh in. But for me right now, it's working just fine as-is and I'm not going to invade the innards and risk breaking something till Pavel or Holger weighs in on this.

    Actually, it is .450V, not .375V, that I set your KT88's to. Not sure where you got the .375V from.
    My bad, Holger et. al. 0.45 VDC is what you set mine for, and where it stays. I was thinking about something else when I typed that and apologize for any confusion to the readers. Actually, 0.45 VDC is also what Andy Bouwman recommends for any of our big-bottle output tubes.
    avatar
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    Post by Guest Mon Aug 19, 2019 10:15 am

    deepee99 wrote:
    erhard-audio wrote:
    deepee99 wrote:
    Bob Latino wrote:
    Dale Stevens wrote:Dee, You're bias @ 0.375 on the KT88 tubes? Thought that was a tad low.  Bob??

    Yes - In my opinion .375 VDC bias voltage per each KT88 or 6550 or KT120 tube in a VTA ST-120 or VTA M-125 is a bit low.

    Bob
    Bob, that's what I thought, too, but it's what Holger recommended and sounds every bit as good (IMHO) as our old bias settings of 0.6VDC/tube. It does make swapping out 6550s for El-34s effortless.
    So I dunno. But I'm not fiddling with anything until Pavel or Holger tell me to crank it up a bit on the master pot -- which they make it a royal PITA to get to, on purpose.
    That may have been what cratered my previous AB boards; I was setting the master at 0.55VDC/tube. That, and I know an un-floated ground to derive power from the filament tap will gum one up. The latter is no longer an issue because the new gadgets come with an isolation transformer on board.
    Maybe one of those guys will weigh in. But for me right now, it's working just fine as-is and I'm not going to invade the innards and risk breaking something till Pavel or Holger weighs in on this.

    Actually, it is .450V, not .375V, that I set your KT88's to. Not sure where you got the .375V from.
    My bad, Holger et. al. 0.45 VDC is what you set mine for, and where it stays. I was thinking about something else when I typed that and apologize for any confusion to the readers. Actually, 0.45 VDC is also what Andy Bouwman recommends for any of our big-bottle output tubes.

    PHEW!!!.....all is good then!...was starting to get worried why it dropped to .375V all of a sudden.....don't do that to an ole schmuck like me!! affraid
    deepee99
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    Post by deepee99 Mon Aug 19, 2019 10:18 am

    erhard-audio wrote:
    deepee99 wrote:
    erhard-audio wrote:
    deepee99 wrote:
    Bob Latino wrote:
    Dale Stevens wrote:Dee, You're bias @ 0.375 on the KT88 tubes? Thought that was a tad low.  Bob??

    Yes - In my opinion .375 VDC bias voltage per each KT88 or 6550 or KT120 tube in a VTA ST-120 or VTA M-125 is a bit low.

    Bob
    Bob, that's what I thought, too, but it's what Holger recommended and sounds every bit as good (IMHO) as our old bias settings of 0.6VDC/tube. It does make swapping out 6550s for El-34s effortless.
    So I dunno. But I'm not fiddling with anything until Pavel or Holger tell me to crank it up a bit on the master pot -- which they make it a royal PITA to get to, on purpose.
    That may have been what cratered my previous AB boards; I was setting the master at 0.55VDC/tube. That, and I know an un-floated ground to derive power from the filament tap will gum one up. The latter is no longer an issue because the new gadgets come with an isolation transformer on board.
    Maybe one of those guys will weigh in. But for me right now, it's working just fine as-is and I'm not going to invade the innards and risk breaking something till Pavel or Holger weighs in on this.

    Actually, it is .450V, not .375V, that I set your KT88's to. Not sure where you got the .375V from.
    My bad, Holger et. al. 0.45 VDC is what you set mine for, and where it stays. I was thinking about something else when I typed that and apologize for any confusion to the readers. Actually, 0.45 VDC is also what Andy Bouwman recommends for any of our big-bottle output tubes.

    PHEW!!!.....all is good then!...was starting to get worried why it dropped to .375V all of a sudden.....don't do that to an ole schmuck like me!! affraid
    'Bout time you came back out of the woods, laddie. You could've caught my egregious error several days ago.
    CletusB
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    Post by CletusB Wed Nov 09, 2022 9:14 am

    Well, a new set of matched KT-88s are on order for my lovely ST-120 and also this time, I've opted to install an AB-Q-ST70-ST120 Auto-Bias board. Is there anywhere on the forum that gives a "theory of operation" of the board? I have a very good idea of such, but want to have it all documented in my manual as this amp will be passed on to my son some day.
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    Post by New2Tubez Wed Nov 09, 2022 9:35 am

    Pavel’s site-
    https://www.audioamp.eu/

    PS, that's my ST120 pictured in post #60


    Last edited by New2Tubez on Wed Nov 09, 2022 11:56 am; edited 1 time in total (Reason for editing : background info)

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    CletusB
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    Pavel's Auto-Bias Board - Page 3 Empty Circuit theory

    Post by CletusB Wed Nov 09, 2022 9:47 am

    New2Tubez wrote:Pavel’s site-
    https://www.audioamp.eu/
    Well, I see installation and setup info but not board-circuit theory, maybe I'm missing something..
    pavlikkkk
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    Post by pavlikkkk Wed Nov 09, 2022 10:26 am

    CletusB wrote:
    New2Tubez wrote:Pavel’s site-
    https://www.audioamp.eu/
    Well, I see installation and setup info but not board-circuit theory, maybe I'm missing something..

