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    Auto-Bias board on Roy's website?

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    pavlikkkk

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

    Post by pavlikkkk on Sun May 06, 2018 4:56 pm

    tubes4hifi wrote:J FIne,
    almost all hi-end modern tube amplifiers are now using auto-bias.  Since we don't have the schematics for either the Pavel design we (Tubes4HiFi) are using, and we don't have the schematics for the PrimaLuna amps,
    there's no way to compare for sure.   This version could certainly be called "adaptive", because if you substitute one tube for another then it's still going to automatically adjust bias to the preset value.
    This is still a "fixed bias" method, which is the best for highest power and lowest distortion.  It is NOT cathode bias.
    We are still using the 10 ohm resistor to measure and compare bias, same as before.   A "cathode bias" method would use an appropriate resistor for the tube type (say maybe 200 ohm 10 watts)
    for an EL34, along with a large 220-1000uF capacitor in parallel with it.  An amp like that has lower power and higher distortion.

    the principle of connecting the AB-2, AB-4 modules is similar to the Primaluna connection.
    The AB-Q and AB-D modules use a different control principle, which is independent of the end-tube excitation.
    Pavel
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    jfine

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

    Post by jfine on Mon May 07, 2018 2:16 am

    Roy/Pavel,

    What happens with the M125 when a tube begins to fail, I understand the other 3 tubes would still run with correct bias, but is there some audible difference, like maybe less volume, etc., that would alert you that the tube failed? It would seem you could run with 3 tubes and maybe not notice?
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    pavlikkkk

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

    Post by pavlikkkk on Mon May 07, 2018 2:56 am

    jfine wrote:Roy/Pavel,

    What happens with the M125 when a tube begins to fail, I understand the other 3 tubes would still run with correct bias,  but is there some audible difference, like maybe less volume, etc., that would alert you that the tube failed? It would seem you could run with 3 tubes and maybe not notice?
    you will notice, of course... distortion will be significant, it can be heard Mad
    Pavel

    mazeeff

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

    Post by mazeeff on Mon May 07, 2018 4:15 am

    I recall a thread on another forum, where folks were talking about the benefits of maintaining consistent bias in a push-pull tube amp. They referred to a term called "Crossover distortion". This distortion was minimized by maintaining the same exact bias on each of the two tubes. Can someone explain how this level of distortion relates to overall THD? I think I also read that if the bias was equalized, that the amp would run in Class A mode for a greater part of the push-pull cycle. I have never quite understood this, but it would appear that the auto-bias board benefits both distortion, and class AB performance. Are these assumptions correct? Is the benefit audible??? For you Math Majors out there, if my bias difference were to be 10% in a push-pull amp, what would the distortion % be? A couple of days ago, I decided to put 4 meters on the bias points in my ST120 to watch the bias drift during warmup. It was surprising the amount of voltage drift that I saw during the 1st hour of warmup (without auto-bias). I saw 1 of my KT120's start at 60ma and work its way down to 55ma, and another that started at 50ma, and worked its way up to 55ma. The auto-bias board would eliminate this difference, without the need to warm up the amp. At this point, I don't see much downside to using auto-bias. The benefits appear to be significant!
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    Kramer

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

    Post by Kramer on Mon May 07, 2018 12:04 pm

    I just got the bias board installed in my st120 amp. I normally am very cautious of my bias levels after losing a tube shortly after assembling the kit. I would check pretty much every time I started the amp.

    After installing and doing a listening test I would say there is definitely a noted difference in the sound. Exactly what I would expect to hear from all tubes consistently at optimal bias levels. I am very happy with the results of installing this board and highly recommend it to anyone considering it. Thanks everyone involved for a great addon to our already amazing amps.


    Last edited by Kramer on Mon May 07, 2018 6:09 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 Wed May 09, 2018 3:58 pm

    in support of the "fuse" addition discussion, anyone who has purchased auto-bias boards from me in the past two months,
    I am offering FREE fuses, email me For anyone else interested in fusing their tube amp output tubes, email me, $1 per fuse,
    these are mini-fuses about 8mm diameter and connect directly between pin 1 and pin 8 on each output tube.
    info@tubes4hifi.com

    mazeeff

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

    Post by mazeeff on Thu May 10, 2018 3:58 am

    Pavel. What is the overall precision of the AB unit? If I set at 50ma, all tubes are set within what %? Once set, how much "difference" must exist, before the AB unit goes into adjustment mode?
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    jfine

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

    Post by jfine on Thu May 10, 2018 9:16 pm

    Pavel again..

