Problem with a circa 1952 Gibson BR-9

[Resonator Guy]

I recently got a BR-9 amp on ebay and .... surprise .... the seller may not have been entirely truthful about it.
I am in the process of replacing the shot field coil speaker, which was supposed to work. But that's not my question.
Inside, there has been some dubious work done that I am correcting. One change has me stumped. From pin 6 of the 6SJ7 preamp tube to the volume control there are two newer .22k@400V caps in series.
My Gibson repair manual has a schem for a BR-9 but with a 6SN7 preamp tube. I know there were a number of different models. The model I have is one with a bar across the grill and that has a gold G in the centre.

Can anyone tell me what cap value should be there. I have it sounding okay but I believe it could sound better.

[Enzo]

Make the amp work first. Even if it has problems, I would wager that it was working at some point configured as it is. Once you have the speaker handled, the filter caps up to date and whatever else, then if it doesn;t sound right, you can chase after that.

A pair of 0.22uf caps in series could have been their way of making a 0.1uf cap when they didn't have one.

And don't, please don't, labor under the misconception that ANY Gibson amp has specific part values. There is one model Gibson amp that I have three different publushed schematics for, and three more schematics that I have hand drawn because the amps did not match the other drawings. So that is SIX versions of the same model number. And this is not unusual for Gibson amps. SO just because some schematic calls for a certain value cap doesn't mean that that same cap isn;t called out as three other values in three other schematic versions. Not only that, but often enough you would find that what was supposed to be for example a GA15 was really a EA45 inside. (I made up those two numbers, so it is just a coincidence if I got them right).

You have the master book? GOOd, I can;t tell you how many times I have flipped through it looking for a certain tube complement to figure out which schematic fit some amp. FOr example, since you have a 6SJ7 and a field coil, it will be older, so I went from the front and voila, look on page 10 at the Mastertone amp. Maybe not quite a BR9, but look, a 6SJ7 front end, and a 0.1uf cap from the plate. And a field coil speaker.

And there are other more complex models with a 6SJ7 front, and I see coupling caps anywhere from 0.1 to 0.05 to 0.001uf. Even if a circuit is otherwise the same, changing such a cap value might make the amp sound better with differnt types of speaker, perhaps keeping an 8" speaker from farting, versus allowing a fuller tone through a 12".


Post script: after reading Jazz post, sure, someone could have stuffed the wrong tube in the socket.

[Jazz P Bass]

Gibson BR9

Are you sure about that 6SJ7?
That is a pentode tube
Pin 6 is the screen!
The 6SN7 is a dual triode.
At any rate, the schematic shows a .02uf cap from pin 6 to the volume control.
It is a coupling cap.
Two .22uf caps in series equals .11uf.
Gibson generally used a smaller cap than .1 to keep the bass tones out of the output section.
They suck power & sound bad in a low powered amp with a smallish speaker.
NoteThe schematic that I have attached has the first half of the 6SN7 mislabeled.
The plate is pin 6, the grid is pin 7 & the cathode is pin 8.)

[Resonator Guy]

Thanks guys. I do have the amp working already, with a spare speaker hooked up to the OT. The field coil just doesn't work. It definitely is a 6SJ7. Doesn't work at all with a 6SN7. The SJ that was in it was real tired and gassy so I put a new one in I had handy. I just thought the series caps looked kind of chintzy. It has recent (?) filter caps, no hum etc and there are two other old caps in there that I am going to replace. I will post my results when it is singing.

[Resonator Guy]

The parts arrived from Tempe this morning, and I replaced two caps and added the power resistor in place of the field coil. The filter caps had been done sometime in the not too distant past so I didn't touch them. I got a vintage CTS 10" speaker with a Gibson logo on it and put it all together this afternoon. It sounds mighty good. Not loud at all but good. Even my Strat sounded nice through it, and that guitar doesn't like my other Gibson amps. There is almost no hum as well, so I'm glad I didn't go for a choke. Only my Gibsonette seems to need one.

On to my Champ scratch build.