An Odd Duck -- Gibson BR-6f Guitar Amp
I'm new here, so hello and glad to meet y'all.
I picked up a Gibson BR-6f awhile ago, as an around-the-house amp, and I'm trying to understand the design in order to know how best to restore it -- or how to nudge it toward its full potential without violating the design's integrity (i.e., grafting on a gnarly extra gain stage equals yikes, while rethinking the value of a plate resistor is not-yikes).
It's a '50s guitar and lap steel amp (the speaker code is 1953) with a lot of interesting behind-the-curve, '40s-specific design features. The speaker is a 10" Jensen field coil model, for instance, and the preamp tube is an 6SJ7 -- a sharp-cutoff pentode.
6SN7 for phase inverter, a pair of 6v6GT in push-pull, 5Y3 recto...
Here's a schematic:
The circuit reminds me, more than anything else, of the really early, octal-tubed Fender Deluxe -- like the Woody version and the first iteration of the tweed amps. The amp sounds. . .well, like it has a ton of potential, but also like it has an urgent need for new caps and tubes, and possibly a new/rebuilt transformer. The sound is muddy and imprecise, but with an unmistakable sort of resonance underneath the muck -- a kind of familiar (but indescribable) quality that attaches to good equipment that's been badly maintained.
Basically, and to be more brief about it: What does this design want to be? I mean, what should a design like this ideally sound like, and what sort of strengths and weaknesses does it inherently have?
The 6SN7 preamp tube is surprisingly gainy, btw. I don't have a clue how to compare pentode gain numbers to triode ones, but it seems to compare pretty favorably to the 12A_7 tribe. The field coil speaker is just sort of bewildering. I know how they work, more or less, but can't imagine what this one ought to sound like compared to an equivalent permanent-magnet Jensen....
I think you have a very nice amp there, Do you have a way to post pictures? IMHO you should shoot for an "as made" restoration. There is no reason I know that this cant be a nice 10-15 watt amp. The fact that it uses a field coil type design means that I would want to know that the speaker was OK early on, so that I could avoid doing a bunch of work and then not being able to get a replacement speaker. Also the temptation to just replace all of the caps should be tempered, because it isn't always nessisary. This might be a gem.
One great thing about youtube, guys just love showing off vintage gear.
I did a quick search and found this.
Gibson is notorious for not following their own schematics, so watch out. I'd replace all the electrolytics, plate resistors, and the power tube coupling caps first. The 6sj7 is kinda like an EF86, and sounds very good. They can be a tad microphonic, but not always. The glass ones seem worse, but I've only had one. Sometimes they were run in grid leak bias, which may look strange...grounded cathode, big grid resistor. You *can* replace the field coil with a suitable choke and use a modern speaker. I just bolt the choke down somewhere in the combo and run well insulated lines (actually, it was a Hammond in my case, but whatever). That way you can keep the vintage field coil (they're fairly irreplaceable) and crank out tunes on something a bit more modern and replaceable.
I love those old Gibson set-ups - and (in my opinion) they sound best with the original field coils.
I agree that you should keep it as close to original as possible; however I'd recommend installing a couple of 1k screen resistors.
Thanks! Let me try to post some pictures today.
I'm just checking in briefly until I have time to reply at greater length, but as a sidenote, I found a really interesting build of a BR-6f clone last night -- a project that you wouldn't think anyone would have gotten around to doing, but it's certainly a big and bounteous world out there.
It's different in a few ways (6K6 power tubes, 6X5GT rectifier), but apparently pretty faithful to the published schematic; and it sounds, surprisingly, like a good hard rock amp -- lots of clear, resonant distortion and very little of the fizzy quality that 12AX7 pre stages can produce.
It's really forcing a reevaluation of what I thought sharp-cutoff pentodes were good for. I didn't know they could do that...
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