    AB modules of the Q series use an integrator with a large time constant to maintain the BIAS voltage. This integrator operates in a voltage window of about 450 mV.
    The integrator still maintains the average voltage in this window. Excitation by an audio signal of any level has no effect on the BIAS voltage.
    Pavel
    Pavel's Auto-Bias Board - Page 3 Ab-q12

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    CletusB
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    Post by CletusB Wed Nov 09, 2022 5:35 pm

    Ahhh, thank you very much for that explanation, very informative. I'm looking forward to installing the board and re-tubing the amp.
    pavlikkkk
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    Post by pavlikkkk Thu Nov 10, 2022 2:36 am

    CletusB wrote:Ahhh, thank you very much for that explanation, very informative.  I'm looking forward to installing the board and re-tubing the amp.
    I wish you success and write your experiences.
    Pavel
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    Post by LeGrace Thu Nov 10, 2022 9:45 am

    I will update my personal experience on surpassing 3 years steady usage this summer. In my case AB-Q-M125 boards. Regarding claims on the audioamp site:

    "AB boards extend tube life, anything up to twice the life" I consider this the no 1 benefit. If anything, claim is conservative, ie have already surpassed 3x with current installed set of KT120's. Translating into huge savings in tube costs, boards rapidly pay for themselves.

    "You'll never have to adjust the bias again, set up once" The level of control is almost uncanny. +/- 0.001v even years later.

    "Balanced output stage for best possible sound" With either of my M125's each tube measures identically for always ideal sonic quality. No more one side drifting off as was the prior norm. Fiddling with pots got old fast, certainly don't miss it!

    IME above claims are 100% valid!

    The leds are a nice touch and bring peace of mind that tubes have biased properly. A common complaint is they end up inside the case. While I cannot observe them directly, I can readily see either a red or blue glow bleeding out through gaps in the case as they are quite bright. I normally watch for the red to blue transition on initial startup and after that simply enjoy the music.

    Eliminating need to use matched pairs of tubes is a great feature as well. I have multiple orphans due to its' mate having flamed out.  Enough that combined with the increase in tube longevity I imagine I won't have to buy any additional tubes for many years to come. Given current supply issues this is huge!

    IMO a truly worthwhile upgrade.

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    Post by pavlikkkk Thu Nov 10, 2022 11:14 am

    LeGrace wrote:I will update my personal experience on surpassing 3 years steady usage this summer. In my case AB-Q-M125 boards. Regarding claims on the audioamp site:

    "AB boards extend tube life, anything up to twice the life" I consider this the no 1 benefit. If anything, claim is conservative, ie have already surpassed 3x with current installed set of KT120's. Translating into huge savings in tube costs, boards rapidly pay for themselves.

    "You'll never have to adjust the bias again, set up once" The level of control is almost uncanny. +/- 0.001v even years later.

    "Balanced output stage for best possible sound" With either of my M125's each tube measures identically for always ideal sonic quality. No more one side drifting off as was the prior norm. Fiddling with pots got old fast, certainly don't miss it!

    IME above claims are 100% valid!

    The leds are a nice touch and bring peace of mind that tubes have biased properly. A common complaint is they end up inside the case. While I cannot observe them directly, I can readily see either a red or blue glow bleeding out through gaps in the case as they are quite bright. I normally watch for the red to blue transition on initial startup and after that simply enjoy the music.

    Eliminating need to use matched pairs of tubes is a great feature as well. I have multiple orphans due to its' mate having flamed out.  Enough that combined with the increase in tube longevity I imagine I won't have to buy any additional tubes for many years to come. Given current supply issues this is huge!

    IMO a truly worthwhile upgrade.
    I am very glad that my AB brought you these pleasant surprises.
    Constant innovations of the AB modules brought this "fruit" in the form of high stability of the working points of the power tubes and the related significant extension of their service life.
    Another option - the use of unpaired and even older, already used tubes is certainly also a great advantage. And the amplifier does not lose any of its excellent reproduction bounce
    Pavel

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    Post by CletusB Thu Nov 10, 2022 7:46 pm

    Silly me for being "on the fence" about this for so long.
    I should have my board here in the shop by the end of next week, as it arrived at mt Miami freight forwarder's yesterday. I can see myself doing the instillation next weekend. Thanks for all the prompt responses guys, that's why I love this forum.  Very Happy

    EDIT
    Now I'm debating weather to remote the LEDs or not. I've got a bunch of tiny blue LED's here in the shop's parts bin, I could easily use, but is it worth the effort?
    pavlikkkk
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    Post by pavlikkkk Fri Nov 11, 2022 3:04 am

    CletusB wrote:Silly me for being "on the fence" about this for so long.
    I should have my board here in the shop by the end of next week, as it arrived at mt Miami freight forwarder's yesterday. I can see myself doing the instillation next weekend. Thanks for all the prompt responses guys, that's why I love this forum.  Very Happy

    EDIT
    Now I'm debating weather to remote the LEDs or not. I've got a bunch of tiny blue LED's here in the shop's parts bin, I could easily use, but is it worth the effort?

    It's definitely worth the effort. The effect is excellent. Just use a gentle soldering technique.
    Pavel
    CletusB
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    Post by CletusB Sat Nov 12, 2022 8:25 pm

    Well finally, I got around to installing a Variac for the ST-120 Cool
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