Paul Kantner, a founding member of Jefferson Airplane, one of the definitive San Francisco psychedelic groups of the 1960s, and the guiding spirit of its successor, Jefferson Starship, died. He was 74. Mr. Kantner, who started as a folk singer, had a mellow baritone voice that blended ideally with the penetrating tenor of the group’s founder, Marty Balin, and the powerful mezzo of Grace Slick, who joined the band after its first album. He played a steady rhythm guitar that anchored the freak-out style of the group’s lead guitarist, Jorma Kaukonen, and the adventurous bass lines of Jack Casady. Mr. Kantner came to be seen as the intellectual spokesman for the group, with an ideology, reflected in his songs, that combined anarchic politics, an enthusiasm for mind-expansion through LSD and science-fiction utopianism. The group was inducted into the Rock and Roll Hall of Fame in 1996. For several years Mr. Kantner and Ms. Slick were a couple. Their daughter, China Isler, survives him; two sons, Gareth and Alexander, also survive him.