BGW 750B output modules

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#101
Yeah I know this is an old thread but I'm trying to keep all the data on BGW 750 outputs in one thread.

So...I have a 750C with a couple of shorted outputs, one each NPN and PNP, Motorola 357407A and 2N3773 to be exact. Everything else checks out fine. Are the OnSemi MJ15024/25 still best subs for this old amp? I'm planning change out the Tantalums and electrolytics while i'm in there.

Thanks!
 
#102
"Are the OnSemi MJ15024/25 still best subs for this old amp?"

Probably.

Motorola used to list the 2N3773/6609 and the MJ15024/25 on the same data sheet, sharing the same curves for gain, etc. The 2N3773 actually was rated at slightly higher SOA at 100V than the MJ15024.
 
#104
djk said:
Some misc remarks:

"input signal of the 318 increases and that will finally overload the next stage."

And that is true of 99% of the amps ever made. Leach degenerates the gain in his input stage so the amp is dominated by the second voltage gain stage.

A Carver M1.0 is a pretty good sounding amp. The front end is about the same as the old Crown M600, a design shared by many of the newer Crown amps too.

The output stage on most of these amps are complementary EF type, either two or three stages.

The Carver and Crown front ends are very similar to the BGW, except they add another complementary voltage gain stage stage and delete the capacitor level shift circuit of the BGW. At one time the capacitors were cheaper than the transistors, now the reverse is true. The caps are now replace with a couple of $0.08 transistors (MPSA42/92 types). See Q201, 202 in the PL3 schematic for reference.

http://www.crownaudio.com/pdf/amps/pl_three_schematic.pdf

"I havent changed any electrolytics at all!!
Do you have any recomendations in this direction "

I don't go overboard with the caps, Panasonic electrolytics with small polypropylene bypass tacked in parallel are what I use. There isn't much room for big film&foil types anyway. I do use a pair of 47µF across the main supply filters, this really affects how the bass sounds. In general, the caps I replace and add cost about $4 for the lot.

"Also the fan is a pain and i may fit a autotemp control "

I don't like the variable speed fans, I prefer the thermo-switch controls (like the BGW has). The ones that vary continuously with temperature and/or signal drive me nuts. The noise they make is much more obtrusive as it is constantly changing. Alesis and Carver come to mind. I use some ball-bearing types that are only 28dB SIL, you slow those down by changing the fan speed resistor and they are very quiet. I find a fixed, low , constant speed easier to mentally 'tune out', and if it gets hot it can kick itself up to hurricane (I probably won't hear than when the low end is hitting 130dB+).

"between the metal /plastic can"

They use the same die in all of them. BGW chose to use only the metal cased hermitic sealed semis (and teflon wire) in the earlier units for superior reliability.

"anyone re biased the output stage into AB instead of AB-B"

I converted a 'B' (quasi) into a 'C' (complementary), not something I would recommend to anyone. I have also added a Baker clamp between the voltage gain stage and the output stage. The clamp allows driving 10dB into clipping without the outputs sticking to the rails.

"should I consider replacing all caps including the power supply ones?"

I do them all except the main power supply ones (unless they are bad). Don't forget to add the bypass films in the signal path, and the bypass electrolytics to the power supply. The films in the signal path clean up most of the solid state 'grundge', and the power supply bypass caps make the bass sound much tighter.

"I also run a SAE X25A "

I like SAE, but never owned the newer ones with paralled outputs, just the series connected ones. The series connected ones are a bit fragile for low impedance loads. They all benefit from the cap thing too.

In general, I like a lot of the old dinosaur amps (other than the weight). While I can buy an amp rated at twice the power for about $300, they don't cut it for me. The BGW750 transformer is rated at 50% more VA, even though it puts out half the power. They just sound bigger. They also play louder, especially compared to switching supply amps in limited power availability gigs.
Click to expand...

> and delete the capacitor level shift circuit of the BGW. At one time the capacitors were cheaper than the transistors, now the reverse is true. The caps are now replace with a couple of $0.08 transistors (MPSA42/92 types). See Q201, 202 in the PL3 schematic for reference.

Its a old Thread but Ctown link dont working, I want replace with transisitors

If somebody can send this schematic for reference Im happy

Cheers
 
#105
Connections

Awesome thread on the 750 series! Thanks!
I just picked up an amp last night and want to connect it for testing. In the future, I'll likely have a pre-proc that provides XLR balanced outs to sub, but for now I don't have it yet.

