Output transformer for 1946-48 Gibson BR-4

[Resonator Guy]

I will soon be receiving a Gibson BR-4 amp. It is missing the field coil speaker so I am guessing it will be missing the OT. I plan on using a permanent magnet 12" 8 ohm Gibson branded CTS speaker that I have and am wondering what would be a good OT to use. I was thinking a Hammond made for a Fender deluxe, both are 2 x 6V6 amps.
What specs should I be looking at for this?
This is just a start. Later I will attempt to find which leads are for the OT and which for the power resistor I will replace the field coil with.

[J M Fahey]

I think your OT choice is fine, a 6V6 is a 6V6.
As of the field *coil*, you'll need, ahem!! .... a "coil" ...
Besides the resistive side, which you already considered, add also a suitable choke.
And I'm sure your amp will be as fine as new.

[Bruce / Mission Amps]

Use a tweed Deluxe output tranny clone....
I think a 470 ohm 5 watt resistor will be fine in place of the field coil "choke".
However, a +3Hy @ 90ma choke, after the first filer cap, with a 360 ohm 2 watt resistor in series will be a bit better for filtering... if you need it.

[olddawg]

You might consider a field coil speaker. I have seen quite a few 12" Jensens on eBay for reasonable prices. They usually come out of Hammond organs that are being parted out. Just a thought. They usually have the OT mounted on the frame.

[Resonator Guy]

The field coil speaker idea would be fine if I could find one without shipping. Prices to Canada get prohibitive. And I do have a nice speaker waiting for it that I know works. As for the choke, I have only needed one so far, in a Gibsonette that wasn't happy with just a resistor. I will have to wait to see what it wants. Thanks for the help and I will post after I have it and get it working.

[Resonator Guy]

I got the amp today and am pretty happy with it, cosmetically. However the joys of Gibson amp collecting continue. It bears little resemblance, of course, to the BR-4 described in the one book about Gibson amps and differs greatly from the schematic in the Gibson manual. It only has two preamp tubes, not three and had a cap can on top rather than discrete caps inside. Of course the can could have been added later but it looks as old as the chassis. I am going to proceed slowly with this one and will ask you folks for guidance as issues arise. I imagine there will be a few.

[Enzo]

It probably fits a different model schematic. What is the tube complement left to right? Does the BR-4 also have a name on it?

[Enzo]

Look for example at models BR6 and BR6F, also older model KEH.

[Resonator Guy]

Enzo, I was looking at a BR-6 schematic and there are similarities. You stay up later than me. I generally shut down the computer at martini time and don't come back on until morning.
The cabinet and faceplate are BR-4, just not the internals. There are no nameplates on this as to model number. The BR-4 also did have a cap can. I can't post pics right now because my editor to re-size them is broke but I will get some up. This should be fun.
Here are some pics. I still can't get my editor to work so these are the bay pics.

[Enzo]

But that's just Gibson. You open us a GA-something only to find it is really an EA-something Epiphone circuit. I have one model - GA17,19,I forget - and I have three different Gibson issued schematics for the model PLUS three additional hand drawn schematics I have made from amps with the same model printed on them. SO six different schematics for the "same model" amp.

And ther are plenty of times where some amp was a transitional item, and there simply is no printed schematic. What you have to do is take all the similar schematics you can find as suggestions. They did tend to do things in similar fashion a lot, so you might find the power amp stage matches the BR4 while the input stage matches the BR6, and so on.

Your input/gain stage is going to be triode or pentode, see what they did with whichever one in other amps of the era.

And speaking of hand drawn, this amp is pretty basic and is going to be relatively roomy inside, so pour a beverage, get out a pad of paper and a pen, and draw the diagram up yourself from the amp. If you can't read a part value, just draw in the part with a ??? by it. We can easily figure out appropriate values. We already know pretty much how a push pull 6V6 stage will look. Start with a similar a schematic as you can find, then look at it as you draw. Make yours look more or less like the factory one. That way if it LOOKS like a wire goes from the rectifier tube to the tone control, you can see that that probably is not really the case.

Looking at the other drawings, I see they used a 1000 ohm field coil in some of them, so consider that, and Bruce's 470 ohms sounds like a good suggestion too. It won;t be critical.

Watch closely how the tube sockets are wired. I am going out on a limb here, but looking at your photos, and assuming the tubes are in fact the correct ones. I see an empty socket, I'd bet for a rectifier tube, by the transformer. Then the two gray ones I assume are the 6V6s? Then the other two. The clear one on the left looks like a 6SL7, and the second dark one loooks like a pentode. If that is the case, I'd bet the pentode belongs on the end with the clear one in the second socket. That is just a guess. Maybe whoever took out the transformer just stuck the tubes into random sockets.

[Bruce / Mission Amps]

It does look suspiciously like an EH-150.... which I have rebuilt a couple without using the EM speaker.

[Enzo]

They all have that family resemblance. I see 750 ohm field coil on that model.

[Resonator Guy]

The empty socket is for a rectifier, 5Y3, I am supposing. The preamp tubes are 6SL7 and 6SN7, and in that order in a BR-6.
I will attempt to do a schematic, and I have managed to read all the cap values. It has been recapped previously and the can disabled. There is a lot of room in there.

[Resonator Guy]

Man, this amp has been on my bench for nearly two weeks and this is the first chance I have had to do anything with it, what with having hi-speed internet installed (finally) and getting a new computer with windows 8. It badly needs new caps, really nasty ones in it. The fuse was also blown and the pilot lamp was burned out. I figured out how the plug for the OT and field coil speaker was wired. You can see in the pic the wires were cut when who-ever removed the speaker and OT. I have a suitable OT from a 2xel84 amp so I patched it in along with a power resistor, hooked it up to my bench speaker and turned it on through my current limiter. As all the tubes lit up I was greeted by a lovely loud hum that I have heard before. So I will await the new caps from AES and will continue doing cleanup work on it. At least it seems to work, which is always the question when one buys something like this. I trust a 47mf cap will be okay to replace the 40 mf one.
Enzo, I have not progressed well with the schematic. I believe I have an age induced learning disability there. I will draw out a section, think it is fine and then find three errors the next day. I never used to have trouble doing flowcharts for systems designs back when I was gainfully employed. But I will give it another shot.
And I still have trouble uploading pictures.

[Resonator Guy]

I have the amp almost working and now have a new question but I will continue this thread.
I recapped the amp and wired in my OT. I used a 1000 ohm resistor in lieu of the field coil as this has worked on most others I have done. I did test with a choke and the results were identical to my tired ears. The speaker is good.
I now get a hum unlike any I have heard before. It doesn't sound like caps or a questionable ground to me. The hum, which isn't horribly loud, sounds like it is coming from the bottom of a well on a mildly windy day. It isn't affected by volume or tone controls. I can plug in a guitar and the guitar sounds nicely over the hum, and isn't bad at all.

Any thoughts on where I should start poking about? This is a brand new type of noise for me.

Also I still can't post pics from my new laptop. It uses Windows 8. The small screen for pics comes up and asks me to browse for a shot. The picture id goes in the appropriate box but when I hit upload nothing at all happens.