Why didn't this amp ever turn into a best seller ?
First, I think the reason lays in this amp's history. No one ever associates Ampeg with guitar amps anymore. Still, in the golden age of rock'n'roll, Fender and Ampeg shared most of the guitar amp market - one on the East coast, the other on the West coast.
Then came the genius idea (but also a commercial suicide) of choosing electronic circuitry. The Ampeg Jet 20 is nothing but a handwired clone of a Fender Brown Face (6G3). That is, Fender's lesser known period: in between the Tweed and the Blackface and so on. 1964, I think.
So, how does is sound? Very raw, rootsy, rock'n'roll, but overall very good! The crunch sounds absolutely beautiful: thick grain, and most important very "organic". The circuitry's simplicity makes its sound depend on tour choice of tubes. Don't hesitate to invest in NOS tubes.
Yet, it's true that the "Low" channel sounds a bit dark, and the "Bright" one is a bit too bright, and the knobs interact with each other, and the stand-by wasn't designed by the book, but hell, how good does it sound!! And this tremolo, wahoou!! It affects the power tubes, so it sounds like... pumping. Beautiful!
Perhaps too dirty-sounding for jazz and not high-gain enough for metal, this amp is versatile enough for blues, rock, punk, garage and probably hard rock. It's also pedal friendly, and loves fuzz and tube screamers. Its power is perfectly suited for crunch sounds with a band, while using the guitar's volume knob you can get beautiful clean sounds.
In spite of the speaker's qualities, I replaced it with a Celestion Gold.
I had to have it serviced three times in 5 years: once to add a variable resistor to allow setting the bias, and twice (the first technician failed to identify the problem) because of an oxydation problem on the tube holders. All small and cheap services. All three technicians were impressed by this little-knowned amp's qualities.
First sold for €900, it soon sold for a mere €450. If you find one for sale, don't hesitate!