Are you sure it's vintage ? The old ones are just called "Jet". "Jet 20" is exclusively a modern amp as far as I know.
If it is vintage, most old Ampegs sound good (at least !) when they are working properly, but some of them from that era use some pretty weird tubes, and I've seen one from around that time with a really flimsy (uncharacteristically so for Ampeg) PCB. I usually try to steer people away from the ones that don't use some sort of available preamp tube and 7591s if not 6V6 or 6L6 power tubes (talking about the small to medium sized combos here), except when they are at stupid low prices or have sentimental value.
Interesting. The guy who wants to trade it to me for a Tele project I have is saying it's truly called the Jet 20 and is from 68-69. He says its an uncommon model that doesn't even look much like an Ampeg. I haven't had a chance to see it; I figured I'd do some research before I took the trouble. It's hard to dig up info, but I appreciate your general comments.
i refurbed a 56 jet back in the 90's. full bore-- it sounded AMAZINGLY big and nasty with the original speaker... totally roared. but backing off the volume knob on the guit... i couldn't ever get anything but a really honky clean tone out of it. finding the tubes was a bit bitchy, but it was pre-internet..
tone circuit didn't seem to do a whole lot, and it was neither great enough in its own right to be wowed by in any setting, nor flexible enough to be really useful... so i sent it on it's way...