1959/60 Tweed Gibson GA-18T Explorer

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The tweed Explorer (and GA-16T) Viscount were very much like Fender Deluxe amps with tremolo, but with tiny output transformers. I haven't seen the cabs, but the schematics and specs for the GA-16T and GA-18T are identical. Possibly one had a better speaker and fancier or larger cab.

[Front Panel]
Control Panel (would be on top)

The gray plastic piece with the jack in it is the customer's home made, speaker out mounting plate.

This amp had been worked on. The PS caps had been replaced with a metal can cap inside the chassis (a short waiting to happen, in my book). A few other caps had been replaced, including the trem oscillator caps. The power tube cathode resistor bypass cap had not actually been replaced; a newer cap had been soldered in parallel with it. Not a great idea. Earlier tweed Gibsons were pure, flying lead, point to point, and a moderate pain to work on. As you can see here, Gibson was now taking a route similar to Fender's. This was still a minor pain to work on, though, as many leads and components blocked access to others. But at least everything was accessable from one side-- unlike with Fender's boards.

An Explorer's Guts Laid Bare

This amp has 2x12AX7, 2x6V6, a 5Y3 rectifier, a lacquered tweed cabinet, and a 10" speaker.

The owner complained of ghost notes. Replacing the PS caps and the power tube cathode resistor bypass caps took care of this. It turns out, however, that there are also intermodulation distortion problems. It probably needs a new OT; I'm awaiting the owner's decision on whether to send the amp back for this.

How does it sound? Much like a tweed Deluxe with slightly later breakup and tremolo. The tremolo sounds as good as I've heard with a simple circuit like this.

[component board] Component board closeup: A bit blurry, but even so you can see that some of the ceramic caps are under resistors, and others are over resistors. Note the small guage, yellow jumper wires on the board. You can get to everything without removing the board, though some of the components would be difficult to work with. [power supply] Power supply closeup: With the addition of the three wire power cord, this is pretty standard stuff. The original, multi-section electrolytic (probably a brown "cigar" cap) has been replaced with an Aerovox multisection, with the band crimped to hold it in place. (I would have wrapped it in electrical tape to avoid any chance of a short if the cap shifted.) This cap was replaced with individual Xicons, as it had leakage issues. You can't tell from this photo, but there was also a black, ground wire that was just flopping around at one end. Another short waiting to happen. I used the loose end as part of the cap ground scheme, solving that problem.
[octal sockets] Octal socket closeup: The sockets are on the bottom side of the chassis when installed, which explains all the dust and crud. This is baked on pretty well, but doesn't poresent any problems. The socket at the left is for the rectifier tube; the others are for the power tubes. The big cap is the original cathode resistor bypass; the smaller, gray cap is parallel. Both are 25V caps; I removed these and used a 50V cap in their place. The resistors here were fine, a pleasant surprise as they seemed to be original. [9 pin sockets] 9 pin socket closeup: This is a bit blurry, but there's not much that's exciting, anyway. It was nice that Gibson stuck to the standard wiring color scheme here and throughout.
[inputs] Input section: Note that Gibson didn't put any resistors on the jacks. At the same time, they also didn't put any on the preamp tube sockets; they're all on the component board. While this is theoretically suboptimal, in this case it didn't seem to cause any problems. But in high RF areas, the first stage grid resistors and grid stoppers might need to migrate to the tube socket. [control pots] Controls (pots): Note the judicious use of spaghetti on bare component leads. I have no idea whether the zip cord (lamp cord) to the tremolo pedal is original or not. It looked old enough, but was in awfully good shape. You can see the new trem caps someone added on the component board (baby blue and yellow), just to the left of the fat, black cap.
[power controls] Power controls: Pretty much bog standard for an amp of this era, other than the three wire power cord someone added. Oddly enough, they left the death cap in when they replaced the power cord. (That's the black cap with the red stripe in the upper middle of the photo.) The bakelite of the power switch was weak from age and heat; when I started to desolder the death cap, it cracked. I replaced it rather than risk it falling apart later in use. Next time I'll just snip the stupid cap out! [transformers] Transformers: I was concentrating on that horribly small OT, and accidentally cropped the PT. The PT is about normal size for an amp like this, which gives you an idea how small that OT is (for reference, it's about the size of the undersized OT on a 5W Kalamazoo Model 1 or 2).


The ".ps" PostScript file may yield the best picture, if you have software to display them. After that, the JPEG is the best - but also the biggest. The larger GIF looks almost as good as the JPEG on most computer screens. The smaller GIF is pretty much just for use on a web page, but lets one see at a glance what the overall structure is. NOTE: The caption says GA-16T, but the schematic was identical for the GA-18T.

ga18t-sm.gif 796x615 70Kb
ga18t.gif 1554x1200 434Kb
ga18t.jpeg 1554x1200 788Kb
ga18t.ps 771Kb

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Last updated: 10 September 2005

Copyright 2005 Miles O'Neal, Austin, TX. All rights reserved.

Miles O'Neal <roadkills.r.us@XYZZY.gmail.com> [remove the "XYZZY." to make things work!] c/o RNN / 1705 Oak Forest Dr / Round Rock, TX / 78681-1514