    The M125 has the option of only running 2 tubes instead of 4. Will the board function properly when 2 tubes are removed like this?

    Actually, will the amp function properly?
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    pavlikkkk

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

    Post by pavlikkkk on Sat May 12, 2018 2:50 am

    mazeeff wrote:Pavel. What is the overall precision of the AB unit? If I set at 50ma, all tubes are set within what %? Once set, how much "difference" must exist, before the AB unit goes into adjustment mode?
    Accuracy is due to tolerance of cathode resistors. On AB-Q is 1% resistors.
    If you leave only two tubes in the amplifier, AB-Q will drive the other two.
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    CletusB

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

    Post by CletusB on Thu May 17, 2018 7:42 am

    Well, I'm thinking about installing the Auto-Bias board in my ST-120 amp (although I just hate fixin things that ain't broke).
    Would be great to see some more pics from people who did do the install.
    If I do it, the TDR board, when removed, will be installed in my Fender Twin-Reverb amp.

    gguarneri

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    I was talking to Pavel

    Post by gguarneri on Sat May 19, 2018 2:03 am

    05/19 2018-01:55:39

    I am new here, but I have bought several ABQ boards from Pavel.  I have one installed in my VTA-120 along with Bob Latino's time delay board and a highly regulated DC heater board for the heaters of the 12AU7 tubes in the amp.  I also have one installed in my Marantz 8B.  As most of you already know, they work great.

    I additionally installed digital meters where the bias check octal sockets usually go, ST70, ST120, VTA120, and a DPDT switch where the stereo/mono switch would go on a ST70, but is not used in the VTA120, so I can easily check the bias two tubes at a time.  The meters only have two significant digits to the right of the decimal point, so they always read .50 all the the time.  I leave the amp on (KT88s) days at a time, and .50 is always on the meters whenever I check.  Ofcourse, before I installed the meters I use to check the bias at the octal sockets with my multi-meter, which has three significant digits to the right of the decimal point, and there would usually be a variance of +/- 2 or 3 in the third digit to the right of the decimal point; but hey, I'm talking 2 or 3 thousandth of a volt here, so that is really insignificant.  Oh yeah, I installed an on/off switch for the 5Vdc power supply, necessary to power the meters, which is mounted to the bottom of the amp's power transformer inside the amp, so that the meters are only lit when I want to check the bias.  I like it.

    Anyway, I am not great with forums: I usually like to read them, but seldom register and post to them.  However, I was looking at the audioamp.eu site, where I buy most of my ABQ boards now, and I noticed the ABS-Q+ plus board.  I want to use this board in my Marantz 8B, but the information at the site said it requires 230 mains voltage.  So I contacted Pavel to ask if the new module could be used here in the USA.  He got back to me with what seem at first to be not so good news.  He said he was sorry, but it could only be used with 230v.  But then he got back to me again, and said it seems they are going to be introducing the latest modules usable with 115/230v.  He said the new ABS-Q+ modules are going to be the next iteration of the modules, and he mentioned availability in as little as a week or two from now.   He mentioned I should blab about it, so here I am blabbing.  I guess I'll see if there are any other threads (if that's what you call em) about auto bias I can blab on.
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    tubes4hifi
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    Re: Auto-Bias board on Roy's website?

    Post by tubes4hifi on Wed May 23, 2018 4:09 pm

    seems no one has posted a photo of the auto-bias board in a working amp, so here's a photo of one I installed in a customer amp yesterday, with fuses on each output tube

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    aguaazul

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

    Post by aguaazul on Thu May 24, 2018 1:49 am

    Roy, Thanks for posting that! Now I can copy your expertise in my amp.  Good Vendor Salute!
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    pichacker

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

    Post by pichacker on Wed May 30, 2018 2:31 am

    Pavel,

    Why should the reference calculation change because of the fuse? If the fuse is connected between the cathode and the autobias board then it is not in the feedback loop of the cathode resistor. The bias sampling voltage still comes from across the 10R resistor so should not change.

