Don’t normally copy slabs out of Wiki but this was most interesting about McGuinn’s Rickenbacker:
During his time with The Byrds, McGuinn developed two innovative and very influential styles of electric guitar playing. The first was “jingle-jangle” – generating ringing arpeggios based on banjo finger picking styles he learned while at the Old Town School of Folk – which was influential in the folk rock genre.
The second style was a merging of saxophonist John Coltrane‘s free-jazz atonalities, which hinted at the droning of the sitar – a style of playing, first heard on The Byrds’ 1966 single “Eight Miles High“, which was influential in psychedelic rock.
McGuinn with The Byrds at a concert held at Washington University in St. Louis (September 1972)
While “tracking” The Byrds’ first single, “Mr. Tambourine Man“, at Columbia studios, McGuinn discovered an important component of his style. “The ‘Ric’ [Rickenbacker guitar] by itself is kind of thuddy,” he notes. “It doesn’t ring. But if you add a compressor, you get that long sustain. To be honest, I found this by accident. The engineer, Ray Gerhardt, would run compressors on everything to protect his precious equipment from loud rock and roll. He compressed the heck out of my 12-string, and it sounded so great we decided to use two tube compressors [likely Teletronix LA-2As] in series, and then go directly into the board. That’s how I got my ‘jingle-jangle’ tone. It’s really squashed down, but it jumps out from the radio. With compression, I found I could hold a note for three or four seconds, and sound more like a wind instrument. Later, this led me to emulate John Coltrane’s saxophone on “Eight Miles High“. Without compression, I couldn’t have sustained the riff’s first note.”
“I practiced eight hours a day on that ‘Ric,’” he continues, “I really worked it. In those days, acoustic 12s had wide necks and thick strings that were spaced pretty far apart, so they were hard to play. But the Rick’s slim neck and low action let me explore jazz and blues scales up and down the fretboard, and incorporate more hammer-ons and pull-offs into my solos. I also translated some of my banjo picking techniques to the 12-string. By combining a flat pick with metal finger picks on my middle and ring fingers, I discovered I could instantly switch from fast single-note runs to banjo rolls and get the best of both worlds.”