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    M125 individual tube bias

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    Too much stuff

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    Join date : 2017-10-23
    Location : Ontario

    M125 individual tube bias

    Post by Too much stuff on Wed Nov 08, 2017 2:52 pm

    Hi. I have a pair of M125's to build and am wondering if there is an official mod out there to individually bias the power tubes. I have VTA M125 CCS driver boards.

    I found this post on another forum:

    http://audiokarma.org/forums/index.php?threads/vta-m125-from-bob-latino.714532/#post-9611772

    I'm wondering if that looks feasible.
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    Bob Latino
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    Re: M125 individual tube bias

    Post by Bob Latino on Wed Nov 08, 2017 6:11 pm

    Too much stuff wrote:Hi. I have a pair of M125's to build and am wondering if there is an official mod out there to individually bias the power tubes. I have VTA M125 CCS driver boards.

    I found this post on another forum:

    http://audiokarma.org/forums/index.php?threads/vta-m125-from-bob-latino.714532/#post-9611772

    I'm wondering if that looks feasible.

    Having 4 bias pots on the VTA M-125 driver board will make no difference in the SOUND of the amp and also would make for a more complex biasing situation if you use just TWO output tubes instead of four output tubes. It would also be nice if the VTA M-125 amps had an LED readout of the bias for the left and right side output tubes. It would be nice if the M-125's had an autobias system. It would be nice if the ON/OFF switch was a push button lighted switch. It would be nice if the M-125's had decals saying on/off and triode/pentode ultralinear and left bias/right bias/ground for the three bias test pins. How about a tube cage ? Unfortunately all the above mentioned would add $$$ to the price of the kit and would add ZERO to the sound of the amp.

    Bob

    Too much stuff

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    Re: M125 individual tube bias

    Post by Too much stuff on Wed Nov 08, 2017 8:16 pm

    Thx Bob. I'm sure it would never end with addons!! Laughing

    I'd be using four output tubes. Does the CCS take care of keeping the tubes equally biased as they age and drift apart.

    Tom
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    Bob Latino
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    Re: M125 individual tube bias

    Post by Bob Latino on Wed Nov 08, 2017 9:01 pm

    Too much stuff wrote:Thx Bob. I'm sure it would never end with addons!! Laughing

    I'd be using four output tubes. Does the CCS take care of keeping the tubes equally biased as they age and drift apart.

    Tom

    No ... CCS refers to the Constant Current Source in the tail of the phase splitter section of the driver stage of the amp. Bias is for the output stage of the amp.

    Bob
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    peterh

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    Re: M125 individual tube bias

    Post by peterh on Thu Nov 09, 2017 2:16 am

    Using matched tubes will be the most cost-efficient way. That is, really matched ones.
    Buy them from a reputable dealer that matches themself and sells with warranty.


    Too much stuff

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    Location : Ontario

    Re: M125 individual tube bias

    Post by Too much stuff on Thu Nov 09, 2017 8:31 am

    Yes, but a few hundred hours in there is no guarantee the tubes will still be matched. I already have bought the tubes from Pcx.

    Back to the original post. I'm wondering if the add to the driver board circuit looks feasible. I'm not an electrical engineer but have no trouble following a schematic. There's lots of room under the chassis to fit the extra parts. I have the skills and equipment to mod the st st chassis as necessary for extra pin jacks and pots. It may not affect the sound, but I would like to be able to monitor the tubes as they age and be able compensate as necessary.

    Thank you for the responses so far. I don't expect VTA to modify there product offerings for every request that comes across. These amps are an excellent value and the unselfish support is much appreciated!

    Tom

    quad44

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    Join date : 2009-06-08

    Re: M125 individual tube bias

    Post by quad44 on Thu Nov 09, 2017 2:49 pm

    Look at my Post No 33 at the following location http://dynacotubeaudio.forumotion.com/t3199p25-dumb-question-on-setting-bias

    That is my solution to your requirement "to be able to monitor the tubes as they age..."

    My configuration is for MKIII Dynacos and I can watch them from across the room as I listen to my music. The only distraction is wanting to get up and and tweak the bias if it is slightly off. In time this itch will disappear.

    Since you are using all 4 tubes on your M125 you will be biasing pairs so your settings will be doubled. I am sure that this reading is covered in the instructions.   My setup uses the chassis mounted sockets to plug into the bias points so hardwiring is not required. There is no under the chassis work required.  

    Have fun!
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    peterh

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    Re: M125 individual tube bias

    Post by peterh on Thu Nov 09, 2017 3:52 pm

    Good quality tubes ( IE JJ ) will age together. I guess mr mcshane will other brands that also
    ages in unison.

    Random ebay tubes will age randomly.

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