Syd was a big experimenter with guitar tones. He used many different pickups, effects, and amplifiers to get his tones.

He mainly stuck to a Selmer stereo amplifier, used with Fender Esquire and Telecaster electrics. He also had a Danelectro electric, although I am uncertain of the model it was.

A big effect of his was the Binson Echorec, a tape-driven echo/delay unit. This was used to get the sweet-sounding echo effects on the solo of Astronomy Domine, a sound which I love (and you can't seem to get echo swells on a lot of today's modern delay pedals, sadly. I rather like that sound).

Stuff like the distortion sound on the beginning of Interstellar Overdrive were really not made with effects; just cranking the amplifier very loud and adding a lot of treble, making the classic 'overdrive' (which I also love the sound of).