NPR Music | Tom Vitale

On a hot, humid afternoon, Bob Stewart has called a rehearsal at his Harlem apartment. Six musicians are in a circle in the living room — on one side, trumpet and trombone; on the other, cello, viola and violin; and in the middle, the elephant in the room — Stewart's tuba.

The musicians are rehearsing for a concert at Lincoln Center at the end of September; it's a release party for Stewart's new album, Connections: Mind the Gap. The band is a double quartet — a classical string quartet, along with Stewart's jazz band of 25 years. Today, he's rehearsing just the strings and horns.

"We're expanding some of the arrangements to really fully involve the strings with the horns in a lot of these songs," he explains. "That's what this rehearsal is about today."

The 69-year-old musician has played with some of the great innovators in jazz — including Gil Evans and Charles Mingus. Stewart says he learned from those bandleaders to let the musicians add their own ideas to the mix.

"It's amazing: However different the sound of each of those people you mentioned is, quite often, their approach to getting there is the same," Stewart says. "They're very open. They're all very generous. And that's the way I try to treat this band – and, therefore, how to make the band sound larger than just my idea."

Stewart's idea was to take the tuba beyond its Dixieland roots, where it was the original bass instrument. By the mid-1920s, it was replaced by the upright string bass.

Stewart's 27-year-old son Curtis plays violin in the band. He says the tuba has a quality a string bass can't produce.

"The difference with the tuba is you can play the note and sustain and grow, or cut it off exactly when he wants," he says. "It has a much more vocal quality, which is funny because you think of strings as being a very vocal instrument. I think that's why the violin and the tuba work so well together: They're different sides of the musical stratosphere."

Bob Stewart's first instrument was trumpet. He grew up in Sioux Falls, S.D., and moved to Pennsylvania, where he studied at the Philadelphia College for the Performing Arts. But he had to drop the smaller brass instrument when he developed problems with his lips.

"There was no way I was going to finish my graduation recital, because I was having some embouchure problems. I switched the Tuba to train new muscle, and then I did my recital on the tuba." He adds, with a laugh: "Hated it."

An early tuba gig brought him to New York, where he found work playing progressive and free jazz. But Stewart says for that music, he didn't have any role models.

"I can't go to someone and ask, 'How do you do that? I have to figure out how to breathe. How do I amplify myself? Do I go through an amplifier? What do I do?'

"It took so many years just to focus on that," Stewart says. "There's a whole melodic side of the horn that I didn't really invest in, because it really took a lot of energy to figure all that stuff out. And now I'm starting to go that other direction."

Stewart's exploration of melody on the tuba ranges from a Thelonious Monk standard to a classical suite commissioned for the ensemble. His son Curtis says this broad mix brings to mind a modern phenomenon.

"The idea of a playlist, and also the shuffle mode on iTunes or Spotify — I think that's very attractive to a lot of, at least, younger people. Because you get to experience a set of music; you don't know what's going to happen next," he says. "It will be a journey, from one song to the next, the ways that you can experience music."

For his part, Bob Stewart says he wants to expand the repertoire for the tuba so that his students at the Julliard School — and the next generation of tubists — will have even more options.