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    Auto-Bias board on Roy's website?

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    sKiZo
    sKiZo

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    Auto-Bias board on Roy's website?

    Post by sKiZo on Tue Feb 27, 2018 3:36 pm

    Did a nice job of hiding it halfway down the page, eh ... ;-}



    Looks like it'd be an easy install too ... just swap the leads from the existing pots on the VTA board and tap the power supply ...

    deepee99
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    Re: Auto-Bias board on Roy's website?

    Post by deepee99 on Fri Mar 02, 2018 3:08 pm

    I have one in my Sachs-modified ST-120. Highly recommend. Glad Roy is now offering them as an option for kit-builders. ARC has been using auto-bias for years so it's not exactly a "new" technology, however these new boards are cutting-edge in quality and performance.
    tubes4hifi
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    Re: Auto-Bias board on Roy's website?

    Post by tubes4hifi on Tue Mar 06, 2018 1:07 pm

    I've been in contact with Pavel for the correct install for the M125, you would need to add additional coupling caps and grid resistors,
    and break the existing connections between the paralled output tubes so that each would be completely independent.
    No problem, ARC and others have been doing this for years.   As is, in an M125, if you have one tube in a pair go into runaway, you'll likely loose both tubes,
    and possibly the transformers.   Won't happen with auto-bias.  


    Last edited by tubes4hifi on Wed Jan 09, 2019 2:02 pm; edited 1 time in total
    deepee99
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    Re: Auto-Bias board on Roy's website?

    Post by deepee99 on Tue Mar 06, 2018 1:14 pm

    tubes4hifi wrote:I've been in contact with Pavel for the correct install for the M125, you would need to add additional coupling caps and grid resistors,
    and break the existing connections between the paralled output tubes so that each would be completely independent.
    No problem, ARC and others have been doing this for years.   As is, in an M125, if you have one tube in a pair go into runaway, you'll likely loose both tubes,
    and possibly the transformers.   Won't happen with auto-bias.   Don't ask, I know Bob doesn't want to do a total redesign of the M125s.
    I have become quite a fan of that auto-bias board. It's tube-specific as to plate volts and bias, yet line fluctuations don't throw it off. Mine's set up for KT-88s. I do check bias from time-to-time, just to keep an eye on the board, but there's never been the slightest variation. I miss the fireworks of a red-plating or dying output tube but think I will survive without them.
    Bob Latino
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    Re: Auto-Bias board on Roy's website?

    Post by Bob Latino on Tue Mar 06, 2018 2:28 pm

    Below is Pavel's autobias circuit as it should be attached in a VTA M-125 amp.




    Last edited by tubes4hifi on Tue Dec 18, 2018 11:33 am; edited 5 times in total (Reason for editing : added yellow traces to show circuit board connections)
    tubes4hifi
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    Re: Auto-Bias board on Roy's website?

    Post by tubes4hifi on Wed Mar 07, 2018 4:49 pm

    stand by, when I return from vacation in late April, I will have an adaptor board for the M125s, and a drop-in board for the ST70 & ST120 amps.
    Start the drooling now . . . . Very Happy

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

    Post by jfine on Wed Mar 07, 2018 5:39 pm

    Man you don't mess around, really appreciate your effort.

    Your support for hifi gear (Roy M. & Bob L.) is second to none.
    Kramer
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    Re: Auto-Bias board on Roy's website?

    Post by Kramer on Sun Apr 15, 2018 2:15 pm

    Wow, came here looking for the name of the bias board to order one and find that one is being custom designed for our amps Smile really appreciate all the effort you guys put into these amps. I will be purchasing one as soon as they are available.
    CletusB
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    Re: Auto-Bias board on Roy's website?

    Post by CletusB on Mon Apr 16, 2018 8:42 am


    Wow! this IS a very exciting development. bounce
    Now the question that's going to keep me awake at night is: do I wanna upgrade to this or do I wanna stay "old-school" and bias the amp myself every couple weeks when I wipe her down?  .....damned if I do, damned if I don't!  What a Face
    deepee99
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    Re: Auto-Bias board on Roy's website?

    Post by deepee99 on Mon Apr 16, 2018 6:23 pm


    buy a $30 *manual reset* GFCI off Amazon to protect your stuff from power failures and brownouts, and worries are over.
    Pavel's boards don't go full-tilt until about two minutes into the show. They are works of art.
    pavlikkkk
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    Re: Auto-Bias board on Roy's website?

    Post by pavlikkkk on Wed Apr 18, 2018 12:33 pm

    For the MT-125S Icath 55-68mA / tube should be used.
    It is important to set Icath without modulation!
    pavlikkkk
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    Re: Auto-Bias board on Roy's website?

    Post by pavlikkkk on Wed Apr 18, 2018 2:35 pm

    Soft-start board is not required. When the amplifier is on, the grids on the pins GRID are maximally biased (-Umax bias supply).
    When the red LED goes out, the tubes begin to slowly open.
    The boards will be sold to you by tubes4hifi.
    Pavel
    pichacker
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    Re: Auto-Bias board on Roy's website?

    Post by pichacker on Fri Apr 20, 2018 5:18 am

    What's the most cost effective way of getting one of these boards in the UK please?
    pavlikkkk
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    Re: Auto-Bias board on Roy's website?

    Post by pavlikkkk on Fri Apr 20, 2018 6:45 am

    pichacker wrote:What's the most cost effective way of getting one of these boards in the UK please?

    In Europe sells boards auto-bias for amp. ST70 and M125 www.audioamp.eu

    please write to pavel@manualy.cz
    Thanks
    Pavel
    deepee99
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    Re: Auto-Bias board on Roy's website?

