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    Auto-Bias board on Roy's website?

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    ericoto

    Posts : 9
    Join date : 2013-12-11

    M-125 autobias board assembly

    Post by ericoto on Sun Sep 09, 2018 7:55 pm

    Just finished M-125 autobias boards. Was a bit harder then I anticipated, I don't see much feedback on installation, but fit is tight. If you choose to try, my advice is First remove C6 & C8 coupling capacitors and sit as close to board as possible when I assembled the M-125 I kept them fairly high, I also covered them with heat shrink tubing. Could it be possible to substitute a small capacitor for C6 and C8? Second, Keep wires longer then you think for connections between A/B and driver board, I had redo 2 as they were too tight though I had plenty of lead. Third, I soldered all the wires on the bias board, then attached to driver screws, made rest of installation easier.  Just my thoughts and I did not see any other feedback. I had all blue lights after installation. Nice soft start up. No listening tests yet. I am an experienced kit builder but by no means an engineer, so it is possible for casual builder.
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    LeGrace

    Posts : 260
    Join date : 2016-08-07
    Location : Ontario, Canada

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

    Post by LeGrace on Mon Sep 10, 2018 5:16 pm

    Interested in this mod for my M125's. Any additional details would be appreciated, pics also if possible.
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    tubes4hifi
    Admin

    Posts : 1479
    Join date : 2008-11-30

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

    Post by tubes4hifi on Sat Sep 15, 2018 10:47 pm

    still no photos from someone with an M125 amp?? I've sold at least 6 pairs of these, someone please post photos showing the install to help newbies . . .

    ericoto

    Posts : 9
    Join date : 2013-12-11

    M125 Autobias board installation

    Post by ericoto on Sun Sep 16, 2018 11:53 am

    Here'ss a photo I took after my recent installation, installation not too bad although is tight, See above notes, would advise removing coupling capacitors and getting as close to board as poosible, I covered mine in heat shrink tubing to prevent any shorts from metal case. Also I found it easier to attach wires to C17 capacitors when replaced than try to solder later, I just soldered close to body of capacitor. Also I attached all wires to the AB board first, labeled, installed board and soldered to M125 sockets. Saved time. Hope this helps I have attached a single photo, only took 1, I did blow the copper top after installing, likely unrelated, case was loose. If I reassemble will post more photos




    New2Tubez

    Posts : 25
    Join date : 2018-03-20
    Location : NY

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

    Post by New2Tubez on Sun Sep 16, 2018 5:44 pm

    Probably not applicable but I got the AB board for my VTA 120 maybe 4 months ago.

    I haven't installed it yet because the room I solder in is currently an un-airconditioned 78 degrees with 69 percent humidity. It was 85 in August. I'm waiting 'til it gets a bit cooler. I also intend to relocate the 4 LEDs to the VTA board (much dimmer ones). As advised, I will heat shrink the K40 PIO caps as well. Will also use a S/S Weber WS-1 instead of the 5AR4.

    It sounds so great that I'm not in a real hurry!

    Hope your's goes well. I'll post once it's done.
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    deepee99

    Posts : 2030
    Join date : 2012-05-23
    Location : Wallace, Idaho

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

    Post by deepee99 on Sun Sep 16, 2018 6:11 pm

    If this is the same board Don Sachs is using in his ST-120 (Kootenay) amps, you will be blown away. I have had mine now since August of last year and it absolutely sings.

    New2Tubez

    Posts : 25
    Join date : 2018-03-20
    Location : NY

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

    Post by New2Tubez on Sun Sep 16, 2018 7:08 pm

    deepee99 wrote:If this is the same board Don Sachs is using in his ST-120 (Kootenay) amps, you will be blown away. I have had mine now since August of last year and it absolutely sings.

    From the looks of the last pic on the KT88 amp, the center green board seems identical to mine. The transformers and most of the other stuff don't, but I'm thrilled just the same.
    Enjoy the sound..!

    msikk

    Posts : 3
    Join date : 2018-04-13

    Problems with Auto Bias install

    Post by msikk on Mon Oct 01, 2018 7:07 am

    Hello all,

    I'm having a bit of a problem modifying my ST-120 w/auto-bias. Here is what I've done:

    1. Removed TDR from circuit per Bob's instructions
    2. Installed board per instructions
    3. Tested board - biases up to .50 vDC w/adjustment (test on C13 and ground)
    4. Blue lights did not come on during test, so I added tubes to check and see if it needed a load. That lasted about 20 seconds until one of the KT-88's started to red plate and I turned it off.

    I think something shorted earlier as it doesn't appear the auto-bias circuit is working properly. I checked the install again and everything looks fine. I currently don't have a scope (just moved overseas). Any thoughts on what to check next would be appreciated. Thanks,

    Matt
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    pavlikkkk

    Posts : 43
    Join date : 2018-03-02
    Age : 71
    Location : Czech Republic

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

    Post by pavlikkkk on Thu Oct 04, 2018 12:54 pm

    msikk wrote:Hello all,

    I'm having a bit of a problem modifying my ST-120 w/auto-bias. Here is what I've done:

    1. Removed TDR from circuit per Bob's instructions
    2. Installed board per instructions
    3. Tested board - biases up to .50 vDC w/adjustment (test on C13 and ground)
    4. Blue lights did not come on during test, so I added tubes to check and see if it needed a load. That lasted about 20 seconds until one of the KT-88's started to red plate and I turned it off.

