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    Auto-Bias board on Roy's website?

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    jfine

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

    Post by jfine on Tue Apr 24, 2018 11:50 pm

    tubes4hifi wrote:next up is the M125 version, a little more involved for install since you need to add two capacitors and two resistors.
    On a stock M125, the output tubes are connected in pairs.   With this board EACH tube will be individually controlled to EXACT bias and balance.  
    I'll supply the resistors, you supply matching caps depending on what you currently have (or let me know and I can order duplicates for you).
    Pictured here is the M125 board under test, and showing the big K40Y caps which are slightly larger than the current standard K42Y (green) caps.
    One photo shows the mini red LED during warmup.  This one came preset for 58ma per tube, I readjusted down to 50ma as per Bob's current recommendation,
    although most KT88 amps are fine with 55-65ma so you could just leave it as is.   That was about 1.5 turns CCW.  Same nice slow 40 second ramp up.

    Interesting, so you can pick/set a preset then it will simply use that. I'm a chicken $hi% when it comes to sticking probes in something powered up (electronics guys). So setting initial bias can all be done with board completely installed, and no tubes?

    About the caps, so are these moved from the existing driver board, or are those 2 extra caps?

    mazeeff

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

    Post by mazeeff on Wed Apr 25, 2018 12:17 pm

    Roy. Most of us have experienced blowing a 10 ohm resistor when a tube blows. With the 10 ohm resistors moved to the auto-bias board, does the 10 ohm resistor still act as a fuse, or are there other components in the 10 ohm path? I think that I have replaced 3-4 10 ohm resistors over the years, as a result of tube failure.
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    tubes4hifi
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    Re: Auto-Bias board on Roy's website?

    Post by tubes4hifi on Wed Apr 25, 2018 12:19 pm

    for the M125s which are running two pairs of output tubes, you need to add two capacitors and two resistors, so that EACH tube can be controlled individually
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    tubes4hifi
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    Re: Auto-Bias board on Roy's website?

    Post by tubes4hifi on Wed Apr 25, 2018 12:26 pm

    yes, the ten ohm resistors on the auto-bias board replace the ones that were in the amp originally, in the same location, so they still have the same function.
    It's nearly impossible to blow one of these out, as they are 1w resistors, and if you have 0.5v of bias running thru ten ohms, that is 50ma x .5v = 25mw.
    To blow out a 1w resistor would require more than 300ma of tube current (300ma x 3v= 0.9 watts).
    Of course with a functioning auto-bias circuit, this would NEVER happen. If the tube goes bad and tries to run-away with current, the auto-bias circuit will set the current thru the tube at zero immediately.
    This is one of the two huge benefits of auto-bias, you should never ever lose a tube due to red-plating. The other benefits of course are never having to set bias, and always having PERFECT bias and balance.
    Perfect output symmetry, minimal distortion.
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    pavlikkkk

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

    Post by pavlikkkk on Wed Apr 25, 2018 1:00 pm

    mazeeff wrote:Roy. Most of us have experienced blowing a 10 ohm resistor when a tube blows. With the 10 ohm resistors moved to the auto-bias board, does the 10 ohm resistor still act as a fuse, or are there other components in the 10 ohm path? I think that I have replaced 3-4 10 ohm resistors over the years, as a result of tube failure.
    Resistors 10 Ohm 1% on the module are carbon. They are not metallized and act as a fuse.
    Pavel
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    tubes4hifi
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    Re: Auto-Bias board on Roy's website?

    Post by tubes4hifi on Sat Apr 28, 2018 7:06 pm

    out of stock on ST70-ST120 auto bias boards now, should have more in about 2-3 weeks.
    Two pairs of M125 boards left.
    Go back to the first page of this thread for an updated M125 schematic with auto-bias connections.
    This photo/drawing hasn't been posted yet and should be helpful for M125 amp installs . . . .

    update 5-5   my bad due to exhaustion (working 16 hours a day every day this past week preparing for a move)
                     change notation on photo - jumper was from pin 6 to pin 6.   And all AB grid outputs go to pin 6 of all output tubes.

    update 5-9   seems I missed one necessary link to remove, see updated photo, the connection from V1-V3 pin 1 and V2-V4 pin 1 must be removed !!
                     very sorry I overlooked this one

    update 5-18   change bias connection to C17 instead of C18




    Last edited by tubes4hifi on Sat May 19, 2018 8:32 pm; edited 7 times in total
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    erhard-audio

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

    Post by erhard-audio on Sun Apr 29, 2018 2:08 pm

    kevinmi wrote:OK, I'm going to add this auto bias board to my ST-120 amp, but I'm a bit confused on one thing-does this work in tandem with the time delay board, or do I have to remove it?

