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Gibson BR-6F Amplifier


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There is a severe lack of information on early Gibson amps, so I thought maybe we can share information on the BR-6(F) models specifically - of course knowing that the sister and cousin models will gladly come into play. I am not an electronics guru, I'm just a tinkerer. I'm using this opportunity to learn more about the world of tubes.


My BR-6F is most likely a 1951 production model. The volume pot and speaker were both made in early '51, and the power transformer was made in late 1950.


I bought this at a guitar shop and it was very low on volume during the demo but the tone and cleans/distortion was kind of how I remember my '58 Les Paul Amp GA-40 that I had back in the 1970's. I've been looking for this sound for a long time. I got this for a good price and figured that I could work on the volume issue.




I opened it up and found that 4 capacitors and possibly 1 resistor had been replaced (it was the only one with a tolerance band on it). The volume problem was solved by re-seating all of the tubes (helped a little) and by re-soldering a broken connection from a pair of the new capacitors (helped a lot).


Checking the resistors in-circuit showed that at least a couple of them were high out-of-spec, one of those by a lot. While it's difficult to determine if a resistor is in-spec or low while it's in-circuit, looking for the ones that are high is easy and the one that was way off showed signs of burning once I got a better look at it. It is a 2.2k cathode resistor and measured 2.9k while installed, but it measured 3.8k out-of-circuit! The new resistors I installed are carbon film, all of the original ones are carbon composition. Carbon film is supposed to be lower noise and I gotta' tell 'ya - there's less noise. There's still some hissing but it's pretty slight and only noticeable at no volume a foot away from the speaker. Now if I can just do something about that hummmmmmm.


That's all for now. I'll post photos tonight.

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There is far more info on these amps on other sites. I figured people here are probably more into bigger more modern amps, not quaint vintage ones. Nuff said, maybe too much on that. Could start an argument...


Re: hum in your BR6-F


Since I have to 'start from scratch' with rewiring my rebuilt field coil speaker and replacement transformer, I have to figure out the 'polarity' of what is supposedly a humbucking winding. I found an article somewhere online (I usually save a copy, but not the source info!) that discussed this. On a few Gibson BR- and GA- series schematics it's hard to distinguish the transformer winding that is in series with the transformer secondary and the speaker voice coil due to the way they drew the magnetic core of the speaker electromagnet through the hum-bucking winding.


Other schematics have a clean and clear drawing but with confusing (to me) annotation.




T1 is the output transformer. "T2" to the right of the transformer winding closest to the 1000 ohms annotation suggests there is another transformer. I guess it has to be the field coil...so one has to think about which 'coil' schematic symbols are where.


The way I see it, the 1000 ohm wnding resistance is the field coil proper, serving dual-duty as 1) the power supply filter choke in a CLC filter, and 2) providing the speaker field coil flux and electromagnetism. I used to think the magnetism was present only when there was output signal, but I forgot that the tube bias is always drawing current through the field coil, with or without signal. It may not vary much since the parallel single-ended versions are Class A...not sure with the push-pull ones...also makes me wonder how different the different versions sound(ed)..the likelihood of having two different circuit versions that are still functioning and original enough to sound as they were supposed to is pretty slim for us. This comparison probably happened in the 50's when they were weren't yet vintage. I think there were bigger & better things to wish for than wishing one had a different version (if they even knew or cared)...that's for future folk (us) to spend 2014 $ on instead of 195x $. We could have worse habits, I guess.


Hmmmm, yeah, back to the hum monologue.


So, what I read elsewhere is that the second pair of wires within the field coil electromagnet structure, which are very low resistance and supposedly only a few turns of wire, need to be connected with the 'polarity' or orientation that causes the lowest amount of hum. Mine, IIRC, measure less than 1 ohm.


If you look at the three inductor symbols that are aligned end to end left-to-right (I really did not want to use the word 'collinear' here, but it helps me to use it...there, I feel better) (1000 ohm coil 'T2', two series-wired inductors, one overlapping the speaker schematic symbol), it took me a while to correctly (I think) recognize the following:


The 1000 ohm part is the higher resistance coil, the 'middle coil' is the low resistance humbucking winding, and the one overlapping the speaker symbol is the 'ordinary' voice coil itself. That middle one can have its two wires swapped so the polarity or phase of that winding only, is reversed, and compare the hum level. Two windings on the same magnetic structure technically speaking are a transformer, but they aren't using it as a transformer here. It's more like a 'coupled inductor', but that's splitting hair(-thin wires) over the convention the draftsman used back then. I see coupled inductors as filter chokes in switchmode power supplies, but don't really understand much about them.


OK, that was one hum suggestion.


Two, an easy one. Make sure all 'ground' or common connections, particularly those relying on chassis hardware connections, are free of oxidation or other corrosion, and tight. Soldered ones, too, but they are easier to find, assess and correct.


Lastly, and this is the hardest and will take some reading, find online a discussion of star grounding as it applies to these amps. I just yesterday saw such a discussion online and it had grounds marked with different colors, yellow and pink/fuschia, I think. The author described the separation of grounds by amplifier stage and went so far as to compare the desired 'star grounds' in a bigger 'constellation ground' context. It may have been an untested recommendation. I had trouble opening the image at a magnification that allowed me to see the whole picture and had to leave, so I shut the computer down. No idea where it was. Maybe ampage.org, as that's one of the last places I was snooping around. If I find it, I'll be back.

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Wow! Great info! Thanks.


Lucky I had a few minutes to read your post Murray.


Yeah, I'm on the fence about the "originality" thing. I want it to function as designed firstly, and I want to be able to use it to its fullest. So swapping out parts so as to restore proper function is my main goal. If that means using more modern resistors, I'm ok with that. The capacitor types are a little more of a concern for me, especially those waxy ones. I absolutely want to maintain the speaker!


Like I said, everything in the amp looks original from the factory except those caps and one resistor. I'd like to match the replaced caps to original if possible. The tubes are all Sylvania and appear to function normally, except the heater voltage is right at the 10% high mark, measuring about 6.93v.


I'd like to clone it twice, once using a field coil speaker, and a second that is setup for a PM speaker. It's a really simple circuit to duplicate but the tubes are the biggest expense, topping out with the metal 6SJ7, which I think is the most important tube in this circuit. Also, it may turn out that the field coil speaker is additionally crucial to the sound of this amp (which I believe is true).

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I'm trying to load some photos of my amp and not having success. Photobucket doesn't like Win8, it keeps hanging and messing up my computer. I only have 500k on this forum for photo usage, not enough space. I then tried to use the Insert Image button above and I get the "You are not allowed to use that image extension on this board" message.


So, no photos. sorry

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