Yes, this connection reliably protects AB-Q against short circuit in an tube.Kramer wrote:Here is how I am planning on wiring them up in case this helps.
Pavel
+20
New2Tubez
Kramer
cci1492
Roy Mottram
msikk
tomlang
ericoto
aguaazul
gguarneri
jwb474
pavlikkkk
mazeeff
Bennyhaha812
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24 posters
Auto-Bias board on Roy's website?
pavlikkkk- Posts : 69
Join date : 2018-03-02
Age : 77
Location : Czech Republic
Pavel
Kramer- Posts : 50
Join date : 2018-02-11
Age : 37
Location : Chicago
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.
ericoto- Posts : 26
Join date : 2013-12-11
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
Thanks
Kramer- Posts : 50
Join date : 2018-02-11
Age : 37
Location : Chicago
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- Posts : 136
Join date : 2012-07-08
Age : 64
Location : Livermore, CA
pavlikkkk wrote:Yes, this connection reliably protects AB-Q against short circuit in an tube.Kramer wrote:Here is how I am planning on wiring them up in case this helps.
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.
tomlang- Posts : 36
Join date : 2009-08-12
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?
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- Posts : 69
Join date : 2018-03-02
Age : 77
Location : Czech Republic
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
Roy Mottram- Admin
- Posts : 1838
Join date : 2008-11-30
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
ericoto- Posts : 26
Join date : 2013-12-11
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.
Roy Mottram- Admin
- Posts : 1838
Join date : 2008-11-30
still no photos from someone with an M125 amp?? I've sold at least 6 pairs of these, someone please post photos showing the install to help newbies . . .
ericoto- Posts : 26
Join date : 2013-12-11
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
msikk- Posts : 3
Join date : 2018-04-13
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
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- Posts : 69
Join date : 2018-03-02
Age : 77
Location : Czech Republic
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
msikk- Posts : 3
Join date : 2018-04-13
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
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- Posts : 69
Join date : 2018-03-02
Age : 77
Location : Czech Republic
Do not insert Tubes. Immediately after switching on, measure the voltage on all four pins of the GRD module AB.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
Send the measurement results to private mail in this forum.
Pavel
Roy Mottram- Admin
- Posts : 1838
Join date : 2008-11-30
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
also the install directions are posted here . . . www.tubes4hifi.com/ABC-M125-install.pdf
pavlikkkk- Posts : 69
Join date : 2018-03-02
Age : 77
Location : Czech Republic
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
Roy Mottram- Admin
- Posts : 1838
Join date : 2008-11-30
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.
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.
cci1492- Posts : 331
Join date : 2016-05-09
Age : 64
Location : NJ
Will the amps eventually ship with 2 sets of instructions, one with the other without?
Roy Mottram- Admin
- Posts : 1838
Join date : 2008-11-30
Bob is selling all amplifier kits, and Roy (that's me) is selling all auto-bias upgrades, so the instructions are separate,
but keep this in mind if you haven't bought either one yet . . .
the four bias pots are NOT installed if you are going to use the auto-bias board.
If you are using PIO caps, they must be insulated and not stick up more than 3/4" off the driver board (K40s are usually too fat, use K42 caps instead)
C17 & C18 should be less than 3/4" tall (Bob still has some older taller ones, new ones are supplied with ST70 & ST120 auto-bias boards)
but keep this in mind if you haven't bought either one yet . . .
the four bias pots are NOT installed if you are going to use the auto-bias board.
If you are using PIO caps, they must be insulated and not stick up more than 3/4" off the driver board (K40s are usually too fat, use K42 caps instead)
C17 & C18 should be less than 3/4" tall (Bob still has some older taller ones, new ones are supplied with ST70 & ST120 auto-bias boards)
Guest- Guest
Hi, does anyone have a photo/example showing they moved the bias lights on the AB board to show through the driver board cover?
I’m thinking the lights would be neat if inserted in the hole that used to provide access to the bias adjustment pots?
Otherwise how do you know if the lights are on/working without unbuttoning the the amp?
Thanks
I’m thinking the lights would be neat if inserted in the hole that used to provide access to the bias adjustment pots?
Otherwise how do you know if the lights are on/working without unbuttoning the the amp?
Thanks
Roy Mottram- Admin
- Posts : 1838
Join date : 2008-11-30
I'll be making a change to the driver board to allow that, but it will be at least a couple of months before I need to order more driver boards
pavlikkkk- Posts : 69
Join date : 2018-03-02
Age : 77
Location : Czech Republic
is that it?jsl1234 wrote:Hi, does anyone have a photo/example showing they moved the bias lights on the AB board to show through the driver board cover?
I’m thinking the lights would be neat if inserted in the hole that used to provide access to the bias adjustment pots?
Otherwise how do you know if the lights are on/working without unbuttoning the the amp?
Thanks
Guest- Guest
I assume this shows where the wires need to originate. What I was asking is has anyone actually done the mod and mounted the lights (or alternative) on the optional driver board cover that sits atop the drive PCB (and inserted the wires through the driver board routed through where the pot adjustment holes are since the bias adjustment pots are removed as part of the AB install as I understand it. I was thinking the wires to the lights could go straight up through each pot hole or at least the pot hole (no pun intended) would provide some space for the led to be mounted about the driver PCB board.
Bennyhaha812- Posts : 15
Join date : 2016-12-06
Age : 50
Location : Western MA.
For sake of saving me from flipping over my amp and putting the multimeter on it, what is the voltage and amperage of the led? I may consider buying some 3mm led units of proper value and helping jsl1234 out with the modifications jumping the power from the AB board at the locations noted by Pavlikkkk. Aside from a variety of other mods this was something he was interested in and since I like to play, why not??? Roy, can you send me a AB board for a st-70/120 and PayPal invoice me please? Thanks.