    just double check on the connections to the Auto Bias with regards to time delayed B+, I think I read in the instructions that if there is a delayed B+, the jumper link needs to be removed and a different B+ connection?, I could be wrong, but you better check that, or maybe Pavel can chime in here.

    mazeeff

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

    Post by mazeeff on Sun Apr 29, 2018 2:55 pm

    Bob. Just curious as to whether you intend to offer this board as an upgrade to your "wired" amps? It would be great if Roy could combine the two boards into one, just to clean it up a bit!
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    pavlikkkk

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

    Post by pavlikkkk on Sun Apr 29, 2018 3:44 pm

    erhard-audio wrote:
    kevinmi wrote:OK, I'm going to add this auto bias board to my ST-120 amp, but I'm a bit confused on one thing-does this work in tandem with the time delay board, or do I have to remove it?

    just double check on the connections to the Auto Bias with regards to time delayed B+, I think I read in the instructions that if there is a delayed B+, the jumper link needs to be removed and a different B+ connection?, I could be wrong, but you better check that, or maybe Pavel can chime in here.

    the maximum delay time (TDR ST120) when using a module's autobias is 20 sec.
    What happens when using TDR?
    Module this function:
    1) After switching on within 20sec. sets the module reference voltage (for example, 0.6V - 60mA for KT88)
    2) There is no voltage on the anode of tubes and cathode resistors - TDR has not connected
    3) On the Grids tubes, do the maximum bias voltage
    4) The module starts to compare the reference voltage (0,6V) with the voltage on the cathode resistors and slowly (within 60 seconds) reduce bias voltage on grids tubes
    5) Since the 0V cathode (TDR is connected not), bias voltage is also reduced to 0V
    6) TDR connected bom
    7) tubes get shock because they are without bias voltage !!

    I strongly recommend to disconnect the TDR !
    The module's delay functions are more sophisticated !
    I hope you understand.. Very Happy

    Pavel


    Last edited by pavlikkkk on Mon Apr 30, 2018 8:42 am; edited 1 time in total
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    Bob Latino
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    Re: Auto-Bias board on Roy's website?

    Post by Bob Latino on Sun Apr 29, 2018 4:05 pm

    Since Pavel designed the AutoBias board, I am going to defer to him... If you have the TDR board on your VTA amp, disconnect and bypass the TDR board. Disconnecting the TDR board is easy to do.

    1. Remove the two wires from that now go from the TDR board to the rectifier tube socket on pins 4 and 6
    2. Remove the two RED wires that go from the power transformer to the TDR board.
    3. Connect those two RED wires that went to the TDR and now connect them to pins 4 and 6 on the rectifier tube socket. You may have to extend the two wires a few inches.

    Bob

    mcgyver74

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

    Post by mcgyver74 on Mon Apr 30, 2018 2:31 pm

    I ordered this board over the weekend for my 120, will let y'all know how the install goes Smile
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    jfine

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

    Post by jfine on Mon Apr 30, 2018 7:05 pm

    So for the M125's, once the boards are completely installed, what is the best way to check the bias? Power it up, do I need any tubes installed, I noticed the positive test pad next to the bias adjustment, but do I just ground the other lead to chassis, etc., is that it?
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    tubes4hifi
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    Re: Auto-Bias board on Roy's website?

    Post by tubes4hifi on Tue May 01, 2018 12:01 pm

    jfine
    connect the filament voltage (6.3vac on AB board to V2 output tube pins 2 & 7, then power up amp (no tubes in amp needed).
    After 40 seconds AB red LED will go out, and bias voltage on test point will ramp up to either 0.40vdc (on ST70 boards) or to 0.58vdc (on M125 boards).
    Test point is round dot right next to C13.  Red meter lead to test point, black meter lead to chassis ground, or ground hole on AB board.


    Last edited by tubes4hifi on Tue May 01, 2018 12:37 pm; edited 1 time in total
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    tubes4hifi
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    Re: Auto-Bias board on Roy's website?

    Post by tubes4hifi on Tue May 01, 2018 12:05 pm

    OK, so I don't have an amp here, but here's a drawing that should make this a simple install in the ST70 or ST120 amps.
    Download the photo from here to see it full size.  http://www.tubes4hifi.com/AB-ST70install3.jpg
    It's color coded, so a B&W print won't help much!
    Connections to R29, 30, 31, 32 are to the opposite end of outputs on VTA board (the ends that used to connect to the bias pots !!)


