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Early'ish pre-1963 is really the years to have for those amps, better tubes & speakers.
I had a 1965 Reverberocket J12 with the trem and echo and it had those oddball tubes, 7868, BK11, etc.
I HAD to swap the original speaker out as well, yuch.
If it has the 6V6's its a good one, the 7591 models don't sound as good and they're harder to find NOS tubes for. I have a two channel 1962 Mercury (M-12) which I love, and they sound very similiat to the 6V6 Jet.
Here's an earlier version of that amp, with the best of all the Jet circuits, which I think they changed in 1963...
The ad says it is a Jet J12A. That has two 6SL7 tubes, and two 7591 output tubes plus a 5Y3 rectifier. The 6SL7 and 5Y3 are plentiful as NOS, and not expensive. The 7591 tubes are very desirable tubes in short supply NOS, and hence expensive. The 7591 is an excellent sounding tube - sort of between a 6V6 and a 6L6 in power capability, but with it's own characteristics. EH makes a modern version of the 7591. Ampegs have not appreciated in value like the Fenders, so the vintage amps are still reasonably priced.
The NOS 7591 tubes are expensive and in short supply! The new EH tubes aren't built as rugged and don't sound as good. And flat out the 6V6 circuit sounds better. You can still find a Ampeg 6V6 circuit for the same kind of money, so I'd (IMHO) look for one of those. Just be patient. No matter what you do make sure that the tremolo section works fully as they have a tendency to die in these amps!