B&W 370 SSB Adapter Mystery

[nq5t]

I seem to have a knack for uncovering undocumented early versions of otherwise standard products.  The last one was a very early TMC GSB-1, which differs in several ways from what is considered the standard GSB-1 as documented in the available manuals.  It also has an interesting mistake on the tuning dial, which may apply to all of the early GSB-1s.

This time it’s a B&W 370 SSB Adapter. 

I’ve been looking for one for a while, and finally turned one up in nice shape.  It looks like any other 370 from the outside.  Even a glance at the top of the chassis looks like a 370. And it’s really well built, too, with a high build quality reminiscent of TMC.  But the one I have does not match the 370 schematic in the commonly available manuals.  It differs in tube layout, the way several internal alignment controls exist or don’t and are laid out, and my early investigation indicates there are quite a few other circuit differences as well.  My hope is that someone out there has a matching manual, or at least a schematic, or at the very least has encountered this 370 version and has some knowledge.  There were minor differences between Run 1 and Run 2 of the “common” 370, but this seems to predate (postdate?) those.  Except for a new cathode bypass cap on the 6V6 audio stage, the circuit is clearly unmolested and original.

The “common” 370, per the manual, has the following tube layout:

(1) V-1 6SN7
(2) V-2 6SN7
(3) V-3 6SJ7
(4) V-4 6V6
(5) V-5 5Y3

This 370 has the following layout with some different tubes and different nomenclature (listed here in the same sequence as the tubes in the “common” 370 manual):

(1) V-1 6K8
(2) V-3 6A8GT
(3) V-5 6SJ7
(4) V-4 6V6
(5) V-2 5Y3

And the tube types seem to be just the beginning in terms of differences.  For example, the 1N34-based detector in the common 370 is not in this one (I’m assuming I'll discover some other detector configuration once I trace things out).

If anyone has come across such a 370, or has any documentation or knowledge please let me know.  I’m not looking forward to reverse engineering the schematic of this thing, but that may be the only option.

Grant NQ5T

[Steve W8TOW]

I'd be interested in hearing about the GSB-1 you have...was that an earlier post here?
73
Steve
w8tow

[nq5t]

I'd be interested in hearing about the GSB-1 you have...was that an earlier post here?

I still owe John Poulton a promised writeup on my GSB-1 for his TMC history website (tmchistory.org).  I'll post a link to it here once I have that done.  I have all the pieces -- some photos, revised module list, revised schematic, etc.  But I need the Round To-it to pull it together.  I've been ignoring getting this done way too long, and will try get it up this weekend.

The differences are relatively minor, but it's interesting to see how things evolved -- probably as a cost cutting measure in at least one case.  It works as well (or as badly, depending on your point of view) as the more common version.

Grant NQ5T

[nq5t]

Here's the writeup on the early GSB-1 unit.  It makes most sense when compared to the chassis diagram and schematic in the usual GSB-1 manual -- such as the manual at BAMA or on Poulton's web site.


http://www.tmchistory.org/tmc_restoration/GSB-1/GSB-1.pdf

[WA2TTP Steve]

Hi Grant,

I have an BW 370 that I got as part of an estate from my dentists mother. She didn't have the manual at the time so I down loaded a version of it from Bama but later she called and gave me a box of manuals for all the equipment I had. I have not compared the original manual for the 370 with the Bama version but will do so when I return home in early April. I'm in sunny Florida until then.

Steve
WA2TTP

[nq5t]

Hi Grant,

I have an BW 370 that I got as part of an estate from my dentists mother. She didn't have the manual at the time so I down loaded a version of it from Bama but later she called and gave me a box of manuals for all the equipment I had. I have not compared the original manual for the 370 with the Bama version but will do so when I return home in early April. I'm in sunny Florida until then.

That would be great, thanks!  I'm checking also with the usual suspects that sell manuals commercially, but haven't gotten a hit yet.

I've block-diagrammed it out, and understand the differences at the macro level, but have yet to start on circuit details.  Just a couple of minor things to do before I try powering it up -- so we'll see what happens

[nq5t]

Youza!!  It lives.  Well, the "Gating Control" (AM passband tuning) is not working properly for some reason, but on SSB it's playing it's little heart out.

370's are more rare than most, and there must be even fewer of this early version.  Seem to be more GSB-1s (because they are easier to find, although usually absurdly priced), and more CE Slicers (A,B) which are fairly common.  But the 370 is an excellent SSB detector.  Good clean audio, no fritz or fuzz, no "odd" behavior, and really good selectivity.  It's certainly in the "cream of the crop" category for these things.  

I currently have it on an SP-600 for testing, although it will eventually reside with an HQ-140X if I can find the table space.  Once again, I'm amazed by the stability of the SP-600 even on 20M ... it'd been on one net/qso for an hour and hasn't budged.  (But maybe it's finally time after almost 20 years to rotate it out for a while).  

In any case, if you happen on to a 370, don't overlook it.   In the days when I had my High School radio club's HQ-140X in the summer time, I would have given my eye tooth (or maybe traded my sister, who might not have been particularly appreciative) for one

Besides the Gating Control thing, the only task left (for the weekend) is to re-flock the speaker grill.

Grant NQ5T

[WA2TTP Steve]

Hi Grant,

I'm finally back from Florida and I dug out my 370 manuals. They both show the common tube line up you indicated. They are little different in that one shows a copulate between the 6SJ7 and the 6V6 and the other one shows discreet parts. Mine has the discreet parts. Also Z3 is shown with .01 coupling caps with a note that there not used on Run 2. The other manual shows Z3 with no coupling caps or note. It had drawn by W2TXP on the lower right. Yours is a mystery.

Mine doesn't have a serial number that I can find.

Glad it's working on SSB.

Steve

[nq5t]

Glad it's working on SSB.

I did finally find the AM passband tuning problem (lead popped off the tuning cap) and got it fixed.  So it's working pretty well all around.  One of these days, when I have nothing better to do, I'll try to reverse engineer the schematic.  It's functionally the same, but quite different in several key details ...

For whatever reason (maybe they didn't sell well, or their owners won't part with them) the 370 is one of the least seen of the sideband adapters.  It's also one of the best playing of the lot, IMO, and benefits greatly from the fact that it has an integral speaker and 6V6 output stage built in.

Grant NQ5T

[nq5t]

I was finally able to locate a manual with the schematic that matches my B&W 370.  The manual doesn't match, but at least it does include the schematic, and operationally the only differences are minor differences in the locations of a couple of alignment points which are easy to figure out by inspection.

Turns out there were 2 "runs" of 370s.  Run 1 is characterized by having a 6K8 as a 1st converter/osc, and a 6A8 as a product detector/osc with a diode detector for AM.  Run 2 is characterized by having a 6SN7 as a 1st converter/osc, 1/2 6SN7 as a buffer amp, 1/2 6SN7 as a BFO, and a two diode product/AM detector.

Early Run 1 370s had factory changes published making some mods in the audio and low pass filter sections.  Mine (serial 518) appears to have had these changed installed at the factory.

Still don't know how many of each were built, or if there are any substantive differences in how the two runs play, but if you encounter a Run 1 unit you can get a manual from SS of Nebraska (the old Hi-Manuals materials) that has the schematic and the circuit updates if they are needed.

The 370 is a very nice SSB adapter -- worth picking up if you see one.

Grant