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Automatic bias board.
Automatic bias board.
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Old 8th February 2018, 07:47 PM   #31
Lingwendil is offline Lingwendil  United States
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Automatic bias board.
Cool!

Might be useful for some low mu types. What's the bias voltage range that it can support?
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Old 8th February 2018, 08:09 PM   #32
kodabmx is offline kodabmx  Canada
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up to 200ma, not sure the max bias voltage but my supply is -80 and it's working fine. I'm using it with KT120 biased at 40ma 540V B+ 280V screen.
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Old 8th February 2018, 08:39 PM   #33
Lingwendil is offline Lingwendil  United States
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Automatic bias board.
Might be worth a try for the 6336A or 6AS7G/6080 types to squeeze a bit more efficiency out of them. Using garter bias with them can throw away 60-80 volts at times!
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Old 8th February 2018, 11:04 PM   #34
kodabmx is offline kodabmx  Canada
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My Stupendous amp is 12.5% efficient... LOL
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Old 23rd March 2018, 03:02 AM   #35
smellyarmpt is offline smellyarmpt  United States
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Default 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.
Attached Images
File Type: jpg AB-Q hook-up 8B.jpg (763.6 KB, 165 views)
File Type: jpg Marantz ABQ-1.JPG (694.0 KB, 165 views)
File Type: jpg Marantz AB-Q & Delay1.JPG (702.5 KB, 166 views)
File Type: jpg Marantz AB-Q & Delay2.JPG (663.5 KB, 165 views)
File Type: jpg P1010002.JPG (619.1 KB, 163 views)
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Old 23rd March 2018, 03:26 PM   #36
kodabmx is offline kodabmx  Canada
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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.
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Old 23rd March 2018, 09:37 PM   #37
smellyarmpt is offline smellyarmpt  United States
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Default 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.
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Old 23rd March 2018, 11:07 PM   #38
kodabmx is offline kodabmx  Canada
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I have no idea, but I think it's simply a 5V line that comes high if the cathode current can't be controlled.


I primarily use SMPS for power these days... The only thing I regulate is the screen supply, as the heaters are all connected to 12V in series/parallel.
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Old 24th March 2018, 02:01 AM   #39
smellyarmpt is offline smellyarmpt  United States
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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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Old 24th March 2018, 01:53 PM   #40
kodabmx is offline kodabmx  Canada
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I keep my HV stable by powering the entire thing from a 12V ATX supply It's good for 12V 62A in 50c ambient with only 15mV ripple...

Here's the thread: Ganged switching power supply for a tube amp.
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