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Old 7th August 2009, 05:50 AM   #1
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Default BGW 500D Bias Adjust ???

Hello All,

This is my first post on this forum. I am exited to be a member. I'm a newbie, so go easy on me please...

I have a BGW 500D I've just worked over. No matter what manual I down load. It is for a BGW 750A/500d. The schematic and alot of the information is for the 750A. I know which resistor to change for bias, and I know which resitor to measure bias across. On page 13 of this early manual, it says, " adjust R20 to obtain 470mv across R28". That clear, and understandable. However, it say's on the schematic, " adjust R20 to obtain 340mv across R28. My question is: WHICH ON IS CORRECT for h$ll sake???

Note: I also replaced the outputs with 2n3773's, and the drivers with NTE162's. Finding the cross reference for those old RCA devices was impossible... I also changed out the pre-drivers with the original RCA's for the NPN's and 2n5415's for the PNP's. I've matched None of the above. I hope I don't have problems. I have replaced all tantalums with Panny fc 10uf 100v units. I have also replaced all resistors with 1% metal films. I used 2.2uf metalized polyester caps as bypass caps for the 12000uf power supply caps (originals were 10000uf). for the polyester caps on the circuit boards, I used metalized polypropylene units. I guess really all that is left of the amps that are stock, are the ceramic disk caps used for RF. I can post a pic or two if you like...

Anyway my main question is that bias adjust. I also did a search on the subject. Couldn't find the bias, but found some helpful tips on DC Offset. The channel I have finished, has a DC Offset of 3.6mv. Not on this channel, the pre-driver, and op amp were ok. On the second board (which I havent fired up yet, waiting for parts) the drivers are not matched, and I'm hoping for the best...

Thanks Folks, Tom
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Old 7th August 2009, 05:37 PM   #2
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Old 7th August 2009, 05:49 PM   #3
llwhtt is offline llwhtt  United States
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Have you tried asking BGW? They are still in operation in SoCal.

Craig
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Old 7th August 2009, 08:08 PM   #4
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I've been trying to reach them for a couple days now, and can't get through... However, I just called one more time. The tech. was a little hard to understand, but he say's, "150mv to 200mv or so.

I find this curious, as there using a 1% resistor there. That being the case, the must won't the bias to be right on.

There is a guy on the forum here from Utah (where I live). From what I have read, he used to do warrantee work on them. I can't email him, or even edit my posts, to correct some of the spelling errors. I'm new, and still on probation here...
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Old 7th August 2009, 08:17 PM   #5
llwhtt is offline llwhtt  United States
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Nelson Pass has a neat little trick for biasing amps that you don't what to set it to. Check out PassDIY and look for the Mosfet Citation 12 mod. All you need is a signal generator, scope and dummy load, I've used it many times works great. It looks at signal and not voltage drops.

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Old 8th August 2009, 08:32 AM   #6
Enzo is offline Enzo  United States
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Bias is simple. You can apply a good sine wave, and set the bias so the crossover notch juuuuusssst disappears. At that point I;d even back off just a hair.

But there is a simple way that is close enough and does not require a scope. The whole point of bias is to set the positive and negative sides just to the brink of conduction so the slightest signal excursion will turn one of them on. if the signal has to move a little bit before the output starts to move, that is the source of crossover distortion. A cold setting.

On the other hand, you don;t want both sides conducting at the same time. That wastes power and runs the amp hot. Like driving down the highway with your brakes on. A hot setting.

The point where the crossover distortion just disappears is the point where the cold turns into hot. it is the point where the signal doesn't have to wait for the output stage. it is also the point just before both sides start to conduct.

When both sides are conducting, current is flowing from one rail to the other. When this happens the current draw from the mains also starts increasing.

SO the simple trick is to monitor the mains current draw. Use a metered outlet, or remove the amplifier fuse or breaker and connect your DMM in its place, set up for AC current measure.

As you turn the bias control up from the coldest setting, the mains draw won;t do much - it'll just sit there. But as you make the adjustment, you will hit a point where all of a sudden the mains current will start to ramp up. That point is when the two sides both start conducting. That is the point where the crossover notch disappears. That is where the bias belongs.

Again, I prefer to back off a little bit cooler just to accomodate drift.

Obviously one could get out a distortion analyzer and... But for getting the amp running, this is good enough for professional audio. I have no idea how many mv across resistor X.
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Old 8th August 2009, 11:28 AM   #7
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Default post....

