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    Auto-Bias board on Roy's website?

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    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.
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    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'm now back from vacation.   Haven't had a chance to update my website, but I have IN STOCK ten sets of ST70 auto-bias boards ($100 with shipping in USA),
    and five sets of M125 auto-bias boards ($170 a pair in USA with shipping).   Email me direct and I'll send you an invoice for payment.
    roymottram@yahoo.com
    www.tubes4hifi.com




    Last edited by tubes4hifi on Sun Apr 29, 2018 1:51 pm; edited 1 time in total
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    tubes4hifi
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    Re: Auto-Bias board on Roy's website?

    Post by tubes4hifi on Sat Apr 21, 2018 8:21 pm

    send me an email for a manual
    roymottram@yahoo.com
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    Bob Latino
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    Re: Auto-Bias board on Roy's website?

    Post by Bob Latino on Sun Apr 22, 2018 9:53 am

    Tubes4hifi AutoBias board shown below how it will install into a VTA ST-70/ST-120 amp.

    Bob


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    aguaazul

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

    Post by aguaazul on Sun Apr 22, 2018 11:32 am

    How does that fit with the Russian PIO Caps? Is there room?

    mazeeff

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

    Post by mazeeff on Sun Apr 22, 2018 1:54 pm

    If I understand this, the bias set point is controlled by the blue pot, which ends up inside the case. Is that correct? If so, that would not work for me, with my ST-120. I run my KT88s at 50ma, and my KT120s at 55ma. Having to crack the case to try different bias points is a problem. Since Roy is fresh from vacation, how about a complete redesign of the VTA driver board, that incorporates the auto-bias function, and drivers into one board. With the bias adjust on top!
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    pavlikkkk

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

    Post by pavlikkkk on Sun Apr 22, 2018 2:05 pm

    mazeeff wrote:If I understand this, the bias set point is controlled by the blue pot, which ends up inside the case. Is that correct? If so, that would not work for me, with my ST-120. I run my KT88s at 50ma, and my KT120s at 55ma. Having to crack the case to try different bias points is a problem. Since Roy is fresh from vacation, how about a complete redesign of the VTA driver board, that incorporates the auto-bias function, and drivers into one board. With the bias adjust on top!
    Nothing prevents you from unsolder a blue pot and attaching a potentiometer 20k LIN (ideal aripot).


    Last edited by pavlikkkk on Mon Apr 23, 2018 4:04 pm; edited 1 time in total
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    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
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    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?

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    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?
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    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
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    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.
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    tubes4hifi
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    Re: Auto-Bias board on Roy's website?

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

    mazeeff - 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.
    so you'd have to move those wires. 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!
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    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.
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    jfine

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

    Post by jfine on Sun Apr 22, 2018 8:03 pm

    deepee99 wrote: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.

    So you've had this exact board in your ST-120? Don did this? But not your M125's correct?
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    pavlikkkk

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

    Post by pavlikkkk on Mon Apr 23, 2018 1:20 am

    jfine wrote: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?

    I'm remake the AB-Q module (LEDs)...
    I have invent how to mount LEDs to the amp panel.
    I removed the SMD LED from the board.
    Into the LEDs pad I drill the holes in 0.8mm.
    I can use the LED with cable to go where I need it..


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    tubes4hifi
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    Re: Auto-Bias board on Roy's website?

    Post by tubes4hifi on Mon Apr 23, 2018 11:30 pm

    photos of installation - hopefully this helps, as many know, I'm a poor technical writer because I know electronics so well (and for 55+ years) that I don't even think about stuff, I just do it.
    So I'll need a tube newbie idiot to help me write up some instructions.   But I'm a visual guy . . .
    1st photo - an octal board in a bare chassis (I'll have the correct shorter C17-C18 caps in a few days, I had to mount these taller ones sideways)
    2nd photo - the auto-bias board stacked on top using 3/4" spacers  (and happy surprise, the volume attenuator fits in there also!)
    3rd photo - another angle on the stack
    for the very observant types, notice no bias pots on the octal driver board . . .



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    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
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    cci1492

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

    Post by cci1492 on Tue Apr 24, 2018 4:15 pm

    Looks so nice. But I guess the $64,000 question for me: is it worth hacking the crap out of my 125s to slip these in? Hands down if I had a new kit I'd be all in.
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    CletusB

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

    Post by CletusB on Tue Apr 24, 2018 5:04 pm

    cci1492 wrote:Looks so nice. But I guess the $64,000 question for me: is it worth hacking the crap out of my 125s to slip these in? Hands down if I had a new kit I'd be all in.
    Well put!
    I too am very perked bounce     ..... but that trend of thought is exactly what's keeping my seat-belt buckled at the moment Laughing

    Twisted Evil ...buy it Cletus, buy it, you know you want  ....druhahahaha Twisted Evil Twisted Evil Twisted Evil Twisted Evil
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    corndog71

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

    Post by corndog71 on Tue Apr 24, 2018 5:08 pm

    Sounds like a good time to start a new kit! bounce
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    cci1492

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

    Post by cci1492 on Tue Apr 24, 2018 5:36 pm

    corndog71 wrote:Sounds like a good time to start a new kit! bounce

    You know, I was thinking that very thing! Put a second system together....maybe. Very Happy
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    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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    tubes4hifi
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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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    tubes4hifi
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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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