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    Auto-Bias board on Roy's website?

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    markeby

    Posts : 2
    Join date : 2019-02-13
    Age : 61
    Location : Near Austin, Texas

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    Post by markeby on Wed Feb 13, 2019 11:14 pm

    pavlikkkk wrote:
    jsl1234 wrote:Hi, does anyone have a photo/example showing they moved the bias lights on the AB board to show through the driver board cover?

    I’m thinking the lights would be neat if inserted in the hole that used to provide access to the bias adjustment pots?

    Otherwise how do you know if the lights are on/working without unbuttoning the the amp?

    Thanks
    is that it?
    Auto-Bias board on Roy's website? - Page 6 Led-st10

    The question I have is: Does the LED have any other functional purpose in the circuit?  I have a lot laying around and I am considering just putting a resister in series to set the brightness lower.
    Does anyone have the schematic?
    pichacker
    pichacker

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    Age : 54
    Location : Near to London - UK

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    Post by pichacker on Thu Feb 14, 2019 5:09 am

    Mark,

    If you look a few messages back you can see that I sketched the circuit out. The position of the LED in the circuit does appear to set the base bias on the upper transistor of the grid drive output stage. It's a sort of constant current generator so in thoery the more voltage dropped across the LED the more current in the circuit.

    Personally the circuit works so well i'd leave it alone. All the LEDs tell you is that the circuit is attempting to control the bias current in the tube. If your LED goes OC then the output pair of transistors could be overrun and burn up.

    See post 29 of this thread http://dynacotubeaudio.forumotion.com/t3925p25-need-help-with-auto-bias-board-malfunction
    tubes4hifi
    tubes4hifi
    Admin

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    Post by tubes4hifi on Thu Feb 14, 2019 1:42 pm

    the lastest instructions from Pavel still seem to be wrong, V1 OUT is actually V1 GRD (grid), same for V2 OUT, V3 OUT, V4 OUT.
    Also, the steps 11-14 are wrong!!   The output tube grids actually connect to the VTA driver board, same as before, not directly to the AB board!
    Should be  
    11. Re-connect the Rg1 wire from the VTA driver board  to pin 6 in the V1 socket
    12. Re-connect the Rg2 wire from the VTA driver board to pin 6 in the V2 socket
    13. Re-connect the Rg3 wire from the VTA driver board  to pin 6 in the V3 socket
    14. Re-connect the Rg4 wire from the VTA driver board  to pin 6 in the V4 socket

    updated/corrected instructions for the ST70-ST120 auto-bias are here on my website
    www.tubes4hifi.com/ST70-AB.pdf

    the instructions for the M125 were correct unless someone else can find an error that I don't see
    pavlikkkk
    pavlikkkk

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    Post by pavlikkkk on Fri Feb 15, 2019 4:28 am

    tubes4hifi wrote:the lastest instructions from Pavel still seem to be wrong, V1 OUT is actually V1 GRD (grid), same for V2 OUT, V3 OUT, V4 OUT.
    Also, the steps 11-14 are wrong!!   The output tube grids actually connect to the VTA driver board, same as before, not directly to the AB board!
    Should be  
    11. Re-connect the Rg1 wire from the VTA driver board  to pin 6 in the V1 socket
    12. Re-connect the Rg2 wire from the VTA driver board to pin 6 in the V2 socket
    13. Re-connect the Rg3 wire from the VTA driver board  to pin 6 in the V3 socket
    14. Re-connect the Rg4 wire from the VTA driver board  to pin 6 in the V4 socket

    updated/corrected instructions for the ST70-ST120 auto-bias are here on my website
    www.tubes4hifi.com/ST70-AB.pdf

    the instructions for the M125 were correct unless someone else can find an error that I don't see

    It is so. Thanks for the repair, Roy.
    Also in the manual were badly numbered power tube positions. Boards AB are now marked correctly.

    Auto-Bias board on Roy's website? - Page 6 Ab_rev10

    The corrected manual is already connected and the schema is correct.
    Correction on www.audioamp.eu
    manual ST70-AB.pdf
    Pavel
    jsl1234
    jsl1234

    Posts : 57
    Join date : 2017-08-28

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    Post by jsl1234 on Sun Mar 10, 2019 8:49 am

    I'm not sure I am posting images correctly but wanted to share how the modified AB board turned out. Several posts back I was asking if the lights could be repositioned on the driver board cover. Through the good work of Ben (VTA Tech on east coast) I was able to get exactly what I wanted (including custom woodwork encasement of my unit as well).

    Jay


    Auto-Bias board on Roy's website? - Page 6 2019-010Auto-Bias board on Roy's website? - Page 6 2019-012
    slate1
    slate1

    Posts : 50
    Join date : 2017-07-26

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    Post by slate1 on Tue Apr 02, 2019 9:04 pm

    tubes4hifi wrote: 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!

    My amp is en route back to me with the auto-bias board installed. Curious as to what this implies re: checking bias from the front holes?  The noted move of the 10 ohm resistor confuses me as to if it’s possible. I’m just thinking, for example, if the rectifier were changed. Would I be able to double check the bias without popping the case. Or - is that irrelevant?

    Noted : I’m an idiot on this stuff!  Ha ha.
    pichacker
    pichacker

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    Join date : 2016-08-01
    Age : 54
    Location : Near to London - UK

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    Post by pichacker on Wed Apr 03, 2019 3:29 am

    Slate....

    You're not an idiot, it's a learning process. With the auto bias board in and functioning then it wil take care of the bias on the output tubes for you. Any changes in line voltage or tube ageing will be automatically compensated for.

    The bias current measurement is performed by measuring the voltage across the cathode resistor on the output tube. On the VTA products this is 10 ohms and was initially wired local to the tube socket. With the fitting of the auto bias board this resistor is removed and replaced by one that is mounted on the auto bias board. Same function different place.

    It is still possible to measure the voltage across each of these resistors on the auto bias board but in reality if things are working to plan the voltages will not vary much and will all be very similar. No adjustment needed.

    Depending upon who/how the board was fitted you may still have the connections to the front sockets so you can monitor the voltage if you wish.

    Hope this helps

    Steve

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