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Automatic bias board.

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and what to use Schottky? The window would smalle. I do not know about germanium diodes in SMD design :)
About symmetry window ..
I think the window should be symmetrical. the course of the saw moves linearly around the "synchronized swimming" reference position.
by changing the voltage of the reference source, reference is set on inverted input UC6 / 10 with function 10mV / mA.
I certainly agree with placing 10 Ohm cathode resistors. Due to thermal loss on reistors I recommend a max. Current of 200mA per cathode.
If someone wants more, they need to be removed from the board and resistors for higher power and connect to the plinth tubes.
Another option is to use resistors with a lower resistance value, such as 5 Ohm. The heat loss is reduced by 4x, but the 10mV / mA equation is no longer valid. but the accuracy of the setting is reduced by 50%. In that case it would be necessary to use cathode resistors with an accuracy of 0.5%.

Thanks
Pavel

Beyond the GRD output is a decoupling resistor (about 100k). When you measure the audio signal at the end tube's inlet, it is totally clean, signal integrator or harmonics are not there.
 
Would this module work if the four tubes are for one channel such as a VTA M125?

What are the overall dimensions of the module? I didn't see that in the listing.

yes, the AB-Q module can be used to control four tubes in one channel.
The module has dimensions 100x57mm.
Pavel

I don't see why it wouldn't work for 4 tubes, one channel. I assume you could also use this to bias 4 mono SE amps.

The dimensions are 100x57mm from audioamp.eu.

I've seen others talk about a 0 - 60 ma current window. Does that mean I can't use this module to set an idle current of 90ma? Or would I have to change the 10R sense resistors to 6 2/3R?
Thanks
The module will work for four tubes. The module is a trimmer that can easily set the cathode current of the tubes 0 - 200mA. Cathode resistors are on the module.
Pavel
 
A problem with these type of auto-bias boards has come to light.

If one tube draws more screen current than the other, then the currents to the OT are no longer balanced. This is because the cathode current is sensed by the board, not the plate current. Some provision is needed to differentially adjust the bias set points for each tube in a pair (while measuring plate current with a meter(s)). At present, there is only one bias adjust pot for all four tubes.

I don't think gm matched tubes check for this issue either. Screen grid alignment with the grid 1 wires affects the amount of screen current drawn. This is a separate issue from grid 1 gm matching.
 
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AB-Q

This problem can not occur. Each cathode section works independently. The set reference level is connect to each section independently. The bias on grids is set independently of other grids. The unique advantage of this AB-Q is that you can use both old and new tubes without pairing them. The cathode current of all tubes will have the same value according to the reference voltage setting. This has been proven many times when mounting dozens of modules!
Pavel audioamp.eu
 
I agree that the cathode currents will all be set the same. The problem (for the OT) is that the plate currents are [cathode currents - screen currents]. If the screen currents are not identical, then the plate currents are no longer identical. The OT is not going to like the resulting DC current offset.
 
I agree that the cathode currents will all be set the same. The problem (for the OT) is that the plate currents are [cathode currents - screen currents]. If the screen currents are not identical, then the plate currents are no longer identical. The OT is not going to like the resulting DC current offset.

This is true, but in reality probably not a problem - think how many amps are manually biased via a measurement on a cathode resistor anyway!

In a UL EL34 amp (for example) with an HT of say 430V, the screen current is around 5mA, assuming the unintentional differences between valves is on the order of a couple of mA, not much to worry about (unless you have finicky cores on your OPTs!)
 
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Circuit checks the current symmetry of both PP OT branches. These streams are the sum of the anode and screen grids tubes (Kirchhoff). And that is fulfilled. The distortion in the tetrod beam caused by the difference in the streams of the screen Grids will occur anyway. I agree that the difference in screen grids streams in UL connections is very small.
 
So I wanted to let you all know that I've got this module up and running and it works flawlessly. I will definetely purchase another one for a different project. It allows the easy use of toroidal power transformers as OPTs, and just on the cost difference between those and Hammond proper tube OPTs the board more than pays for itself. Combine that with "set it and forget it" or the fact you can pair a 6L6 with a KT88 in a pinch to still have output and you're gold!
 
AB-Q auto bias board

Okay, bought one of these boards from a gentleman in the Czech Republic. He sells them on eBay under the user name (Kotra). Not absolutely sure of the spelling of that name, but anyway, a search on eBay under AB-Q for four tubes will most likely return the item. All I can say is, this board is excellent!

I bought it to install in my Marantz 8B. While I can't say if it is a ripp-off of the Tent Labs unit, as some have suggested here, I can say it is much less expensive and way less bulky.

My Marantz 8B has very limited space, and even with stand-offs that elevate the board above other components in the amp, there is still plenty of room to install this board. The Tent Labs board would absolutely not work under any circumstance in an 8B. Inaddition, the board looks absolutely great installed in the amp. Why did I buy this board? Well let me see if I can explain.