    Post by deepee99 on Fri Apr 20, 2018 5:11 pm

    To cut back to the chase: Pavel's boards work. No way I'd go back.
    tubes4hifi
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    Re: Auto-Bias board on Roy's website?

    Post by tubes4hifi on Sat Apr 21, 2018 2:23 pm

    I have IN STOCK ST70 auto-bias boards ($110 with shipping in USA),
    and sets of M125 auto-bias boards ($190 a pair in USA with shipping).  
    roymottram@yahoo.com
    www.tubes4hifi.com




    Last edited by tubes4hifi on Wed Jan 09, 2019 2:16 pm; edited 2 times in total
    tubes4hifi
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    Re: Auto-Bias board on Roy's website?

    Post by tubes4hifi on Sun Apr 22, 2018 3:03 pm

    3/4" space between boards, so plenty of room for those fat PIO caps
    jfine
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    Re: Auto-Bias board on Roy's website?

    Post by jfine on Sun Apr 22, 2018 4:59 pm

    Question, (M125's)

    When auto bias board is installed, is it still possible to check the bias thru the existing front holes, or are these disconnected, or is there some relatively easy method to check to make sure bias stays consistent?

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

    Post by jfine on Sun Apr 22, 2018 5:01 pm

    Also I read somewhere that auto bias tube amps can result in less distortion somehow, if so, how does that work?
    pavlikkkk
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    Re: Auto-Bias board on Roy's website?

    Post by pavlikkkk on Sun Apr 22, 2018 5:07 pm

    jfine wrote:Also I read somewhere that auto bias tube amps can result in less distortion somehow, if so, how does that work?
    Here's how it is described:
    https://www.audioamp.eu/en-clanky-1.html
    tubes4hifi
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    Re: Auto-Bias board on Roy's website?

    Post by tubes4hifi on Sun Apr 22, 2018 5:41 pm

    simply put, because the auto-bias board constantly monitors tube current conditions for each tube, the amplifier is ALWAYS operating at it's ideal operating point,
    and the outputs (push and pull) are always EXACTLY correct, so distortion is always at it's absolute minimum.
    tubes4hifi
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    Re: Auto-Bias board on Roy's website?

    Post by tubes4hifi on Sun Apr 22, 2018 5:45 pm

    it's a ONE TIME ADJUSTMENT !!   Set it and forget it for the next 20 years!!
    jfine  - yes, still possible to monitor if you want, but now the 10 ohm cathode resistors are on the auto-bias board instead of on the output tube sockets.
              But really, no need to monitor, since they will ALWAYS be EXACTLY correct, except during warm-up, when they ramp up the current slowly,
               protecting those tubes!


    Last edited by tubes4hifi on Wed Jan 09, 2019 2:19 pm; edited 1 time in total
    deepee99
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    Re: Auto-Bias board on Roy's website?

    Post by deepee99 on Sun Apr 22, 2018 6:42 pm

    All I can tell youse guys (again) is that it works. And for the lazy amongst us, it's a tube-saver. For the hyper amongst us and for the audio geeks, it's a sound-saver. I've put that board through some pretty heavy-duty use in my Sachs VTA ST-120. Should be required equipment on all VTA amps.
    tubes4hifi
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    Re: Auto-Bias board on Roy's website?

    Post by tubes4hifi on Mon Apr 23, 2018 11:51 pm

    installation notes (still working on a REAL document)
    1 - remove the 10 ohm cathode resistors from each output tube
    2 - remove and replace C17 & C18 on the VTA driver board with supplied shorter caps
    3 - remove R37 & R38 on the VTA driver board
    4 - remove bias pots from VTA driver board (pliers, easy twist breaks them out) or else cut the traces to the pots from the bias supply (photo supplied with instructions)
    5 - connect filament voltage from V3 output tube pins 2 & 7 to auto-bias 6.3v terminals
    6 - connect bias voltage from C17 (+ and - leads) as per drawing
    7 - with NO tubes in the amplifier, power up amp and check bias setting on AB board (0.40vdc for ST70, 0.50vdc for ST120 or M125 amps)
    8 - connect negative bias voltage and ground from VTA driver (junction of R39/C18) to AB board bias-in & bias-GND
    9 - connect AB board GRID pads to R29-R32 on VTA driver board (photo supplied with instructions)
    10 - connect AB board CATH pads to each output tube pins 1-8 (where the old ten ohm resistors were connected)(photo supplied with instructions)
    11- turn on amp and enjoy !  no bias to ever check again !   (the auto-bias board will start up with zero tube current and slowly ramp up to set bias in 30-60 seconds)


    Last edited by tubes4hifi on Sat May 19, 2018 11:46 am; edited 1 time in total
    tubes4hifi
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    Re: Auto-Bias board on Roy's website?

    Post by tubes4hifi on Tue Apr 24, 2018 5:48 pm

    wish I had a completed amp to test this in, but too busy selling preamps! Very Happy
    OK, so I had to at least do a simple test! I hooked up 6vac to the board, and put a test lead on the bias set pad.
    The ST70 auto-bias boards have all been preset at 0.40vdc.
    The red LED in the center of the board will light up and stay on for about 40 seconds, meanwhile, the bias starts from zero volts to keep the tube from conducting any current,
    then finally starts ramping up, ending up at 0.40vdc, and the red LED goes off. Nice ramp.
    The board in the photo is for an ST120, so I turned the bias up to 0.50vdc, which was about 1.5 turns CW on the bias pot.
    Disconnected power, and watched it ramp up again, settling at the preset after 40 seconds.



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