The Dynaco Tube Audio Forum

Dedicated to the restoration and preservation of all original Dynaco tube audio equipment - Customer support for Dynaco VTA tube amp kits, all Tubes4hifi.com products and all Dynakitparts.com products


    Auto-Bias board on Roy's website?

    Share
    aguaazul
    aguaazul

    Posts : 98
    Join date : 2012-07-08
    Age : 58
    Location : Livermore, CA

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

    Post by aguaazul on Wed Jun 06, 2018 1:15 pm

    aguaazul wrote:I have a DS2 - SP-14. My Latino ST-120 is on the bench waiting for a couple of spare hours to put it together, along with the Octal board.

    Got it all wired up this morning. Using fuse holders for easy replacement.



    Will test tonight.

    Aguaazul

    Hosted picture is kind of huge. Here is a link to a google photos album.


    AB Board - Fuses w/ easy fuse change out
    pavlikkkk
    pavlikkkk

    Posts : 24
    Join date : 2018-03-02
    Age : 71
    Location : Czech Republic

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

    Post by pavlikkkk on Fri Jun 08, 2018 6:50 am

    pichacker wrote:If pin 8 is the cathode and pin 1 unused then the MOV should go the other side of the fuse. Ie pin 1 to ground.

    [Pin 8 Cathode] -- [FUSE] -- [Pin 6] -- [Bias Board and MOV to ground]

    That way the MOV clamps the input to the bias board and the fuse then fails.

    Yes. The MOV V8ZA1P reliably switches to 15V overvoltage. Tested.
    Pavel


    pavlikkkk
    pavlikkkk

    Posts : 24
    Join date : 2018-03-02
    Age : 71
    Location : Czech Republic

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

    Post by pavlikkkk on Sat Jun 16, 2018 2:51 pm

    Kramer wrote:
    aguaazul wrote:
    pavlikkkk wrote:
    pichacker wrote:If pin 8 is the cathode and pin 1 unused then the MOV should go the other side of the fuse. Ie pin 1 to ground.

    [Pin 8 Cathode] -- [FUSE] -- [Pin 6] -- [Bias Board and MOV to ground]

    That way the MOV clamps the input to the bias board and the fuse then fails.

    Yes. The MOV V8ZA1P reliably switches to 15V overvoltage. Tested.
    Pavel
    So what is the advantage of setting it up with the MOV vs just a fuse across pins 1 & 8?

    It's my understanding that with just the fuse a output tube short could still travel back to the AB board and cause damage. The varistor will sit there doing nothing until the voltage increases during a tube short where it will open up and the high voltage will be directed through to ground instead of the AB board preventing damage.

    Yes, the varistor normally does nothing at all.
    aguaazul
    aguaazul

    Posts : 98
    Join date : 2012-07-08
    Age : 58
    Location : Livermore, CA

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

    Post by aguaazul on Sat Jun 16, 2018 7:40 pm

    pavlikkkk wrote:
    Kramer wrote:
    aguaazul wrote:
    pavlikkkk wrote:
    pichacker wrote:If pin 8 is the cathode and pin 1 unused then the MOV should go the other side of the fuse. Ie pin 1 to ground.

    [Pin 8 Cathode] -- [FUSE] -- [Pin 6] -- [Bias Board and MOV to ground]

    That way the MOV clamps the input to the bias board and the fuse then fails.

    Yes. The MOV V8ZA1P reliably switches to 15V overvoltage. Tested.
    Pavel
    So what is the advantage of setting it up with the MOV vs just a fuse across pins 1 & 8?

    It's my understanding that with just the fuse a output tube short could still travel back to the AB board and cause damage. The varistor will sit there doing nothing until the voltage increases during a tube short where it will open up and the high voltage will be directed through to ground instead of the AB board preventing damage.

    Yes, the varistor normally does nothing at all.

    Why the move from pin 1 to pin 6?
    Would the MOV from pin 1 to ground work also?
    tubes4hifi
    tubes4hifi
    Admin

    Posts : 1508
    Join date : 2008-11-30

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

    Post by tubes4hifi on Sat Jun 16, 2018 8:08 pm

    Pavel just used pin 6 as a convenient tie point, but personally I don't like that because then you lose the tie point for the 1K grid resistor,
    I rather just add a terminal strip in there somewhere if you want to add the MOVs.
    tubes4hifi
    tubes4hifi
    Admin

    Posts : 1508
    Join date : 2008-11-30

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

    Post by tubes4hifi on Tue Jun 19, 2018 12:31 pm

    There are many ways to make an electrical connection that has the same results.  I don't disagree with the electrical connection that Pavel has suggested, I just don't prefer using pin 6 of the tube,
    I'd rather keep the grid resisitor where it is.   And just as a fuse in not a necessity, the MOV is not a necessity, but each has it's place in making a better solution. (the AB is not a necessity either).
    So below is a diagram of how I would personally connect things.  It could be done without the terminal strip, but is easier for most to just add one terminal strip between each pair of output tubes.
    Left channel on one side of the amp, Right channel on the other side of the amp.   Just my personal preference.  Not right or wrong.
    Kramer
    Kramer

    Posts : 43
    Join date : 2018-02-11
    Age : 31
    Location : Chicago

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

    Post by Kramer on Thu Jun 21, 2018 9:04 pm

    Here is how I am planning on wiring them up in case this helps.

    pavlikkkk
    pavlikkkk

    Posts : 24
    Join date : 2018-03-02
    Age : 71
    Location : Czech Republic

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

    Post by pavlikkkk on Fri Jun 22, 2018 12:30 am

    Kramer wrote:Here is how I am planning on wiring them up in case this helps.

