Wow, for $20 you couldn't go wrong!
If this is just the amp head I think you may "technically" have a B-42X, although the info inside the amp may very well say B-22X. Here's an excerpt from "Ampeg - The Story Behind the Sound" (pg. 150) that explains why I think that:
Ampeg continued its pattern of frugality with the B-22X Jupiter 22 and the B-42X Jupiter 42. Both amps shared the same chassis and circuit. These amps were closely related to the B-25/B-25-B circuit, but were a bit of an upgrade, since the Jupiters offered reverb, vibrato, and tremolo. The Jupiters were rated at 55 watts and had a tube complement consisting of a 5AR4, two 7027As, four 12AX7s, a 6CG7, and a 7199. Each of their two channels sported two inputs (bright and normal), volume, treble, bass, Ultra Hi and Ultra Lo. There were separate controls for tremolo, vibrato, reverb, and 'after-beat' - an effect similar to the 'repeat percussion' found on other Ampegs.
The main difference between the two Jupiter models was their configuration. The B-22X was an upright 2x12 combo amp, while the B-42X was a piggyback rig consisting of an amp head and a 4x12 speaker cabinet. Altec 417B speakers were available as an option on both Jupiters. Ampeg would repeat this pattern of using the same chassis for a piggyback/combo amp with the V-4/VT-22 and the V-2/VT-40."
It looks like that amp was introduced in 1969 and discontinued in 1971.
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