‘THE MAKE-BELIEVE BALLROOM,” the radio program on WNEW-AM that has been a bellwether of mainstream, traditional pop music for half a century, from its beginnings with Martin Block to the curatorship of the late William B. Williams, may be in for a change of emphasis under its new host, Steve Allen. Not that Mr. Allen’s taste in music is at variance with the program’s continuing style – the Glenn Miller music that was at the heart of Martin Block’s programming or William B. Williams’s devotion to ”the chairman of the board,” Frank Sinatra. But as Mr. Allen remarked a couple of days ago, ”I’m not a D.J. – I’m an entertainer.”

”Since I took over the program in January,” said Mr. Allen, who was the original host of the ”Tonight” show on television in the 1950’s, ”The show has been evolving. It was not consciously planned. I came into a situation in which the station’s musical taste agreed with mine. It was a program with a minimum – actually zero – of garbage.”

For his first day as host, Mr. Allen put together four hours totally devoted to the music of Jerome Kern.

”I spent a week picking out the greatest recordings of Kern’s best compositions,” Mr. Allen said. ”But as the days went by, the percentage of music on the program has dwindled while there’s been an increase in comedy, which has always been part of my work as an entertainer.”

Now, starting his fourth month, Mr. Allen sees ”The Make-Believe Ballroom” as 70 percent comedy and 30 percent music. The shift in emphasis is partly due to his own inclinations as a performer and partly to the presence on the program of Mark Simone, whom Mr. Allen called ”my Ed McMahon.”

The program is on the air from 10 A.M. to 2 P.M., Monday to Friday, and alternates between New York and Los Angeles. This week and next, Mr. Allen is in New York. Then for two or three weeks he moves to Los Angeles, where he lives. When Mr. Allen is in Los Angeles, Mr. Simone does the first two hours – 10 A.M. to noon – in New York. He picks out the records and it’s his show. At noon, Mr. Allen joins in from Los Angeles and there is banter back and forth, interspersed with records that Mr. Simone has programmed.

When Mr. Allen is in New York, he is on the air all four hours and he picks the records, but Mr. Simone is still on hand as a foil and a prod.

”This show has the hippest, funniest listeners in the world,” Mr. Allen said. ”They write brilliantly funny things in the letters and, of course, we use them on the air.”