    Yes, this connection reliably protects AB-Q against short circuit in an tube.
    Pavel
    Kramer
    Kramer

    Posts : 43
    Join date : 2018-02-11
    Age : 31
    Location : Chicago

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

    Post by Kramer on Fri Jun 22, 2018 1:39 pm

    Wired the fuses and varisters in using the configuration shown above. Tested to be sure everything was correct and installed the tubes. Fired the amp up and everything is working properly.
    avatar
    ericoto

    Posts : 14
    Join date : 2013-12-11

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

    Post by ericoto on Fri Jun 22, 2018 2:38 pm

    Just received my auto bias boards for pair of M125's, does anyone have any photos of it installed in an M125, IS the notch on the board to accommodate c16 and c17 on driver board? What do I do with the PIO capacitors on the driver board?

    Thanks
    Kramer
    Kramer

    Posts : 43
    Join date : 2018-02-11
    Age : 31
    Location : Chicago

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

    Post by Kramer on Fri Jun 22, 2018 6:43 pm

    ericoto wrote:Just received my auto bias boards for pair of M125's, does anyone have any photos of it installed in an M125, IS the notch on the board to accommodate c16 and c17 on driver board? What do I do with the PIO capacitors on the driver board?

    Thanks

    Bob posted a photo earlier in the thread that shows one installed in a M125, it's a bit close up but it may help with the orientation. The installation into the VTA120 required replacing two capacitors to shorter caps of the same value to properly fit over the driver board. The standoff screws should pull the board away enough for the POI caps to fit ok but results may vary, I used a strip of thick electrical tape on the side of the POI caps facing the A/B board just in case.
    aguaazul
    aguaazul

    Posts : 98
    Join date : 2012-07-08
    Age : 58
    Location : Livermore, CA

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

    Post by aguaazul on Sun Jul 15, 2018 5:54 pm

    pavlikkkk wrote:
    Kramer wrote:Here is how I am planning on wiring them up in case this helps.

    Yes, this connection reliably protects AB-Q against short circuit in an tube.
    Pavel


    I did a variation on this theme:



    I like the idea of the fuse holders on a single terminal strip. These are the old rat shack terminal strips. The tabs are a little tight for the fuse holders, so I did a little delicate file work to put a tiny notch in the insides of the tabs.

    avatar
    tomlang

    Posts : 35
    Join date : 2009-08-12

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

    Post by tomlang on Thu Aug 16, 2018 8:57 am

    This seems like a great development.  I appreciate the effort the designer (Pavel) goes in engaging the community with this.  Some questions:

    1. Are the tubesforhifi st70 board and the AB-Q board the same except for the footprint of the pcb?
    2. Is there any effort to put protection components for the output tubes i.e. varistors, etc. on the board itself?
    3. Finally is this is a fully assembled board?
    pavlikkkk
    pavlikkkk

    Posts : 24
    Join date : 2018-03-02
    Age : 71
    Location : Czech Republic

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

    Post by pavlikkkk on Thu Aug 16, 2018 12:43 pm

    tomlang wrote:This seems like a great development.  I appreciate the effort the designer (Pavel) goes in engaging the community with this.  Some questions:

    1. Are the tubesforhifi st70 board and the AB-Q board the same except for the footprint of the pcb?
    2. Is there any effort to put protection components for the output tubes i.e. varistors, etc. on the board itself?
    3. Finally is this is a fully assembled board?

    Answers to questions:
    1)The AB-Q PCB is electrically the same as the PCB AB-Q-ST70-ST120.
    2)Varistor should not be on PCB, nor fuse. In case of a short circuit, there is a high voltage on these components and a the short can damage the PCB.
    3)The boards are fitted with all the components except varistors and fuses.
    Pavel
    audioamp.eu
    tubes4hifi
    tubes4hifi
    Admin

    Posts : 1508
    Join date : 2008-11-30

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

    Post by tubes4hifi on Fri Aug 31, 2018 1:25 pm

    the auto-bias connection to bias from the driver board is to C17, so yes, it's possible to remove C18 and R39, but not necessary
    avatar
    ericoto

    Posts : 14
    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.
    tubes4hifi
    tubes4hifi
    Admin

    Posts : 1508
    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 . . .
    avatar
    ericoto

    Posts : 14
    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



    avatar
    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
    pavlikkkk
    pavlikkkk

    Posts : 24
    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
    avatar
    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)
    pavlikkkk
    pavlikkkk

    Posts : 24
    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
    tubes4hifi
    tubes4hifi
    Admin

    Posts : 1508
    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-install.pdf

    pavlikkkk
    pavlikkkk

    Posts : 24
    Join date : 2018-03-02
    Age : 71
    Location : Czech Republic

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

    Post by pavlikkkk on Wed Jan 09, 2019 1:34 pm

    Here are the new detailed AB-Q assembly instructions ST70-ST120 and AB-Q MK125:
    AB-Q M125


    AB-Q ST70-ST120



    Last edited by pavlikkkk on Fri Jan 11, 2019 3:32 pm; edited 2 times in total
    tubes4hifi
    tubes4hifi
    Admin

    Posts : 1508
    Join date : 2008-11-30

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

    Post by tubes4hifi on Wed Jan 09, 2019 1:58 pm

    thanks Pavel, these are excellent manuals and should make things clear to anyone and everyone.
    This thread has gotten quite long, and MOST but not all of the older posts have become irrelevant, so I'm going to edit and delete older postings to clarify and update the subject.

    Sponsored content

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

    Post by Sponsored content


      Current date/time is Wed Jan 23, 2019 4:33 am