    Bennyhaha812

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

    Post by Bennyhaha812 on Tue May 01, 2018 7:14 pm

    So I just had one of the new golden lion kt88 short out on me and redplate blowing out my bias resistor! Thankfully I had one left from the set Bob sent me when I replace the Sovteks. I had a bad tube that was arcing and had moved it around and worn out a few of my bias resistors. I now own a Hickok 600a and can check my tubes before using them. One of the Sovteks are now being used until I can replace the golden lion. So will this board fit it the stock amp case or am I going to have to finally take it all apart and build a custom case with Maida style regulators to use this? Smile
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    pichacker

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

    Post by pichacker on Wed May 02, 2018 6:13 am

    How robust is the auto bias board to tube flashovers?

    If a tube goes short, or draws high current for any reason, then there is a possibility of a high voltage being presented to the cathode path. On a stock version this fuses the 10 ohm resistor. On the auto bias board this would of course make its way to the sensing circuit that would normally only be looking for 0.6V. With a defective tube we would have little control using the grid bias voltage.

    mazeeff

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

    Post by mazeeff on Thu May 03, 2018 7:27 am

    pichacker wrote:How robust is the auto bias board to tube flashovers?

    If a tube goes short, or draws high current for any reason, then there is a possibility of a high voltage being presented to the cathode path. On a stock version this fuses the 10 ohm resistor. On the auto bias board this would of course make its way to the sensing circuit that would normally only be looking for 0.6V. With a defective tube we would have little control using the grid bias voltage.

    I share the same concern. Perhaps someone with the board can describe the part used to control the cathode current. That part would need to be able to withstand 500v on it, given a tube failure. A part number or schematic would be helpful in determining the max voltage of the part. I would also like to convince myself that the circuit prevents blowing the 10 ohm resistor, as Roy pointed out earlier. When these 10 ohm resistors blow, they generate an immense amount of heat. That heat may jeopardize the circuit board, if the 10 ohm resistors continue to act as fuses, in the event of tube failure.
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    corndog71

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

    Post by corndog71 on Thu May 03, 2018 9:19 am

    You can always add fuses between the cathodes and their respective auto-bias connection.  But you can't just throw in any ordinary fuse.  You need a high breaking ceramic fuse like this https://www.digikey.com/products/en?keywords=%20F4603-ND   This will work for each power tube of an ST70, ST120, and each paralleled pair of tubes of the M125.

    I know what some are thinking.  Why use a $1.50 fuse when you can just let a 0.10 cent resistor burn up?  Well, if you've ever smelled a resistor go up smoke then you know it's not a pleasant experience.  Then you have to unsolder it and resolder a new one.  It's a convenience to simply replace a fuse.  Granted there's not a lot of room in the classic dynaco chassis.  This is another reason why I prefer a custom layout.
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    pavlikkkk

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

    Post by pavlikkkk on Thu May 03, 2018 11:49 am

    corndog71 wrote:You can always add fuses between the cathodes and their respective auto-bias connection.  But you can't just throw in any ordinary fuse.  You need a high breaking ceramic fuse like this https://www.digikey.com/products/en?keywords=%20F4603-ND   This will work for each power tube of an ST70, ST120, and each paralleled pair of tubes of the M125.

    I know what some are thinking.  Why use a $1.50 fuse when you can just let a 0.10 cent resistor burn up?  Well, if you've ever smelled a resistor go up smoke then you know it's not a pleasant experience.  Then you have to unsolder it and resolder a new one.  It's a convenience to simply replace a fuse.  Granted there's not a lot of room in the classic dynaco chassis.  This is another reason why I prefer a custom layout.

    Watch out for the one thing with moduls AB-Q Series!
    If you use a fuse,the fuse has a resistor of 1.24 Ohm.
    1.24 Ohm is added to the resistance value of the 10 Ohm cathode resistor.
    Altogether, it is 11.24 Ohm in the cathode against the GND.
    The setting of the cathode already output does not match Ikat = Uref / 10 (mA, mV) !

    If you want to set the correct cathode current, you can do it only by measuring the voltage on a 11.42 Ohm clocked "value resistance" cathode.
    You set without signal !
    Pavel
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    corndog71

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

    Post by corndog71 on Thu May 03, 2018 12:56 pm

    pavlikkkk wrote:
    corndog71 wrote:You can always add fuses between the cathodes and their respective auto-bias connection.  But you can't just throw in any ordinary fuse.  You need a high breaking ceramic fuse like this https://www.digikey.com/products/en?keywords=%20F4603-ND   This will work for each power tube of an ST70, ST120, and each paralleled pair of tubes of the M125.

