Actually, Tom Petty was playing bass for the Dwight Twilley Band briefly before getting the Heartbreakers together. They alternated on the show with the Spinners.
Tom Petty & the Heartbreakers was the house band on Wacko. I saw him for the first time ever on this show, in 1977. On one of the episodes I saw, Tom Petty and his band performed the song "Breakdown".....I was sold....