Worth the price of admission??
Hi Dan: Yes, but not much more; the minimum is about what it was worth at the beginning of the year. Among Gibson amps, only the venerable GA40 Les Pauls are holding their values. Some of these GA77s were fitted with a 15" speaker, you might want to confirm the size and ask him what make / model is in it now. It looks like the reverb and trem live on Channel 2 and this model pre-dates channel-switching.
Techno-jabber and assuming the amp matches the public domain schematic - Gibsons are notorious for not matching their schematics. The amp uses the poorly-understood 6EU7 in the preamp; except for Sovtek reissues, the tube is NOS/used only. It's a twin triode like a 12AX7 but the heaters are wired differently and the pinouts are different; that is, the 12AX7 tubes are not drop-in replacements for 6EU7s. Unlike the 12AX7s that come in flavors; RCA, Telefunken, JJ ... the 6EU7s don't; GE, National (Yugoslavian), and that's about it.
The 6EU7 is a reliable, sonically-neutral tube ... it just doesn't have the cachet of the 12AX7. But since this isn't a giant among vintage amps, re-wiring 6EU7 tube sockets to accept 12AX7s won't harm its value and, in fact, might increase it. 6EU7s run $12-$22, about what 12AX7s but there's no such thing as a 'killer' make/model of it. This amp also uses a 7199; I think it's functioning as a mixing amp but whatever it does, its a rugged but sort of expensive tube; $40-$50. The used models labeled 'Dynaco' out of Dynaco ST70s are $75-$100.
Speaker; the take-out, assuming it's 12", is a Jensen P12S; it wasn't the original but it was a poor substitution; it can't handle the power but would work fine in a single-ended / twin 6V6 / 15 watt amp. Seller says it has a generic higher power probably ceramic speaker; in other words, there's nothing in the choice of speaker to help justify or boost the asking price. So ... if you don't like it, you're going out of pocket for something that you do.
Blue boxes: bias supply at bottom, resulting negative bias voltage shown at the top. The amp has no bias pots or test points like the Guild Thunder Heads / Birds / Whoppers so it needs matched tubes - a little more expense and offers no direct means to piddle with the bias. Among ampistas with Obessive Compulsive Disorder, screwing with the bias amounts to something to do for an evening.
Green box/V5 - Red box/V4: this is the tremolo oscillator - it jerks DC voltage up and down and that flucutation is transmitted to V4. Worth mentioning is the 'Factory Adjustment' tag on a potentiometer between the V4 cathode and ground; I guess it's supposed to be adjusted to achieve 2.4v on the cathode but ... uh ... we here at Juanzampz get nervous when we see proprietary stuff ... same goes for the
LDR above V4; a Light Dependent Resistor ... sort of a
Gyro Gearloose design. In short, V4 and V5 make that little 'light bulb' go from bright to dim; doing so causes the resistor half of the gadget to decrease in resistance shunting power to ground and then increase in resistance as the light comes back on. The result is the dc power heading chart north to that 220K resistor goes up and down causing the line signal - heading left to right - to go: "...owawowawowa ..."
So.....wassa matta with that? One of the valuable features of the amp depends on a proprietary gadget and a semi-conductor that's approaching obselescence ... if it isn't already. Mad Dog's Ampeg Gemini and several Fender designs use an optical-coupler a/k/a 'roach' but the after-market is still supporting those amps ... without a part # or cross-reference, I can't say the same for this model Gibson.
Finally, the other half of V5 (upper center) is the phase splitter sending the positive half of the wave form to one half of V6 and the negative half to the other half of V6; that is, V6 acts like a true driver; a little more costly way to do it and likely to produce a good result. Otherwise, the amp looks great; tolex, control panel, knobs and so on. The smooth brown tolex a little easier on the eye than the tree-bark stuff seen on later models. Good luck! CJ