Myth no. 1: Only fusion music and jazz/rock will draw a crowd to a large auditorium.

Myth no. 2: Los Angeles is simply not a jazz city.

If only for destroying these long-held illusions, the Playboy Jazz Festival earned itself a niche in history. At the Hollywood Bowl, where six years ago a jazz festival thudded to a $50,000 loss, a totally responsive audience of 13,500 flocked to the bowl Friday. The Saturday show drew the first capacity house (17,200) in the history of the Bowl's erratic relation ship with jazz.

Hugh Hefner took his pipe out of his mouth long enough to offer an instant analysis: "We combined two vital elements-George Wein's long experience as a producer, and Playboy's promotional and advertising campaign, with support from the mayor, the City Council, everyone at the community level. I expect this to become an annual event."

The ambiance matched the music. Astonishingly, when Joni Mitchell's Charles Mingus tribute, well performed and received, segued (via the revolving stage) into Benny Goodman's set, instead of anticlimax that might have ensued, this was the climactic point of the entire evening.

It couldn't have been nostalgia; most of his listeners had never seen the veteran clarinetist before. The sometimes lethargic Goodman, who just turned 70, created with an unspectacular ad hoc combo of a brand of energetic, swinging music that recalled his pristine years. I have rarely heard him more inspired, even during the Charlie Christian era.

The evening had begun on just the right note, with the Young Tuxedo Brass Band strutting around the Amphitheater to color the scene. After sundown the clelebratory mood was heightened by the brisk sale of green glow-in-the-dark necklaces, and by an audience as broad in tastes as in age.

In one of Friday's more memorable moments, Sue Mingus, the bassist's widow, introduced a band of his alumni "to preserve the spirit of his music," as she put it. This group's revival of "Porkpie Hat," the Mingus dedication to the late Lester Young, was followed by Joni Mitchell's own lyricized version.

Joe Williams, 18 years away from the Basie band, proved in his latest reunion that he and the Count are still less than a heartbeat apart. Waymon Reed, a Basie alumnus, now Sarah Vaughan's husband, rejoined the band's trumpet section to accompany Vaughan, along with her new rhythm rection sparked by Mike Wofford at the piano.

The Saturday marathon show (3-11p.m.) had a more contemporary orientation. Whether Report's declamations, probably audible well into Ventura County, swung thunderously on "Birdland." The group lacks some of the character of the days when it had an additional percussionist. The bassist Jaco Pastorius, and a phenomenal solo, lent new respectability to the word distortion.

Chick Corea and Herbie Hancock at their grand pianos had the unenviable job of following Messrs, Zawinul and Shorter: for both it was a case of "Fingers, don't fail me now." The fingers came through.

The mainly Brazilian band of Flora Purim and Airto amd the Afro-Cuban distillations of Willie Bobo were heard back to back. Purim and her group, augmented by percussionists borrowed from Caldera and Chicage, turned "Five Hundred Miles High" into a fiendishly rhythmic orgy. Bobo's spirited set was weakened by the addition of a mediocre soul singer, Errol Knowles.

Bill Cosby, a discreet emcee, allowed himself only two indulgences: He sat in on percussion with Bobo, and went through the motions of a vibes duet with Lionel Hampton.

The Hampton orchestra, mainly local pros assembled for the occasion, cruised through simple blues charts that were, for them, a piece of cake. Most of the excitement was generated by Hamp's unique presence.

The festival ended lamely with one of those meaningless all-star jam sessions. Men like Dizzy Gillespie and Gerry Mulligan were wasted on one or two tunes. Dexter Gordon lumbered awkwardly through "Body and Soul." Stan Getz came off smelling like a rose by choosing an unusual song, "No More," and playing it exquisitely. Jam-session concept finales should be banned at the federal level, but the impact of these two nights was so tremendous, the organization and general performance level of the festval was so high, that you could forgive it for ending with a whimper.