External Combustion – the second album and first as band leader of the Dirty Knobs – is proof that lightning can strike twice. His first record, Wreckless Abandon, was released in November 2020 to a great reception, gaining attention from Broken Record, Vulture, WTF with Marc Maron podcast, LA Times, Rolling Stone, American Songwriter, Billboard and many more.

The Dirty Knobs made External Combustion in three weeks over the summer of 2021, and "The band became this spontaneous type of combustion”, Campbell recalled, recounting how the band became more intuitive the longer they played.

Campbell claims he was never offered a solo deal in his four decades with Tom Petty and the Heartbreakers, despite also writing and producing for artists like Roy Orbison and Don Henley. "I wouldn't have known what to do with it," he adds quickly. "I was Tom's partner. Lyrics and singing – he could always do it much better. But I was writing and recording more music than Tom could deal with. That's when I got the Dirty Knobs, which gave me a chance to try singing. So I started woodshedding. And then when my life changed (with Petty's death in October 2017) it was, 'Time to do this now.'"