He used an amp called a Versatone. It was designed with two tube amps going into one speaker, sort of a weird biamp concept. It was meant to be a very full range clean sound. The designer brought one by to the studio where the Airplane was working (Baxter's, maybe?) to show it around and Jack checked it out and immediately dimed it. Of course, that meant he had to have it and the rest is history (there's a brief section on Baxter's, Wild Tyme, I think, where it just explodes with some power chords). There was a bunch of discussion in the Guild Forum of the old Dudepit about these amps. Fred Hammon, of Dark Star pickups owns one and Dan Schwartz (mgod) has a few (as well as Phil Lesh's custom Guild M-85, but that's another story, well worth checking out).
Jack still uses the Versatone for larger gigs in conjunction with other amps for the clean tone, apparently. You can see it in live shots of various eras of his playing.
The SWR Interstellar Overdrive was an attempt to recreate this effect in a rackmount unit.
HTH
Edwin
