NEARLY every band has to deal with personnel changes sooner or later, but what do you do when the person leaving can't be replaced? That's pretty much the situation Northern Son guitarist and vocalist Jeff Carmassi found himself in when his longtime friend Tyler Florence told him this summer that he was quitting the band. Florence was more than just a bassist—he and Carmassi had founded the band together in 2009, and it was obvious the band wasn't going to be the same without him. "It was tough," says Carmassi. "We're superclose friends. For him to not want to do it anymore was rough for the first couple months. But there's no hard feelings."

Adding to the pressure was the fact that in a short time, Northern Son's popularity had skyrocketed.

"You have a lot of expectations when things take off fast. There was a point when we were pushing so hard. We had to step back and say, 'We haven't even been a band for a year.'"

But underneath the ambition, there's a gritty sense of determination to Carmassi's musical vision. Even when absorbing the shock of Florence's departure, he never considered breaking up Northern Son, and the other members—guitarists Jonathan Sontag and Oj Montevirgen, keyboardist Sean Ghavi and drummer Brent Nunn—back him up. "It was never an option that the band was going to stop," he says. "We were all committed.The band's primary influences include Tom Petty, Bruce Springsteen and Wilco, and like all of those musicians, the key to Northern Son's immense appeal is their ability to coat simple, slightly rootsy songs in gorgeous, surprising arrangements. It's a big, powerful sound with instant accessibility, and on their newest single, "Spin," they show even more pop prowess, showing glints in the melody of two NorCal hitmakers, Third Eye Blind and San Jose's own Smash Mouth. Losing Florence has been a setback, but they've connected themselves to another South Bay success story by bringing in former Strata bassist Hrag Chanchanian. And Carmasi is ready to push hard again, to bring Northern Son's music to as big an audience as possible. Mainstream success? "None of us feel that's a pipe dream," he says.