Gibson Explorer GA-18(T?)

[Rhodesplyr]

I've got a Gibson Explorer on the bench from late 1961, according to the date codes on the potentiometers. It's a 2 x 6V6 with tremolo and a 10" speaker (missing the original speaker). I can see an "18" on the chassis, but the T, if it had one, is obscured by corrosion.

It's not vital for general repair purposes, but it doesn't match either the GA-16T or GA-18T schematics; it seems more like a hybrid of the two, generally closer to the 16T.

Notable component values: 2nd stage cathode resistors is 2.2k, grid stoppers are 10k. 6V6 cathode resistor is 270.

Did Gibson ever publish an accurate schematic of this model, or is it an undocumented transitional model?

[Enzo]

Gibson rarely published an accurate schematic of anything. COmponent values were always in a state of change, so were circuit configurations. I have one model somewhere in the GA-teens area that I have SIX schematics for. Three are published by Gibson, and the other three I drew from the amps themselves. All different, and not just a couple resistor values.

As far as I can tell, for the most part, if the knobs came out the same on the panel, Gibson would put most anything inside. A GA something opens up to find the circuit is really an EA something, or vice versa.

Hybrid is as good a word as any. Like most amp makers, they didn't reinvent the wheel over and over, so you may find the input stages from one and the power amp from another and the trem from a third, and so on. The thing to do is look for the section you are fixing. If you are working on the trem, who cares if the power amp circuit is wrong? Or they may take a circuit based on 6EU7 and rewire the sockets for 12AX7 on some of the amps. The amps don;t care, and the circuit doesn't care, just the socket wiring is diferent.

SO if you are looking for THE schematic so you can restore it to an "accurate" configuration, good luck. The ones they made in June could be really different from the ones they made in July.

[bob p]

Gibson rarely published an accurate schematic of anything.

You made me laugh. I remember working on an old Gibson amp for a local guitarist who runs a recording studio. He had some oddball Gibson that belonged to his grandfather. I never was able to figure out exactly what the name of the circuit was. I had always thought that I just couldn't find the right schematic. I hadn't considered that there might not be one!

[Rhodesplyr]

The same is true of the Premier/Multivox amps. There are, I think, four different schematics for the Premier 120 floating around the internet. I've worked on two of them, and neither matched any of the schematics--or each other. The schematics are a rough guide.

[Jazz P Bass]

Don't even bring up Valco amps.

[Enzo]

What about Valco amps?


Oh... sorry...

[Jazz P Bass]

Edit: Valco Amp 'schematics'.
Tough to find.
Usually incorrect.

[Rhodesplyr]

I've usually been able to find Valco schematics--just never under the actual name/model of the amp :-)

[Enzo]

C'mon, Jazz, it was a joke. You said don't bring them up, so I brought them up.....

[Jazz P Bass]

Hee Haw.

[custom_amp_30]

That may be that the circuits changed from the original schematic as production changed. Gibson use to be good at publishing some of their older schematics on the Howard W. Sam's Photo facts.

I've pulled at least four schematics from the older publications.. Examples: Model GA-200,GA-80, GA-70 and the GA-6 (Lancer). I would check the years of the GA18 production Years and see if they listed the (1959- 1961). The schematic I used to repair my GA-18 was the GA-16T which matched almost to the final resistor..If memory severed me well. My son owns a 1959 GA-18 and I own a 1960 GA-18T.

[Enzo]

Gibson published tons of schematics. Unfortunately they made changes even more often. I have the big Master Service book they published - full of drawings and specs. Much of it is available here:
Gibson Amp Heaven Schematics - Maestro Skylark Les Paul Junior Lancer Gibsonette Titan Scout Falcon Reissue

The telling story in my experience is that I have three different Gibson issued schematics for one model, and three more hand drawn - all different - for amp with the same model number. That is six different schematics for the same model number. I hope to god that is all the variations on it. Oops, I already told that story in post #2.


The schematics are bound to match some units, you just never know when.