    I think something shorted earlier as it doesn't appear the auto-bias circuit is working properly. I checked the install again and everything looks fine. I currently don't have a scope (just moved overseas). Any thoughts on what to check next would be appreciated. Thanks,

    Matt


    1. Check that the cathodes KT88 to AB are connected correctly. The original cathode resistors must not be connected
    2. Check Grid tubes connections over by 100k resistors
    3. do not insert the tubes
    4. Turn on the amplifier
    5. Measure voltage at Pin BIAS on AB
    6. Write what you have measured

    Pavel

    jwb474

    Posts : 29
    Join date : 2014-07-13

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

    Post by jwb474 on Fri Oct 12, 2018 4:22 am

    I unearthed a couple of MK3s. They were my 1st Dynaco Tube build. These have Auto-Bias that I got from the Audio Amateur or Glass Audio. It is a very simple design. After slowly bringing up the voltage on the caps, the amp came right up to and regulated the bias at 70ma. It may be an old design, but it still works and I never had any issues with it. As usual every time I would read about a mod in the magazines, I would have to try it out. The amps also have DC heater on the 6an8 along with elevated heater. This pair of MK3s will get the 2CH auto-Bias I got from Roy.

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    pavlikkkk

    Posts : 43
    Join date : 2018-03-02
    Age : 71
    Location : Czech Republic

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

    Post by pavlikkkk on Fri Oct 12, 2018 5:21 am

    jwb474 wrote:I unearthed a couple of MK3s. They were my 1st Dynaco Tube build. These have Auto-Bias that I got from the Audio Amateur or Glass Audio. It is a very simple design. After slowly bringing up the voltage on the caps, the amp came right up to and regulated the bias at 70ma. It may be an old design, but it still works and I never had any issues with it. As usual every time I would read about a mod in the magazines, I would have to try it out. The amps also have DC heater on the 6an8 along with elevated heater. This pair of MK3s will get the 2CH auto-Bias I got from Roy.


    It's about this:


    this connection uses the AB-2 module.



    Pavel
    www.audioamp.eu


    Last edited by pavlikkkk on Fri Oct 12, 2018 7:45 am; edited 1 time in total

    jwb474

    Posts : 29
    Join date : 2014-07-13

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

    Post by jwb474 on Fri Oct 12, 2018 6:40 am

    It's about this:


    this connection uses the AB-2 module.

    Pavel
    www.audioamp.eu[/quote]

    Man is that close. I actually found my Design/Mod notes for these MK3s. I am still looking for the actual article in which I go this idea from. Your circuit may be the actual circuit from the article in which as usual I made some changes.




    Here is a photo of my science fair project that featured one the these MK3s before they underwent mods.



    Brings back a lot of memories. Bet the members here have similar memories too.

    ericoto

    Posts : 9
    Join date : 2013-12-11

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

    Post by ericoto on Sat Oct 13, 2018 5:45 pm

    What was in the "Advanced Anatomy Set" from Southern Biologic?

    msikk

    Posts : 3
    Join date : 2018-04-13

    Problem w/ST-120 Install

    Post by msikk on Sun Oct 14, 2018 5:05 am

    Pavel,

    As I suspected, I damaged the first board. Installed a replacement and all 4 blue lights illuminate. Bias is set at .5VDC and holding steady. Voltage across each Grid/Cath is holding steady at 4.3VDC. Pin Bias on AB from C17 to ground is -76.4VDC.

    I have not yet put tubes in the amp. I read through the whole thread and didn't see anything I should measure other than what I did above. Thanks for the reply and please let me know if I need to measure anything else,

    Matt


    Last edited by msikk on Mon Oct 15, 2018 5:48 am; edited 1 time in total (Reason for editing : Updated text)

    jwb474

    Posts : 29
    Join date : 2014-07-13

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

    Post by jwb474 on Sun Oct 14, 2018 5:27 am

    ericoto wrote:What was in the "Advanced Anatomy Set" from Southern Biologic?

    If I remember right (Remember this was back around 1968) if was the Human model that was clear that had all of the internals that you could remove the various layers (You could even exchange parts to male and female).
    You have probably seen them in Biology labs and in the movies. I believe it was about 3ft high. There was all the "cue cards" that explained what each part did. It was neat, wish I still had it. It was like the visible V8 model. I still have the visible Radial engine.
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    pavlikkkk

    Posts : 43
    Join date : 2018-03-02
    Age : 71
    Location : Czech Republic

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

    Post by pavlikkkk on Mon Oct 15, 2018 1:00 pm

    msikk wrote:Pavel,

    As I suspected, I damaged the first board. Installed a replacement and all 4 blue lights illuminate. Bias is set at .5VDC and holding steady. Voltage across each Grid/Cath is holding steady at 4.3VDC. Pin Bias on AB from C17 to ground is -76.4VDC.

    I have not yet put tubes in the amp. I read through the whole thread and didn't see anything I should measure other than what I did above. Thanks for the reply and please let me know if I need to measure anything else,

    Matt
    Do not insert Tubes. Immediately after switching on, measure the voltage on all four pins of the GRD module AB.
    Send the measurement results to private mail in this forum.
    Pavel
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    tubes4hifi
    Admin

    Posts : 1479
    Join date : 2008-11-30

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

    Post by tubes4hifi on Mon Nov 05, 2018 3:43 pm

    seems this photo never got posted, auto-bias in an M125 amp . . .
    also the install directions are posted here . . . www.tubes4hifi.com/ABC-M125-4.pdf


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