Lionel Hampton, the first genius of the vibraphone, genial swingmaster and Republican activist, frowned at the way the drummer of the Howard University Jazz Ensemble was keeping time.

"I want to show you a different way to play that," said the master to Robert Taylor, who reluctantly gave up his seat.

Hampton, who first gained fame as a member of the Benny Goodman aggregation in the 1930s, counted off "Moment's Notice" and demonstrated how the passage should be performed. He played a bit stiffly but rolled and accented in the right spots.

He was rehearsing the ensemble for a benefit concert he will perform tonight at 8 at Cramton Auditorium for the D.C. chapter of the Howard Alumni Association. (For ticket information, call 686-6693.)

Looking more like an investment hanker in his three-piece suit than a jazz musician who has set theaters and dance halls afire with rhythm. Hampton next asked the 19-piece band to breeze through another section of the piece.

This time after the saxophones skipped through a passage, trumpeter Wallace Roney, a freshman student who turned down scholarship offers from the Eastman School of Music and the New England Conservatory to attend Howard, served up a crackling solo.

Hampton turned quickly to face Roney, flahsed a toothy grin and applauded when the young man finished. Later, he said, "He's (Roney) in there. He knows what's happening. And that girl (Gerri Allen) playing piano is great, too. I played with her last year."

Earlier, he had shown Allen some chords to play behind his solo. "I like the way she picks up things real quickly," Hampton glowed. "She's got a great future."

Hampton will bring his own group in for the concert. Howard music professor Kirk Stuart is writing an extended blues pieces for the vibraphonist. The last time Hampton performed in Washington was at the White House Jazz Festival in June. That was when he tried to shoo Pearl Bailey off the bandstand as she took the microphone to sing "In the Good Old Summertime" while the Hampton-led group played "Flyin' Home."

"I really didn't know who she was," laughed the musician. "She got up there singing all that old-time stuff just about the time we were really getting into some good playing. She was singing, something different from what we were playing. We couldn't even follow her."

He sure followed the Howard band. Hampton bent over the vibes as the group swung into "Groovin' Hard," rolled his tongue over his lips, smiled and pelted the keys. His melodies still dance as gracefully as they did a generation ago.

"We hope to have an outstanding event," he said. "My hand is going to play most of the concert. But I'll play a couple of numbers with the Howard band and they'll play a couple by themselves.

"I'm glad to see Howard exploit its resources. They have a terrific music department. I talked with President (James) Cheek and he said he wanted to keep these services going. This is something we didn't have in the '40s and '50s.

"Jazz used to be spelled 'jass.' Whites called it 'jackass' music. But we're getting some respect for the music now. We still don't have enough."