The New York combo is on tour promoting its second album, ''Travelers & Thieves"

”We used to jam forever,” says John Popper, lead singer and harmonica player of the proudly old-fangled Blues Traveler. ”We’d play one song for three hours, easy.” In these days of sampled instruments, that boast should hold about as much appeal as a Country Joe & the Fish LP. But a reputation for jamming has actually helped Blues Traveler, a New York combo that makes music the classic-rock way — with a brawny, infectious mix of riff-heavy guitar and bluesy harmonica. And they’re reaping rewards for it.

The group formed in 1987 in Princeton, N.J., and released its first, eponymous album in 1990. Its second, Travelers & Thieves (A&M), is selling briskly and the band is headlining a national tour. They’re also playing opening-act gigs with spiritual forefathers like the Allman Brothers Band and Jerry Garcia. In fact, Blues Traveler’s crowds often look suspiciously like Deadheads.

Popper was introduced to the blues by, of all people, the Blues Brothers (”I wanted to be a comedian at the time”) before graduating to the likes of Elmore James. But despite its name, the band — comprised of Popper, 24; drummer Brendan Hill, 21; guitarist Chan Kinchla, 22; and bassist Bobby Sheehan, 23 — uses blues as merely a starting-off point for a melange of rock styles. ”We don’t sound like a Mississippi Delta band,” Popper says, ”but we play what we mean, and that’s the spirit of the blues.”