Hi Pascal; that's an early-mid '50s Gibson GA40 Les Paul ... these days, it's widely considered to be the best-sounding amp Gibson ever produced. Can't tell much from the pics but the very clean ones go $1,600-$1,900 ... there are at least two GA40LPs owned by our BBers ... one of which - if he hasn't sold it yet [he's been talking about it] - is a later model with black over tweed cosmetics
(Seen here, page 2; see also page 4) but the same circuit as the CL model; 2X5879 (channel 1 and 2 preamps), 1X6SQ7 (tremolo oscillator), 1X0725 (phase inverter/driver), 2X6V6, and a 5Y3 rectifier.
I think the secrets of the amp's fine tone are (1) 5879s in the preamps; the 5879 is a pentode, not a twin-triode like a 12AX7 ... it's built like an output tube and they produce a hot, smokey tone. My GA20T that you listened to ... it has a 5879 in channel 2. (2) an output transformer that is larger than those found on both Gibson 'student' amps with lower 'GA' numbers and the GA78/79/83/90-series 'stereo' amps. (3) A post-phase inverter master volume control shown in the blue circle below:
The Master Volume means that the 5879s can be run to near-distortion and then still have the chance to modulate volume just ahead of the output tube. PPIMV not adopted by most other mfrs until the mid/late '60s. Assuming the CL amp is in average to good condition, it's a reasonable price.
On that other Gibson amp you posted last week ... the black one with a Fender badge, I replied to your post and I think it's a GA30. The CL post was messy; the seller didn't know anything about it; wanted people to tell him what it was so he could figure out what it was worth ... anyway, I sent him an offer on it ... I don't expect to hear back. Cheers! John