There John Tesh sits on the Entertainment Tonight set, cool and calm, a shock of dark blond hair spilling onto his forehead as he dishes up the latest dirt about Hollywood. Watching him night after night, it's hard to imagine that inside Tesh the creative urge burns. But it does, it does. While the show's male viewership may be thinking about co-anchor Mary Hart's legs, Tesh, no doubt, is thinking, "Hmm, should the second chord in the opening theme be a C sharp diminished or an F dominant seventh?"

It has yet to be an ET "Inside Story," but here's the deep scoop on John Tesh: TV's seemingly bland white-bread wonder is leading a double life as a new age, jazz-fusion composer of no little renown. His score for CBS's coverage of the Tour de France won a 1987 Emmy and became the basis of his first album, 1988's Tour de France, which sold 85,000 copies. His second and current album, Garden City, has sold 50,000 copies, risen to No. 5 on Billboard's new age chart and was voted best pop-rock instrumental and best independent jazz album by critics and the public at the New York Music Awards. Tesh, who has also scored the theme for NBC's NFL Live, is now getting a six-piece band together for a 1O-city tour this summer.

"Yeah, music is really my first love," says Tesh, 37, settling into the studio he has built into his three-bedroom Santa Monica condo. The $1 million workplace, complete with a 24-track recorder and an $80,000 Synclavier (a keyboard that produces sounds ranging from breaking windows to the string section of the Berlin Philharmonic), is where Tesh repairs every afternoon after taping FT. "Television has been a way to pay the bills while I've been trying to establish myself," says Tesh, who makes more than $200,000 a year.

Tesh's ET colleagues don't seem to mind playing second fiddle to his music. "John's the most amazing, complex anchor I've ever met," says executive producer David Nuell. "He's just got so many interests. You don't often sec that in this business." And Tesh is not quite the stiff he sometimes seems onscreen. "Once he gets on the air, he gets really serious," says Mary Hart. "But he's pretty much a cutup. At times, he's like the brother to whom you want to say, 'Oh, John, did you really say that?' " In fact, off the FT premises, much of what Tesh says might sound a mite freaky to his TV colleagues. Flipping into jive, he frequently begins his sentences with "Hey, man," and liberally sprinkles the conversation with "cool," "groovy," "righteous" and "dude."

This is John Tesh?

Believe it. The cool dude was born in Garden City, N.Y., the youngest of three children. He inherited a passion for music from his late father, a textile chemist who played violin. By high school, Tesh was proficient on trombone, trumpet, piano and electric organ, "When I went to clean his room, I thought I was going to hang myself with all the electrical wires in there," says his mother, Mildred, a retired nurse.

A music and communications major at North Carolina State University, Tesh got a job as deejay at the campus radio station, which led to a weekend anchor job at a Durham TV station. When the weekday anchor pulled anchor, Tesh applied for the job, landed it and left college.

His rise thereafter was steady. He anchored in Orlando, then Nashville (where the weatherman at his station was a guy named Sajak, and the anchor at a rival station answered to Oprah), New York and finally at CBS Sports. Hired to become what he calls the next Jim McKay, Tesh began circling the globe reporting on athletic events, all the while keeping up with his music. While covering the Tour de France bicycle competition in 1985, Tesh got permission to write the theme song for the broadcast. No simple task, that. Using a keyboard and a Macintosh computer, Tesh did some of his scoring in the network van, which was racing as fast as 80 mph in pursuit of the cyclists. In any case, voila! a music career was set in motion.

Tesh was pedaling along nicely at CBS until Laurence Tisch took over the network. Amid the firings and defections, Tesh decided to make a move in 1986. Meanwhile, ET had decided to make a change in anchors and called Tesh, who showed up for the audition in a flowered shirt. Despite the fashion crime, he got the job. And although Tesh would like more time at the keyboards, he has managed a harmonious blending of the two careers.

But there's discord in Tesh's personal life. He and his wife of eight years, ballerina turned actress Julie Wright, 31, began divorce proceedings in April. "I guess people just grow apart," says John. Tesh's dual career is one problem. He tapes ET from 9 A.M. to 1 P.M. each day, then works in his studio until 10 P.M. "I don't think I've been a very good husband lately," he says. "You've seen my schedule. It's really intense. But when you're given opportunities in music, it's hard to turn them down."

With music taking up more and more of his life, Tesh thinks now and then of giving up TV. But for the moment, it's a safe bet he'll stay put at Entertainment Tonight. After all, when else during the day does a busy dude get to sit down?

—Joanne Kaufman, Patrick Cole in Los Angeles