The GA-80T was a tweed-covered Gibson. The started covering their amps in tweed in 1959. This style lasted until 1961.
The GA-80T is credited as being the first amp to use push button tone shaping with a series of "Vari-Tone" buttons, much like the ES-355 guitar had. The 1950s and 1960s were a time of great innovation at Gibson. This innovation is well known and respected in their guitar designs, but their innovation in guitar amplifier technology goes largely overlooked...probably due to popularity of a certain other California guitar and amp maker of the time.
The GA-80T, as with most of their higher end amp models, featured Jensen AlNiCo Special Design speakers. This one is a 15" P15P.
The amp is 2-channels. Channel 1 is just Volume and Tone. Channel 2 incorporates the aforementioned Vari-Tone tone-shaping buttons, a traditional tone knob (when engaged), and a (now fully operational and beautiful) tremolo with Depth and Frequency controls.
Some of the most innovative, fascinating, and toneful amps of the 1950s bear the Gibson badge. With the rush of collectors to acquire Fenders of the era, these often get undeservedly shunned or overlooked. It might also be the case that many of the best designs were made in such small numbers that few know much about them! Furthermore, Gibson's marketing of their amps has been historically abysmal. Where the design teams have shone, the marketing teams have faltered. Plus, you don't see these reissued like the great Fenders of the past. In fact, Gibson barely has anamp line at all these days, and they haven't had since the 70s. It's a shame.
A previous service had done this amp some favors - a few fresh caps - but it had also screwed around with the tremolo circuit so severely, it did not work. Some components in the tremolo circuit were completely out of place, some were put in BACKWARDS! The previous service tech had given up and simply clipped the tremolo footswitch out of the circuit altogether and left it non-working.
They did one interesting mod, which I left in place... They rewired one of the 5879 pentodes to accept a 7199 triode/pentode combined tube, and had changed some of the values of the caps and resistors surrounding it. Had to redraw that part of the schematic. It was a marathon, to say the least, but it was worth it. This amp sounds AMAZING!
For those of you thinking of working on old Gibson amps,don't be surprised if you encounter discrepancies between the original schematics and what you actually find in the circuit. I found a few components that looked factory which were different values, and even one resistor, which was called for on the schematic, which didn't appear to have ever been installed at the factory. This was probably the result of an early revision in the circuit.
To see more great vintage Gibson, Valco, Danelectro and other off-beat and rare amps, go to Guitologist.com.