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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Auto-Bias board on Roy's website?
mazeeff- Posts : 155
Join date : 2014-01-06
Age : 69
Location : Grand Rapids, Michigan, USA
jfine- Posts : 152
Join date : 2017-06-19
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?
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?
pavlikkkk- Posts : 69
Join date : 2018-03-02
Age : 77
Location : Czech Republic
Accuracy is due to tolerance of cathode resistors. On AB-Q is 1% resistors.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?
If you leave only two tubes in the amplifier, AB-Q will drive the other two.
CletusB- Posts : 234
Join date : 2018-02-11
Age : 69
Location : Trinidad & Tobago
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.
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- Posts : 2
Join date : 2018-05-19
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.
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.
Roy Mottram- Admin
- Posts : 1839
Join date : 2008-11-30
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
pichacker- Posts : 103
Join date : 2016-08-01
Age : 60
Location : Near to London - UK
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
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
pavlikkkk- Posts : 69
Join date : 2018-03-02
Age : 77
Location : Czech Republic
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
pichacker- Posts : 103
Join date : 2016-08-01
Age : 60
Location : Near to London - UK
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
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
pichacker- Posts : 103
Join date : 2016-08-01
Age : 60
Location : Near to London - UK
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
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
pavlikkkk- Posts : 69
Join date : 2018-03-02
Age : 77
Location : Czech Republic
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
Roy Mottram- Admin
- Posts : 1839
Join date : 2008-11-30
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 !!
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
Roy Mottram- Admin
- Posts : 1839
Join date : 2008-11-30
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.
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.
Roy Mottram- Admin
- Posts : 1839
Join date : 2008-11-30
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.
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)
Kramer- Posts : 50
Join date : 2018-02-11
Age : 37
Location : Chicago
Roy, thanks for clearing that up and explaining the method in more detail. Very nice wiring on that latest board install.
pichacker- Posts : 103
Join date : 2016-08-01
Age : 60
Location : Near to London - UK
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
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
Roy Mottram- Admin
- Posts : 1839
Join date : 2008-11-30
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.
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.
pichacker- Posts : 103
Join date : 2016-08-01
Age : 60
Location : Near to London - UK
Fitted the board last night along with the cathode fuse/mov protection circuits. It works a treat 
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.
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.
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.
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.
aguaazul- Posts : 136
Join date : 2012-07-08
Age : 64
Location : Livermore, CA
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- Posts : 69
Join date : 2018-03-02
Age : 77
Location : Czech Republic
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- Posts : 69
Join date : 2018-03-02
Age : 77
Location : Czech Republic
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- Posts : 136
Join date : 2012-07-08
Age : 64
Location : Livermore, CA
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?
Roy Mottram- Admin
- Posts : 1839
Join date : 2008-11-30
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.
I rather just add a terminal strip in there somewhere if you want to add the MOVs.
Roy Mottram- Admin
- Posts : 1839
Join date : 2008-11-30
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.
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- Posts : 50
Join date : 2018-02-11
Age : 37
Location : Chicago
Here is how I am planning on wiring them up in case this helps.