How can I confirm that the 1/4" jacks are balanced or unbalanced on my GBW 750c? I'm using it with home components and was going to whip up a quick RCA-XLR, but I know that is a compromise at best. I've read somewhere that the 1/4" are unbalanced already, so is that a better solution?

Thanks!
 
#107
Last night I grabbed the manual and then confirmed that I have the two kept caps to make this unbalanced. I checked the pins shorted and will be good there.

Does anyone know what the expected voltage is for inputs in this configuration? We had the gain turned all the way up when I auditioned it.

Also, if I ever did want to go balanced, any idea where I could find the transformers?

Thanks again.
 
#108
Djk,
I also wanted to mention that your contribution to this thread and specific guidance was a big contributor to my decision to get this amp. I'm running through sourcing replacement caps now and will be working it over soon. Looking at Nichican Muse KX series, but am also considering poly for smaller values in the signal path where space may allow. I was looking at handmadeaudio.com as friends have had great luck there with great price and service.
 
#109
I've listened to the amp again on speakers that I know with a source that I know. It sounds good - not great. I believe it lacks some transparency and sparkle up top. Further, it seems to lack control in the lower registers. I've taken the amp apart and cleaned it up. Caps have been replaced once before and based on the caps used, I would say this was probably in the early 90's - a guess. A couple are leaky(including C3), so I'm ordering replacements all around.

I can easily do the Elna SilmicII or Nichicon Muse KZ electrolytics, but was looking to try to get a poly into C1 and C3 if at all possible. Challenge is the clearance over the transformer - it's less than an inch. Anyone have recommendations that I can check out?

Has anyone found a small voltage and small profile poly cap for this application?

Also, I see recommendations for 47uF caps bypassing the main 21KuF caps. Assuming a 100v rating, is there anything wrong using electrolytic here? With a 47uF, I don't think I could fit a poly in there. I've also had a recommendation of bypassing with a 210uF and then a 2.1uF - essentially 1% step-downs. In my reading, there appears to be no specific formula for selection for value that is universally accepted. The individual with the greatest experience with this unit has had positive results with the 47uF application, so I'll probably go that route.

Thanks!
 
#111
Looking closer at the schematic, I see a bypass cap in the amp module with a value of .1uF 100v. These are (in my amp anyway) orange drop 225 caps (polyethylene). Seems the bypass application is already there, but the values are very different. This is where my reading has suggested a mix of experience, science, and magic seem to drive the values selection, but less science than in many other applications.

One school of thought is that the bypass cap is not "in" the signal path and therefore is less critical. Further, it's not an electrolytic and should not require replacement due to age. Vote, leave it alone.

Another is that it is generally. Accept edit hat polypropylene is better than polyethylene and I could swap them out for either 715s or even the popular Vishay 1837s.

Lastly, should I simply change the value of the existing bypass cap? ...or add the 47uF at the large 21k caps for a reason I don't yet understand? ...but why?

Thanks again!




Sent from my iPad using Tapatalk HD
 
#112
Amp module updated

Just a quick update for those who are interested or may read this thread in the future. I updated the caps in my amp with Nichicon Muse KZ bypassed with Vishay 0.01uF foil caps and am listening to it now. I did get a Polypropylene to fit at C1. The BGW 750C is a nice amp and a great find for <$200. Below is the result. Not elegant, but it sounds nice.

BGW750_Phase1Update.jpeg
 
#117
While I'm listening to the amp, I'm also considering a 220uF or 440uF(pair of on-hand 220uF in parallel) cap in parallel with C18/24 (0.1uF 225 orange drops). I've also seen this applied at the main filter caps, but the recommendation was to get it as close to the board as possible. I figure even if I have to extend a lead an inch to get it in there, it would be better in this position. Sound about right? Has anyone done this before on this amp?

Note that in general I like the sound of this amp full range - it certainly holds it's own and is a fun and easy to work mod-project. That said, it's intended to drive a pair of 12" subs in a home theater and had a good reputation for bass capability. Any specific recommendations given the intended use as a sub amp?

Thanks!
 
#120
@ Pingrs

Hi, indeed you're quite right, so thanks for pointing it out. Just goes to show, trying to do 3 things @ once isn't always wise 😀 And appologies to djk too.

@ tpremo55

Yeah could be flux/soldering iron burn etc, but i've seen many resistors over the years that looked like that which had been overloaded. Anyway it was just a heads up 😉
 
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