    Cathode ---> Fuse ---> X 10R X ---> Gnd.

    https://photos.app.goo.gl/bhA0jyiIDbOEr0n93

    If the bias current is altered by measuring the voltage across the 10R (X above) then the fuse will not make a noticable difference surely?

    On a different note regarding the removal of the power on delay and solid state rectification. If the bias board holds the output valves biased off until they have warmed up, then with no power on delay, solid state rectification, and no load, the main resovoir cap will be charge to peak voltage until normal operation is established. As I have yet to receive my board (still in the post) I have not made any measurements.

    I also note that there is a terminal for "HiV" on the auto bias board. Looking at the instructions it appears that this could be connected to the HT line to control the operation of the bias board and start the power on delay process once HT has been established? Am I reading this correctly? If this is the case then what are the voltage ranges allowable on this pin please?

    Just being a devils advocate but I note the mini fuses are rated for 250V. If an output tube suffers a breakdown or flashover from anode to cathode then in thoery full HT (minus drop across opt) could be briefly applied to the fuse.

    Steve
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    pavlikkkk

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

    Post by pavlikkkk on Wed May 30, 2018 11:24 am

    pichacker wrote:Pavel,

    Why should the reference calculation change because of the fuse? If the fuse is connected between the cathode and the autobias board then it is not in the feedback loop of the cathode resistor. The bias sampling voltage still comes from across the 10R resistor so should not change.

    Cathode ---> Fuse ---> X 10R X ---> Gnd.

    https://photos.app.goo.gl/bhA0jyiIDbOEr0n93

    If the bias current is altered by measuring the voltage across the 10R (X above) then the fuse will not make a noticable difference surely?

    On a different note regarding the removal of the power on delay and solid state rectification. If the bias board holds the output valves biased off until they have warmed up, then with no power on delay, solid state rectification, and no load, the main resovoir cap will be charge to peak voltage until normal operation is established. As I have yet to receive my board (still in the post) I have not made any measurements.

    I also note that there is a terminal for "HiV" on the auto bias board. Looking at the instructions it appears that this could be connected to the HT line to control the operation of the bias board and start the power on delay process once HT has been established? Am I reading this correctly? If this is the case then what are the voltage ranges allowable on this pin please?

    Just being a devils advocate but I note the mini fuses are rated for 250V. If an output tube suffers a breakdown or flashover from anode to cathode then in thoery full HT (minus drop across opt) could be briefly applied to the fuse.

    Steve

    You're right. If the cathode resistors remain on the module (the original), then nothing has to be recalculated  is true Uref = Icath x 10 (I measured it).
    If you dismantle the resistors, add the fuses and mount them to the tube plinths, connect the AB-Q CATH input to the cathodes. Then you have to recalculate it Uref.
    Further,
    After switching on the amplifier (without anode voltage delay) grids Tubesand have the highest negative voltage from the bias source.
    After about 90 sec. tubes are gradually opening.
    It is not necessary to connect the HiV to anode voltage delays.

    Remark the fuse voltage rating is not correct, but I think the fuses fulfills the function even if the 250V voltage is exceeded.

    The maximum allowed voltage at HiV is 400V


    Pavel
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    pichacker

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

    Post by pichacker on Thu May 31, 2018 3:02 am

    Pavel,

    Thank you for the prompt response. Looking forward to receiving my board soon.

    If anyone lifted the resistor on the board and added the fuse there this would be counter productive to the initial task of attempting to protect the bias board from a high voltage. The fuse on the valve base is probably the best place in my opinion.

    I was wondering if it would be advantageous to add a "sacrificial" zener between the cathode connection to the bias board and ground (possibly at the tube base). Under normal circumstances the zener would not conduct but under fault conditions it would prevent the higher than normal voltage spike entering the board, clamping the voltage for long enough for the fuse to blow.

    https://photos.app.goo.gl/ozWaSeRoNkj6D7GN2

    Cheers

    Steve


    Last edited by pichacker on Thu May 31, 2018 4:47 am; edited 1 time in total
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    pichacker

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

    Post by pichacker on Thu May 31, 2018 4:56 am

    Pavel,

    Wasn't suggesting removing the resistor, I appreciate this is needed for the system to function.
    Just thought that the best place would be on the board as you have designed it.