------post a schematic ( if not the all thing make a copy from bias stage till output

----- changing outs to 2N3773 should be a waste of time nad money there is by far better and more reliable transistors but for sure not a drop in replacement it might take something more except replacing tranistors but the result will by a hell of a lot better

----- if you have installed anything ...... anything that has NTE tag on it remove it as fast is possible and get your shelf a few decent transistors

happy regards sakis
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Old 8th August 2009, 02:03 PM   #8
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adjust R20 to obtain 470mv across R28". That clear, and understandable. However, it say's on the schematic, " adjust R20 to obtain 340mv across R28. My question is: WHICH ON IS CORRECT for h$ll sake???

------------------------------------------------------

The 340mV specification.

The voltage is measure across one of the driver emitter resistors and is optimum for a class AB+B amplifier.

Crown and Phase Linear of the same vintage call for the same bias method and voltage value.

Modern 2N3773 are an excellent choice for outputs as they have 4Mhz speed and 150W SOA at 100V

Predriver and Vas are of the 2N3440/2N5415 family (later models use MJE340/350).

Outputs?

I've used mostly MJ15022/24 (which are from the 2N3773 family), later fully complementary output stage models take the MJ15024/25.

I object to the NTE162, but not because it is an NTE part. It has the wrong characteristics for this application (it was designed for HV switching, not linear use).

http://www.nteinc.com/specs/100to199/pdf/nte162.pdf

Another pair of 2N3773 will be OK in this circuit.

A later BGW schematic:

Click the image to open in full size.

Signal path upgrades (replace all small electrolytics and add 0.1F film bypass):

Click the image to open in full size.
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Old 8th August 2009, 05:03 PM   #9
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WOW!!! I appreciate all this information. I'm running underbiased right now then (263mv). Alfter talking to that tech. I was becoming Baffled .

I'm beginning to see a pattern emerging. NTE is Crap apparently. I'm out of 2n3773's djk. I can get more, but is there anything you suggest that would be the best, with my current layout??? As these transistors are where most the sound quality comes from, then the class B's kick in.

On a side note: I purchased a new LM318h metal can op amp off evil bay. Now, as you know, these are in a round pin configuration, with pin 8 being next to the index tab. On my BGW 500d the pin layout for this op amp is in the form of an 8 pin DIP situation. BGW just carefully bent the pins of the 318 to fit this arrangement. That's easy enough to do, so I did it, and was horrified when I found pin 8 on the opposite side. I thought of everything, then bit the bullet, and desoldered the TO-3 transistor socket (again), and mounted the 318 on the bottom of the board, instead of the top. So what I'm saying, is the pinout was backward from the one BGW had installed. I did this manuever in such a way, as to be able to remove the 318, if the need arises, without desoldering or removing the TO-3's. Man! I sure hope they didn't change things somewhere down the line, and indexed that pin 8 to pin 1 instead... I'm still trying to figure out, if that arangement might have been made special for Brian W. ?

I've attached a couple pictures of the channel that I am working on. Keep in mind, it is under construction. I haven't had the chance yet to clean the flux up spotless, thats my usual procedure.

This amp will be running the bass section in a two way actively crossed over system. The system is in my front room. Note, I have replaced alot of parts for Hi Fi puposes. I also bought 2 new double ball bearing fans, one is in the BGW amp, the other will exhaust the rack area. Both fans are controlled by the BGW 500d. at idle, my guess is there moving around 10-15cfm of air. Then if the amp chooses to hit 130* or over, the fans will kick up to 62cfm. I have alot of vintage gear going into this system. I have rebuilt every single item. I have 100's of hours into them, and a bucket full of parts. What a job, and it doesn't help when you have a litte O.C.D. I'm such a perfectionist, thing never seem to get completely done, or at least as perfect as I would like...

I appreciate all you folks time, in replying to my thread...

P.S. When I was removing the pre-driver heatsink cups, they were siezed in the heat sink, and a broke a couple. So I cleaned them up, and used JB Weld to atach them again. I had to buy new brass bolt to JB Weld them to. I found out the hard way. These cups are NOT conducting. In other words one of the ones I had built was grounding the collector of a pre-driver. I had to redo this one. These cups are insulated, who'd of thought it. Well on look at the schematic should have told me this, but I overlooked that Duh!!!

Best Regards, Tom

Click the image to open in full size.

Click the image to open in full size.
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Old 8th August 2009, 10:34 PM   #10
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[Add a pair of 47F 100V power supply bypass caps across the main filter caps.] Quote by djk...

P.S. As I mentioned above, I used 2.2uf metalized polyester caps for bypassing the power supply caps. Would you change those out, and replace them with say Panasonic FC Series caps (47uf 100volts)??? Just curious, as I beleive you mentioned, they sound very good as far as bass is concerned... Or perhaps, I could bypass the 47uf bypasses with the mentioned 2.2uf's. Now were opening up a whole can of worms on bypassing the bypass caps. It's kinda controversial...
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