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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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    M125 Re do and autobias install.

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    Rec


    Posts : 68
    Join date : 2020-03-11

    M125 Re do and autobias install. Empty M125 Re do and autobias install.

    Post by Rec Mon Feb 20, 2023 5:38 pm

    I just totally redid my tubes4hifi m125s. Auto bias is fantastic , easy add and pavels support was great!

    New laser cut plate, recycled picture frame for case. Ditched the rectifier and quad cap. Mundorf caps. Bunch of other stuff from these forums. ( That’s why I posted my used m125 chassis up.)

    It’s crazy to hear how the amp just sounds so much better once bias locks in.M125 Re do and autobias install. A8690210
    M125 Re do and autobias install. 86652e10

    Bob Latino, rustybutt, LeGrace, New2Tubez, BryceJ and rwbaker like this post

    LeGrace
    LeGrace


    Posts : 388
    Join date : 2016-08-07
    Location : Ontario, Canada

    M125 Re do and autobias install. Empty Re: M125 Re do and autobias install.

    Post by LeGrace Mon Feb 20, 2023 6:02 pm

    Super cool mods! 100% in agreement around benefits of auto bias, best thing I ever did to my M125's.
    avatar
    Rec


    Posts : 68
    Join date : 2020-03-11

    M125 Re do and autobias install. Empty Re: M125 Re do and autobias install.

    Post by Rec Tue Feb 21, 2023 9:52 am

    Thanks! Maybe these notes will help others.

    Here is an underside shot. I made the regulated bias circuit seen elsewhere. This stays fixed at -67v regardless of b+ to driver board. I did notice that I would not hold bias on all 4 tubes above about 400v. I imagine with the existing bias supply there might be enough range to bias the tubes, eg negative bias range does not drift up with b+ with the regulation. I added 66ohm dropping resistor (as per the Weber wz68) after the diodes and now the voltages are dialed in and all 4 always bias. My line voltage drifts between 115 and 120. M125 Re do and autobias install. C3f5a910

    LeGrace and New2Tubez like this post

    LeGrace
    LeGrace


    Posts : 388
    Join date : 2016-08-07
    Location : Ontario, Canada

    M125 Re do and autobias install. Empty Re: M125 Re do and autobias install.

    Post by LeGrace Tue Feb 21, 2023 11:37 am

    Wow! cheers
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    rebellovw


    Posts : 78
    Join date : 2022-01-23

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    Post by rebellovw Wed Feb 22, 2023 3:36 pm

    Awesome - I redid mine - no autobias - very happy so far.
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    BryceJ


    Posts : 23
    Join date : 2019-04-25

    M125 Re do and autobias install. Empty M125 Re do

    Post by BryceJ Thu Feb 23, 2023 7:20 pm

    I really like your restyled M125s. They look very nice.
    pavlikkkk
    pavlikkkk


    Posts : 69
    Join date : 2018-03-02
    Age : 76
    Location : Czech Republic

    M125 Re do and autobias install. Empty Re: M125 Re do and autobias install.

    Post by pavlikkkk Fri Feb 24, 2023 3:01 am

    Rec wrote:Thanks! Maybe these notes will help others.

    Here is an underside shot. I made the regulated bias circuit seen elsewhere. This stays fixed at -67v regardless of b+ to driver board. I did notice that I would not hold bias on all 4 tubes above about 400v. I imagine with the existing bias supply there might be enough range to bias the tubes, eg negative bias range does not drift up with b+ with the regulation. I added 66ohm dropping resistor (as per the Weber wz68) after the diodes and now the voltages are dialed in and all 4 always bias. My line voltage drifts between 115 and 120. M125 Re do and autobias install. C3f5a910

    very good work !

    pavlikkkk likes this post

    pavlikkkk
    pavlikkkk


    Posts : 69
    Join date : 2018-03-02
    Age : 76
    Location : Czech Republic

    M125 Re do and autobias install. Empty Re: M125 Re do and autobias install.

    Post by pavlikkkk Sun Mar 12, 2023 1:14 pm

    pavlikkkk wrote:
    Rec wrote:Thanks! Maybe these notes will help others.

    Here is an underside shot. I made the regulated bias circuit seen elsewhere. This stays fixed at -67v regardless of b+ to driver board. I did notice that I would not hold bias on all 4 tubes above about 400v. I imagine with the existing bias supply there might be enough range to bias the tubes, eg negative bias range does not drift up with b+ with the regulation. I added 66ohm dropping resistor (as per the Weber wz68) after the diodes and now the voltages are dialed in and all 4 always bias. My line voltage drifts between 115 and 120. M125 Re do and autobias install. C3f5a910

    very good work !

    Just to be sure, I am still sending the latest version of the manual for this installation and I wish you a pleasant work and a pleasant listening experience:
    https://www.audioamp.eu/_obchody/teston.web5.cz/prilohy/2/modul-pro-rizeni-pushpull-koncovych-stupnu-pro-dyn-62400b.pdf

    avatar
    Rec


    Posts : 68
    Join date : 2020-03-11

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    Post by Rec Sun Mar 12, 2023 1:38 pm

    Thanks ! I went off the map a bit and put the caps in an easier position to change out in future (the mundorf mkp sound good tho) also put a ss power supply on a pcb. The original pio caps are now in an se kt88 amp and are sounding great. Everything is sounding great. Working on a tube cage design.
    pavlikkkk
    pavlikkkk


    Posts : 69
    Join date : 2018-03-02
    Age : 76
    Location : Czech Republic

    M125 Re do and autobias install. Empty Re: M125 Re do and autobias install.

    Post by pavlikkkk Sun Mar 12, 2023 2:17 pm

    Rec wrote:Thanks ! I went off the map a bit and put the caps in an easier position to change out in future (the mundorf mkp sound good tho) also put a ss power supply on a pcb. The original pio caps are now in an se kt88 amp and are sounding great. Everything is sounding great. Working on a tube cage design.  

    It's a good thing all four Mundorfs are the same.
    You also solved the location of the LEDs nicely, which have adequate brightness, as can be seen in the photo. Congratulations!
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    Rec


    Posts : 68
    Join date : 2020-03-11

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    Post by Rec Sun Mar 12, 2023 4:40 pm

    Led are 3mm flat top mounted in hole in chassis secured with thermal glue. I soldered a header to ab board and used leftover ribbon cables with connectors to led.

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