In December 1979, in a benefit concert at the Alvin Theatre, about a dozen Broadway stars of the past and present strode to the microphone to sing some of the songs they made famous. John Raitt, Alan Jones, Jack Gilford, Michael Moriarty, Delores Wilson and others received waves of enthusiastic applause from the packed house. But when a short, stocky, barrel-chested man with thick eyeglasses and a nose like Jimmy Durante's shuffled to center stage, the audience didn't merely cheer: it erupted. And when 75-year-old Jan Peerce finished his two arias, he was prevailed upon to give the only encore of the evening. Appropriately enough, his choice was "If I Were a Rich Man" from _Fiddler on the Roof_, the show in which he made his Broadway debut at the age of 67. Although Peerce has been one of America's most beloved singers for almost half a century, it was not for sentimental reasons alone that he was treated with such acclaim that evening. He still has one of the clearest, strongest, sweetest tenor voices in the business, and his repertoire is enormous. Besides arias and showtunes, he performs ballads, German lieder, French contemporary songs, cantorial and oratory music with equal facility. In order to keep his voice in top form, he now limits his concerts to about 50 a year, but last summer, on a tour of Australia, he did 17 concerts in 21 days. "I vocalize every day of my life, I keep observing the laws of decent living, and I face every booking as it was my first," he says in a recent telephone interview, contacted at his Westside apartment. "I believe in the adage that the show must go on, but you must not go out at the expense of your health, or impair the quality of your voice by singing against nature." This fall will find him doing a one-man show at Carnegie Hall. In addition to his regular schedule of cross-country concerts, he makes cruises of the Caribbean several times each year aboard the SS Rotterdam. His parents were Orthodox Jews who had immigrated from Russia, and they were able to afford violin lessons for him by taking in lodgers at the Lower East Side apartment where he grew up. Born under the name Jacob Pincus Perelmuth, he began his career working primarily as a violinist and bandleader in the Catskills. In 1929 he married his childhood sweetheart, Alice Kalmanowitz, and three years later was discovered by the great showman Samuel "Roxy" Rothafel, who hired him as a featured singer at the new Radio City Music Hall. "People on Broadway said I belonged in opera," recalls Peerce, "and opera people said I belonged on Broadway. But when Roxy gave me my break, things began to happen. And then came Toscanini. He hired me to sing with his NBC Symphony of the Air. And when he accepted me, that sort of clinched things. People said, "If he's good enough for Toscanini, this guy must be good.'"