Hi Dan ...I thank you ... and Coastie thanks you ... :wink: Gibson ... changing amps design and cosmetics the way most of us change socks. According to Wally Marx, the eBay amp is a '54-'56 and he goes on the say the early models ... like this one ... had 8" speakers and the later ones had 10". The auction text doesn't say but it'd be useful to know. Like that juan guy's post said, it's a Class A / single-ended design with twin 6V6s running in parallel; blue is signal in and red is signal out just like a GA8.
The two differences between the GA8 and GA9 are the preamp tubes and the field coil speaker (green box on right). The GA8 had a 12AX7 preamp like the tubes in the GA18 but the GA9 has a 6SJ7 pentode. Both the early GA8s and 9s had field coil speakers and both models later went with more modern speakers. The other difference between the early and late GA9s (and GA8s) is this early model doesn't have a tone control.
These amps do the hot, rich, thick thing to perfection. Many Gibson models used pentodes in the preamp including the GA40, GA83, and GA90 although they dropped the 6SJ7 in favor of the 5879 somewhere along the line. The knock on all preamp pentodes is that they can become microphonic. The 6SJ7s are around as either NOS or used and don't cost very much. There's no such thing as a 'premium' 6SJ7 so it'd be worth going through a few to get the quietest ones. Also, the parallel design means that the 6V6s, unlike push/pull amps, do not have to be closely matched ... although if somebody had a pair of ... say ... matched Brimars ... this amp would sing .. :wink: