Gibson EH185

[GHR]

Hi all, I'm trying to identify the inductance of the GQ13 choke shown on this schematic for the Gibson EH185
amp. http://www.gibson.com/Files/schematics/EH-185%20Amp.pdf

It is used as part of the bass tone control and a medium sized iron core inductor from the few
guts shots I have found online. choke | Flickr - Photo Sharing!

Is there perhaps an old Gibson parts list anywhere online that might identify the part
by it's value(inductance and resistance) rather than just the part number?
This seems to be the only Gibson amp of that time that has this tone control circuit and the second 1940s version switched to a different preamp tube complement and a single-knob, regular tone control with a
0.0075uF cap.

I keep googling for Gibson parts list or the like, but the word "Gibson" is too common overall
and gives me thousands of irrelevent hits including for "Mel Gibson" .

And when it doesn't go irrelevent it find Gibson guitar parts not amp parts. :-(

I'm quite interested in this tone control and how it works. It seems it might be something interesting to
add to an amp. T-Bone Walker used the EH185 so it must have been a good sounding amp.

I've tried to research this type of tone control and found thordarson had a tone control unit with a "humbucking tone control" choke which shows up in a 1938 Thordarson catalog with
specs 22H-0mA(5mA max)-230R, but I cannot find any other information.

http://www.theused.com/manuals/thord...ne_control.pdf


Any information would be appreciated.
Thanks,
Henry

[Enzo]

Good luck, you'll be lucky if the schematics even match the circuits in many Gibson amps.

That GQ-13 is their part number, if you can find that.

[GHR]

Yeah, I know it's a long shot. That's what I'm actually looking to do; match a part number with what the part actually is.

I realise Gibson schematics have MAJOR errors. That schematic has all almost all the resistors marked in MegOhms instead of KiloOhms including marking the PS dropping resistors as 10M when they should be 10K. The pots are marked 500M so I guess the best thing to do is find out what that GQ-13 would have been replaced with. That's why I was hoping to find some kind of parts list, catalog or some such from back in the days when these things would have been worked on.

That GQ13 would not have been replaced much since it's seeing zero DC and the AC voltage is, or should be, low too.
I wish I could find someone with a real EH185 who would take an inductance and DCR reading, but I guess that's wishful thinking... :-(

[JoeM]

Fairly large choke.

[yes, that's my EH-185]

[GHR]

Hey, that's cool! Does it sound as good as it's rumored to? Can you tell me anything about that GQ13? Any way to measure it's inductance and DCR?

Also, the schematic seems to show a 200R cathode resistor and it say 20V so that would mean the tubes are idling at 50mA each ignoring screen current. This is a little off from the datasheets so is it maybe an error as Gibson schematics are prone to? Any details on this from your real world amp? I'd be much abliged.

[JoeM]

I wish I could report on how my amp sounds, but it was acquired a few years ago in non working condition. It took me some time to track down the schematic since my particular amp shows it as an EH-150. After some research, apparently several of these were made before Gibson had new model no. labels, and used ones from the EH-150. Strangely, the schematic shows M as an abbreviation for K(ilo) (1,000), so the values look real screwy. I have heard these before though. One of my teachers and mentors, Tony Rizzi had one. At least his version was very dark sounding, but a great jazz sound. Tony was a big Charlie Christian fan (Some of you may have heard of Tony's 5 Guitar Band). I remember him telling me he was sort of going for a Tenor Sax kind of sound with it.

I've thought of restoring it one day, but I'm not sure how to deal with replacing the field coil speaker, the one in it is nearly destroyed.

[Enzo]

Those Ms on the very old schematics did not mean meg ohms, they were not in error, at least not that way. The M was the Roman Numeral M for 1000. Just as in older drawings you saw mf insyead of uf next to caps. Today we'd read that as millifarads, but back then there were no caps large enough to be in millifarads, and the small m stood for "micro."

50ma tube current sounds high when you think in terms of modern class AB amps with 450 volts on the plates, but look at how low the B+ is. 270 volts. Less the 20v on the cathode, and you have 50ma through the tube with 250v across it. That means the tubes are dissipating 12.5 watts each. I don't see that as much of a stress. Go back to the data sheets and look up typical currents at 250v on the plate. Once again, I don;'t see this as an error.

I don't have a recall of Gibson schematics being full of errors, what I DO see all the time is circuit changes during production life. SO a schematic may not - and often doesn't - match the unit at hand, but it is due to changes, not mistakes. There is one model, I forget now which without looking, GA19, GA17, something in there, and I have SIX different versions in my files, each with its own schematic.

[GHR]

Those Ms on the very old schematics did not mean meg ohms, they were not in error, at least not that way. The M was the Roman Numeral M for 1000. Just as in older drawings you saw mf insyead of uf next to caps. Today we'd read that as millifarads, but back then there were no caps large enough to be in millifarads, and the small m stood for "micro."

50ma tube current sounds high when you think in terms of modern class AB amps with 450 volts on the plates, but look at how low the B+ is. 270 volts. Less the 20v on the cathode, and you have 50ma through the tube with 250v across it. That means the tubes are dissipating 12.5 watts each. I don't see that as much of a stress. Go back to the data sheets and look up typical currents at 250v on the plate. Once again, I don;'t see this as an error.

I don't have a recall of Gibson schematics being full of errors, what I DO see all the time is circuit changes during production life. SO a schematic may not - and often doesn't - match the unit at hand, but it is due to changes, not mistakes. There is one model, I forget now which without looking, GA19, GA17, something in there, and I have SIX different versions in my files, each with its own schematic.

Really interesting!
What's strange is that Gibson has that schematic hosted on their website and hasn't updated it to modern nomeclature since anyone who isn't familar with M being used in place of K would thing it is an error as I and several other people at two other forums where I've posted the link to that schematic.

I don't have a datasheet for the 6L6/G(remember old 6L6s) class A w/ 250V plate & screen and cathode biased. The one I have shows fixed bias only.

[Enzo]

Doesn't matter how the bias is created, the tube doesn't know the difference. All th tube knows is how much voltage is there between plate and cathode and what voltage the control grid sits at relative to the cathode. 250v across the tube is 250v. And the plate dissipation is a function of voltage and current.

Gibson may post these old schematics, but they are not going to waste their technical people's time redrawing 60 year old amp schematics. Modern techs may not know all the terminology on antiques, but once you find out, you ar not going to forget. Everything we know, we at some point didn;t know.

I don't know that this amp runs class A, I suspect it doesn't. I don't know whose data sheets you used, I just use my RCA book. Many places sell reprints of the RCA RC-30, and well worth the $30 or whatever they go for. And you can also download the thing.