    I know what some are thinking.  Why use a $1.50 fuse when you can just let a 0.10 cent resistor burn up?  Well, if you've ever smelled a resistor go up smoke then you know it's not a pleasant experience.  Then you have to unsolder it and resolder a new one.  It's a convenience to simply replace a fuse.  Granted there's not a lot of room in the classic dynaco chassis.  This is another reason why I prefer a custom layout.

    Watch out for the one thing with moduls AB-Q Series!
    If you use a fuse,the fuse has a resistor of 1.24 Ohm.
    1.24 Ohm is added to the resistance value of the 10 Ohm cathode resistor.
    Altogether, it is 11.24 Ohm in the cathode against the GND.
    The setting of the cathode already output does not match Ikat = Uref / 10 (mA, mV) !

    If you want to set the correct cathode current, you can do it only by measuring the voltage on a 11.42 Ohm clocked "value resistance" cathode.
    You set without signal !
    Pavel

    Sorry, I didn't mean to complicate things. Embarassed
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    pavlikkkk

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

    Post by pavlikkkk on Thu May 03, 2018 1:09 pm

    corndog71 wrote:
    pavlikkkk wrote:
    corndog71 wrote:You can always add fuses between the cathodes and their respective auto-bias connection.  But you can't just throw in any ordinary fuse.  You need a high breaking ceramic fuse like this https://www.digikey.com/products/en?keywords=%20F4603-ND   This will work for each power tube of an ST70, ST120, and each paralleled pair of tubes of the M125.

    I know what some are thinking.  Why use a $1.50 fuse when you can just let a 0.10 cent resistor burn up?  Well, if you've ever smelled a resistor go up smoke then you know it's not a pleasant experience.  Then you have to unsolder it and resolder a new one.  It's a convenience to simply replace a fuse.  Granted there's not a lot of room in the classic dynaco chassis.  This is another reason why I prefer a custom layout.

    Watch out for the one thing with moduls AB-Q Series!
    If you use a fuse,the fuse has a resistor of 1.24 Ohm.
    1.24 Ohm is added to the resistance value of the 10 Ohm cathode resistor.
    Altogether, it is 11.24 Ohm in the cathode against the GND.
    The setting of the cathode already output does not match Ikat = Uref / 10 (mA, mV) !

    If you want to set the correct cathode current, you can do it only by measuring the voltage on a 11.42 Ohm clocked "value resistance" cathode.
    You set without signal !

    On the contrary, I'm glad you wrote it, thank you.
    It's for the other ..

    Pavel
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    erhard-audio

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

    Post by erhard-audio on Thu May 03, 2018 3:25 pm

    pavlikkkk wrote:
    jfine wrote:Question, (M125's)

    When auto bias board is installed, is it still possible to check the bias thru the existing front holes, or are these disconnected, or is there some relatively easy method to check to make sure bias stays consistent?

    I'm remake the AB-Q module (LEDs)...
    I have invent how to mount LEDs to the amp panel.
    I removed the SMD LED from the board.
    Into the LEDs pad I drill the holes in 0.8mm.
    I can use the LED with cable to go where I need it..



    it's difficult to see on the photo, which end/side of the SMT LED is the anode. I am currently building an amp for a client using the Auto Bias module and I have four LED's on the top plate for external visual indication so will need to know which end is the anode, thanks Pavel.
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    pavlikkkk

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

    Post by pavlikkkk on Thu May 03, 2018 4:22 pm

    erhard-audio wrote:
    pavlikkkk wrote:
    jfine wrote:Question, (M125's)

    When auto bias board is installed, is it still possible to check the bias thru the existing front holes, or are these disconnected, or is there some relatively easy method to check to make sure bias stays consistent?

    I'm remake the AB-Q module (LEDs)...
    I have invent how to mount LEDs to the amp panel.
    I removed the SMD LED from the board.
    Into the LEDs pad I drill the holes in 0.8mm.
    I can use the LED with cable to go where I need it..



    it's difficult to see on the photo, which end/side of the SMT LED is the anode. I am currently building an amp for a client using the Auto Bias module and I have four LED's on the top plate for external visual indication so will need to know which end is the anode, thanks Pavel.


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    jfine

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

    Post by jfine on Thu May 03, 2018 6:14 pm

    corndog71 wrote:I know what some are thinking. Why use a $1.50 fuse when you can just let a 0.10 cent resistor burn up?

    So is this the final answer, without a fuse the resistor frys, so is that the extent of the flashover problem?

    Bennyhaha812

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

    Post by Bennyhaha812 on Thu May 03, 2018 6:40 pm

    Pretty sure I read earlier that if a tube dies power is immediately clamped off preventing the blowing of the resistor by this bias board so no more blown resistors! I wish I could remember where I read this and link it......

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