When using my 8B, which has individual bias pots for each EL-34 tube, I got very annoyed having to often reset the bias. Often, when I turned on the amp and biased it, 15 minutes later the bias would be off. I also would keep the amp on days and weeks at a time as I have music constantly playing in the back round and for discreet listening moments. I would check the bias habitually, and it often needed to be readjusted: Not much, but it was often somewhat off.

Therefore, I started reading up on biasing and auto biasing amps. One I came across that I can recall was a model made by VAC (Valve Amplification Company). Remember the Marantz reissues? However, that amp made by VAC under the VAC name was $20,000.00. Well I may be crazy, but not that crazy, though tempted. I also learned about the Tent Labs unit around the same time, but quickly became disappointed because the unit was cumbersome and just to large to install in a Marantz 8B. I don't really recall how I learned about the AB-Q unit; since I was searching all things auto biasing for tube amps, I probably did a search on eBay and turned up the seller and the unit he was offering.

The seller, in his adds on eBay had a very good description of the AB-Q unit. He also had a link in his eBay add that takes you to a Euro Amp website which more fully explains the unit along with other items produced and sold purely for tube amp enhancement. All very interesting stuff.

Continuing, since I was now hell bent on have a tube amp with an auto biasing circuit, I contacted the seller after first having done a stupid thing.

I purchased an Orange Amps' Divo OV4 unit at a cost of over $300.00. Actually, it was a Divo OV2 that says Divo OV4 on it, but it only came with two octal sockets for two tubes. I did not care. I wanted auto biasing, so I bought it to try it out and figured I'd buy the extra octal connectors later. Fat chance: Try buying them separately. Also, the Divo OV4, though it has some excellent features which the AB-Q board does not have, namely certain protection features, really is a large unit meant to be installed in the case of a large guitar amplifier of 100 watts for four tubes; the OV2 is for 50 watts two tubes (octal). I bought the unit, OV4, and decided to just put it aside. I don't really know what I will use if for since, as noted, I can't seem to get the extra sockets. Well, I digress, so let me get back to it.

As I said, I really wanted auto biasing for my Marantz 8B, so my tenacity finally paid off when I discovered (Kotra) and the AB-Q board on eBay.

Before I bought the board, I read everything he had to say about it in his adds on eBay, and checked out his link in the adds. I contacted him though eBay. This was a pain because you can see how much I like to type, and eBay very much limits you as to how much you can say in a single eBay contact email. But suffice it to say, since he was the seller he was allowed to contact me back and give me his direct email address, or perhaps I contacted him through his website. I don't remember which now.

None-the less, once I had his direct email address we communicated extensively about the unit and my amp. One of the concerns I had after reading his descriptions of requirements for installing the board, was whether or not the bias supply of the 8B was adequate. And even though I was fairly sure how to hook-up the board into the circuitry of the amp, I was still reticent about buying it: During our communication, I asked if I could send a pdf. of the 8B's schematic for him to take a look at so he could address my issues. He was more than willing to look at it and address my concerns. He sent me back the schematic, which he reproduced anew, that showed me how to enhance the bias supply of the amp with the addition of a single capacitor and diode into the bias circuit, and that further showed me exactly how to hook-up the board to the amp. Not complicated at all.

So, I bought the board, and in very little time I installed it. If you read his information regarding powering the board, I had both options he mentions available to me. I ultimately choose to install a tiny toroidal transformer to provide the 6.3Vac to power the board. The reason for this was that I had already installed a small, small B+ voltage delay/slow ramp-up board into the amp which uses 6.3Vac from the filament winding of the amp's power transformer. There was really no reason for using the small, small toroidal since the Marantz's filament winding is vigorous and would not be taxed at all by the small millamp(mA) current requirements of both of these board together, but I just chose to use the separate toroidal because I did not want anything else on the filament winding of the power transformer. It was psychological actually, and besides, it looked cool. Well to the chase now.

The board works flawlessly: I have the amp on days at a time, some times a week or two at a time, and the bias is always spot on as confirmed by the amp's bias meter. It absolutely does not vary, ever! The sound quality was also improved is all aspects, from the lows to the highs, especially the highs. Though the improvement to the high may also be attributable, in part, to certain modification I did on my Marantz model 7 preamp. More about that later. Further, the beauty of the AB-Q auto bias board is also in the fact that the reference voltage set by the trim-pot on the board, depending upon the type of tube used, I use 6L6s and EL34s in the amp, always sets the bias meter's needle exactly to the bias point marked on the Marantz 8B's bias meter even though the reference voltage requirement for the two types of tubes is different.

Finally, let me just close here by saying, if you have an amp such as the Marantz 8B, the Dynaco ST-70, various two tube Dynaco mono blocks, or any number of other fixed bias tube amps, such as the Tubes 4hifi VTA-120, this thing will work wonders. I am not exaggerating.

