SAN FRANCISCO — Keeping a big band together these days is difficult unless it's either part of a larger institution, such as Lincoln Center or Carnegie Hall, or carrying on the legacy of a legendary bandleader like Ellington or Mingus (which presents a different set of challenges).

But special projects and weekly gigs have allowed bassist Marcus Shelby to maintain his 15-piece Jazz Orchestra for nearly two years.

Shelby's trio, active for more than four years, has recorded such tracks as "Slappin' 7th Avenue With the Soles of My Shoes." About two years ago, however, after completing his most recent trio project, Sophisticate (released on his own Noir label), Shelby began thinking about what he wanted to do next.

"I'd written for small combos and trios and done some chamber work," said Shelby, 33. "But I'd never done much orchestral writing for jazz. So I began developing music and putting together an ensemble of great local musicians." Shelby took advantage of the many musicians he'd worked with as music director of the Oakland-based Savage Jazz Dance Company and as a scorer for independent film projects.

Unlike a community big band that shows up only for gigs, Shelby's group is committed to improvement. "It's been part of our life for the last year," he said. "We rehearse once a week and have maybe two or three gigs at different places. We've been lucky, because it's a management nightmare to keep a big band together."

The Marcus Shelby Jazz Orchestra's highest-profile gig has been at Yoshi's in Oakland, California. But the band regularly plays Thursday nights at Bruno's and Saturdays at Butterfly, a pair of San Francisco restaurants.

The orchestra's four trumpets, five saxophones and clarinets, three trombones and a rhythm section of piano, bass and drums allows him to explore big-band music by the likes of Ellington, Strayhorn and Mingus. It also affords him, his musicians and other friends in the Bay Area jazz community the opportunity to compose and arrange for a large group.

Shelby was born in Anchorage, Alaska, but grew up in Memphis, Tennessee, surrounded by the blues and gospel music. He joined his junior high school's jazz band when his military father was reassigned to Sacramento, California, took up double bass and continued to sing and play in church.

He moved to Southern California after turning 20. "I learned about music publishing, production and songwriting in L.A.," Shelby said. "I had always written music, but I never knew it could make money for me." In Los Angeles Shelby wrote music for theater and such films as John Singleton's "Higher Learning" and Desmond Nakano's "White Man's Burden."

He also discovered drummer Billy Higgins' World Stage jazz workshop in 1990. "I hung out there day and night," Shelby recalled.

Over the next few months he found a group of young musicians with common aspirations to assemble a small combo and write and rehearse music for it.

The resulting group, Black/Note, came together over a period of about seven months and included drummer Willie Jones III (who currently plays in trumpeter Roy Hargrove's quintet). The group released four albums and played together for six years.

In 1991 Shelby won the Charles Mingus Scholarship to the California Institute of the Arts and studied with bassist Charlie Haden and flutist James Newton. Two years later he became music director for the Jazzantiqua Dance and Music Ensemble.

Settling in San Francisco in 1996, Shelby now divides his time between Northern and Southern California. He works with both the Savage and Jazzantiqua companies, composes, participates in the San Francisco Jazz Organization's educational outreach programs, runs his record label, and leads his Jazz Orchestra and trio.

"I'm glad he's well-established in the Bay Area," said Newton, who remembers him as a dedicated CalArts student.

"Yeah, I love music," Shelby said. "But we've got to keep on moving forward. I try to do that within the language of jazz."