    If the zener voltage is at a level much greater than the nominal voltage developed by the standing bias, or music peaks, then it will sit there doing nothing until a fault occurs. I'd rather change an off board fuse and possibly a zener than an op amp on a pcb.

    Cheers

    Steve
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    pavlikkkk

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

    Post by pavlikkkk on Thu May 31, 2018 3:55 pm

    pichacker wrote:Pavel,

    Thank you for the prompt response. Looking forward to receiving my board soon.

    If anyone lifted the resistor on the board and added the fuse there this would be counter productive to the initial task of attempting to protect the bias board from a high voltage. The fuse on the valve base is probably the best place in my opinion.

    I was wondering if it would be advantageous to add a "sacrificial" zener between the cathode connection to the bias board and ground (possibly at the tube base). Under normal circumstances the zener would not conduct but under fault conditions it would prevent the higher than normal voltage spike entering the board, clamping the voltage for long enough for the fuse to blow.

    https://photos.app.goo.gl/ozWaSeRoNkj6D7GN2

    Cheers

    Steve

    Probably the best thing would be to use a varistor, for example LITTELFUSE V8ZA1P.
    It would not have to change after the short circuit.


    Pavel


    I tried it, it works perfectly.


    Last edited by pavlikkkk on Wed Jun 06, 2018 4:56 am; 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 Sat Jun 02, 2018 8:59 pm

    to Kramer, and others regarding . . . The image posted looks like the AB cathode connection is going to pin 8 on both sockets and the fuse used to connect pin 8 to 1.

    The cathode connection should go to the unused pin 1 and the fuse should be used to bridge the connection to the cathode at pin 8.


    On all octal output power tubes normally used such as EL34, KT88, etc, the cathode is pin 8, not pin 1.  In some tubes pin 1 is not connected, in other tubes it is connected to the suppressor grid,
    in some tubes pin 1 and 8 are internally connected.  So the cathode connection is actually pin 8.   In most all amps pin 1 and pin 8 are connected externally.
    Now my first thought would be to connect the fuse from pin 8 to ground, but my associate and long-time tube tech Don Sachs has recommended this connection
    from pin 1 (suppressor grid) to pin 8 (cathode) to protect against INTERNAL tube shorts caused by overcurrent/internal overheating.
    Also, it should be obvious that if the fuse is only rated at 250v then that is NOT a problem, because the cathode and suppressor should never have more than about -60vdc on them.
    And if they do go above 250v, why is that a problem?  The fuse should blow!!

    Also, as for the fuses I am using/recommending, they are only 1/4 ohm, far less than most other similar fuses, this changes the bias reference voltage by only 2.5% rather than 15%, not enough to bother with

    Also, for those who don't seem to read the instructions (or maybe don't understand them) the HV connection on the auto-bias board is normally NOT used,
    it would only be used if you were delaying the B+ turn-on by MORE THAN ONE FULL MINUTE.  
    If you have a TDR installed, the delay is only 20 seconds, so the HV connection on the AB board is NOT used !!


    Last edited by tubes4hifi on Wed Jan 09, 2019 3:11 pm; edited 1 time in total
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    Re: Auto-Bias board on Roy's website?

    Post by tubes4hifi on Sat Jun 02, 2018 9:09 pm

    here's a photo of my third auto-bias install in an ST120 amp, in this photo I haven't put fuses in yet, but this is a cleaner amp build than the photo I posted a couple weeks ago.
    About 45 minutes this time rather than 90 minutes on the first one I did, practice makes perfect and cuts time!  Also the old 10 ohm cathode resistors are just cut off and not connected on one end.
    12 EZ wire connections, and pop out the old bias pots and R37-R38 and that's it, simple.