I bought a second AB-Q from same seller to put in (have put in) my Tubes4hifi VTA-120 (which I built from a kit). It works great there: My digital VOM meter in the front octal sockets.....(VTA-120 has individual bias pots for each KT88. I use a DPDT switch in place of stereo/mono....not used in VTA-120 amp....to switch between the two tubes on each side of the amp when checking bias at front octal sockets)......Always shows .500mVdc as specified. Sometimes there is a +/-2 digit variation in the third digit to the right of the decimal point when I check bias on the amp.

So one last time, the AB-Q auto bias board provides real time constant bias monitoring and real time instantaneous auto correction of bias variance. It does this without any problems and under dynamic load conditions such as when playing the amp at whisper volume, or loud, really loud volume. I hear no distortion at all at all volume levels I care to play the amp at.

WHAT IT WON"T DO: It won't protect the amp or tube should there be a component failure (resistor, capacitor, etc) or tube failure. I found this out after completing my VTA-120, and I installed one of four KT88s that had a hole in the glass envelope. Main amp fuse blew but no damaged occurred, not even to the AB-Q board itself. I solved that problem to be sure transformer would be protected, and board, by installing fusing in the cathode circuits from the AB-Q board to shut down cathode current to the individual tubes should a component or tube fail. This way the amps main fuse will not blow. And please don't tell me using a fuse will affect the sound. It will not/has not, and besides, the fuse is not actually in the output, plate circuit of tube.
 

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It's a nice board, but I like his other one even more, the AB-4. It has a TTL error port you can use to trigger a protection circuit, too. My AB-Q blew up. One of the opamps vapourized a pin! I've sent it back to Pavel to figure out what's wrong with it, and he said he would repair it.
 
AB-4: Auto Biasing (Tube monitoring/protection)

Sorry to hear about your AB-Q blowing up. I too would like to learn more about Pavel's AB-4 board. I have to do more research on it, but I do like the idea of the TTL protection circuit. The AB-Q offers little in that regard. The best I could do was to put small 5x20mm fuses, 200mA, in line with the cathode circuit. I know, from what I red, that that should shut down all current flow in the individual KT88s and El34s in my various audio amps without blowing the main amp fuse.

You know, its funny: I went to the AR website last night because I have a Reference 75SE and I wanted to see if I could get a schematic for the amp. I might like to install and AB-Q/AB-4/TentLabs board into it. But lo-and-behold, what do I see: They now make an amp that has auto biasing with fuse protection for each individual tube. I just thought that was funny, especially the fuse protection for the individual tubes.

I am sure they put the fuses in line with the cathode circuit because that is the only way to do it if using simple fuses to insure complete current shut down without excessive screen grid current still frying the tube.

Anyway, yes it is funny: The amp they now put these features into is by far not the most expensive one they make, but I was thinking how crude to use cathode fuses. That is something I'd do modifying a kit amp or an old Marantz 8B; I would have thought Audio Research might have come up with something a bit more exotic: Circuit breakers at least. That is what I wanted to do, but I could not find breakers too use.

My Reference 75SE now has a retail price of almost $17,000.00.....I paid way, way less used...... and AR still doesn't put such things as auto bias or simple individual tube protection into it.

Anyway, again, I just though it was funny: AR now fuses the cathode circuit on their latest $10,000 entry amp. Who would have thunk it..lol?


PS: I gotta check to see what chip he uses for the TTL protection on the AB-4.

Oh, by the way, have you ever checked out Nurochrome? They make a great HV regulator which I installed in my preamp, and an interesting DC filament regulator which I am waiting on to install for the driver and inverter tubes of my Marantz 8B. (6BH6/6CG7/6FQ7). Just thought I'd mention it.
 
I avoid them because they are more complex than linear power supplies, and I am not sophisticated enough to feel comfortable with them. More efficient, but proper implementation required to assure good noise suppression. Though, I guess a good prebuilt unit can be as easily purchased as a linear PS.

Anyway, I have a LV regulated, adjustable linear power supply, with an ultra-low noise voltage regulator in my preamp. It can put out the 18.9Vdc exactly that the series/paralleled wired filaments in my preamp require.

For the HV in my preamp, I use the 21st Century Maida Regulator sold by Nurochrome (Canada). Keeps HV absolute precise no matter how much my mains jump around.

Anyway, later tonight I am going to contact Pavel about his AB-4 board. I am thinking about taking out the AB-Q board I installed in my 8B, and replacing it with that. Kind-of expensive, but I really want to see if it will be an improvement. I specifically want to know if I use this board, does the B+ delay board I have in the amp become redundant. So if I can use this AB-4 board, and I can take out the B+ board, I'd have a second location to mount the Nurochrome DC filament board I am waiting on. The second location also already has the stand-off I am going to need to mount the board into the amp.

I am not an engineer, or any thing like that, but I do like reading about this stuff, even the somewhat very technical articles which I can sometime mostly understand. But what I really like is the results I have gotten with the installation of these boards. I mean, I read-up on theory as to why for this or that regarding tube amps, and than it is great to find people who know a lot more than me have already done the design work, and make products available. I tried some design work myself using a program called Eagle. However, I did not really enjoy that.
 
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