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

    Post by tubes4hifi on Sat Jun 02, 2018 9:55 pm

    here's a photo of it working, 456vdc, this ST120 amp has the TDR which was set for around 20 seconds, and bias is at 55ma per output tube,
    This amp is using a rectifier tube, so that rectifier is running at MAX current spec, highly suggest a WZ68 rectifier.



    Last edited by tubes4hifi on Sun Jun 03, 2018 4:38 pm; edited 3 times in total (Reason for editing : updated photo)
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    Kramer

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

    Post by Kramer on Sun Jun 03, 2018 2:09 pm

    Roy, thanks for clearing that up and explaining the method in more detail. Very nice wiring on that latest board install.
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    Re: Auto-Bias board on Roy's website?

    Post by pichacker on Mon Jun 04, 2018 2:49 am

    Pavel,

    I like the suggestion regarding using a MOV. Only the fuse and a tube to change in the event of a flashover.

    My board got delivered on Friday and whilst looking over it, and sketching out the circuit, I noticed that the cathode end of the 10R resistor is very close to an adjacent track.

    https://photos.app.goo.gl/gv2txwwmSw9EQJVw9

    I am definately going the MOV/Fuse route when I install the board to lessen the chances of a flashover to this track, also if the Zener method was used and the Zener shorted then of course this would remove any feedback monitoring of the cathode current. Result would be the bias board would turn the tube hard on!

    I'll post some more photos as I go on.

    Roy,

    looking at a combination of instructions and photos I can see in one of them that there are dual ground connections. One from the gnd point next to the HiV to the common star ground and then another from the bias connections. I can see on your later photos that the common ground point one is not used.

    Since we do not want to create a loop, which ground connection do you find best to use. To the star point or from the bias supply? I'm leaning towards taking the ground from the driver board bias supply.

    Also have you noted any hum introduction when one side of the heater windings is grounded?

    Cheers

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

    Post by tubes4hifi on Mon Jun 04, 2018 12:11 pm

    PicHacker,
    there is only one ground connection to the board, the HV and GND connections are NOT used.
    Ground is from the driver board ground or can be from the amp chassis ground, either works since they are the same other than about 3" of wire.
    I haven't found any hum added with the filament connection.
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    Re: Auto-Bias board on Roy's website?

    Post by pichacker on Tue Jun 05, 2018 4:29 pm

    Fitted the board last night along with the cathode fuse/mov protection circuits. It works a treat Smile

    It felt a bit like performing open heart surgery on an old friend as I had to de-solder and lift the driver board out to remove the pots and lay down the bias caps to avoid fouling. Also for us over here in Europe I had to remove the imperial screws and replace them with metric ones to allow me to screw on the 4 spacers for the bias board to mount to.

    Connecting up went without any issues and I used a single ground wire to the common point on the driver board bias supply. For the filament connections I used the side nearest to the input terminal which also happened to be the side that does not feed the centre tube. This allows for the centre tube to still have a floating filament supply.

    For my cathode protection circuits I decided to mount the components on a three lug piece of tag board secured under a mounting screw. One for each output tube. That way any local fault currents will have the shortest way back to chassis and also allowed the output tubes to still have the pins linked as original. No real need to do this but just my preference.



    (Must get around to uprating the 470R on the main cap as it runs a little too hot for my liking)

    Once set up, the tube bias current is rock steady. I have yet to put it through its paces with extremes of line voltages or changing from SS to tube rectification but I have no doubt that it will cope without any issues. I have sketched out the circuit and it is very well designed. I was actually surprised at the length of the time constant used in the bias regulator.

    I kept my HT switch on delay circuit still in operation and it applies HT at about the same time as the red LED on the bias board extinguishes. There are still many seconds after this event before the tubes ramp up to normal operating current. No perceived issues.

    I attempted power cycling to see what would happen, there were no issues here either, as the bias board reset relatively quickly and always ramped the current up smoothly. I would not recommend trying this with a tube rectifier installed.

    Well done Pavel, a nice design which appears very stable with about the right time constants and gain in the control loop.



    Amp is dead quite with no hum from the speakers.  cheers


    Incidentally as I don't use the front mounted sockets in my amp I connected the cathode leads to the monitor points on the driver board. That way I can connect my fluke to the topside when the amp is together and measure the bias voltage to confirm things are still in order.

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