Ludwig van Beethoven sat on the piano - a plaster bust of him, slightly smaller than life-size - looking quizzical, perhaps because the five-piece band was playing "S Wonderful."

Except for Gershwin (and Richard Rodgers and other modern Americans in the band's repertoire), the scene could have been right out of Old Vienna. The tables in the high-ceilinged, wood-paneled room were lit with candles, occupied by black-tied men and women in masks and evening dresses, with waiters slipping quietly from table to table bearing hors d'oeuvres and refilling the champagne glasses.

Washington's beau monde was well represented, with such luminaries as Charles the First, Francis the Second and a real Russian nobleman in his formal array with a large cape and a small dagger. It was the Fasching-Redoute sponsored by the Beethoven Society of Washington, and for most of the evening the people present must have thought they were there simply to have a good time.

Fasching, in German-speaking countries, is the annual event we usually call "Mardi Gras" in English. A Redoute is a ball at which the women wear masks. Many of the masks, Friday night in the Chevy Chase Circle home of Dr. John G. Threlfall, were decorated with feathers, lace or sequins.Some were carried like lollipops on the end of long sticks - notably one wronght in copper by sculptor Richard Conroy.

Diana McLellan, Washington reporter, wore a red velvet mask surmounted by a large, ornate, beaded tiara of oriental design that almost totally concealed her Ear. It was, she said, taken by mysterious means from the Forbidden City of Peking toward the end of the Manchu Dynasty. Two long, Chinese-red tassels hung from its sides.

Two years ago, the Beethoven Society was Mrs. Clarence Milton Fisher and a few friends, who all call her Maria. Then, as friends of friends began to come in and other heard of what was happening and asked to join, it grew. Now, the Beethoven Society is Maria Fisher and a mailing list of 350 names: "I have no committees; nothing, I do everything myself from soup to nuts. I address the envelopes and lick the stamps. Anyway, I'm having fun, and so are my friends."

The Beethoven Society, in a word, has evolved into a group of people who enjoy what Maria Fisher enjoys doing. That includes about half a dozen activities per year - dinner dances and concerts, competitions for young musicians, an annual Beethoven's Birthday Party and performances of Handel's "Messiah" at the National Presbyterian Church. Some of the people in the Threlfalls' large, handsome ballroom Friday night were there simply because they like ballroom dancing (which was available in abundance, and which is hard to come by in the age of disco and rock), some because they enjoy good parties, and this was an excellent tone. Most were there because Maria Fisher has a new bee in her bonnet and plans to exploit them.

I've invited some friends that we can squeeze," she admitted privately before making her big announcement. They seemed for the most part affluent and therefore eminently squeezable, and since Maria Fisher is notorious for her way of taking from the rich to give to the poor, they should have known that something of the sort was coming up. "When they see me coming," she admits, "they know I want something."

What she wanted this time was a subsidy for a series of pops concerts she plans to offer on Sunday afternoons next season in a downtown hotel. "I want them to be inexpensive and available to whole families; we will have a separate room with baby-sitters for the very small children and a lounge where the fathers can smoke their cigars and watch the football games on television if they prefer. I don't want it to cost any more than taking the family to the movies, and I'm going to need contributions to make this possible."

She wants this for a number of reasons. The Beethoven Society has had several music competitions for singers and instrumentalists in their teens and the winners have been aided with scholarship money. Now they need the experience and exposure of public performance, and these concerts, can give them that kind of opportunity.

But she also wants to enlarge the audience for classical music in Washington, and for that she enrolled an experience collaborator - educator and conductor Richard Weilenmann. As music director of the Washington Civic Opera, Weilenmann already has an impressive track record in the tricky work of finding new audiences for the classics, and he is now getting ready to put together a Washington orchestra that will be a counterpart of Arthur Fiedler's in Boston.

Notable features of the new project, he expects, will be "a relaxed atmosphere," an audience of people who can't normally afford to go to concerts at the Kennedy Centre long intermissions to let people socialize and give the events more of a party atmosphere, and an air of informality, with children allowed to sit on the floor up near the orchestra if they want to.

"We are not going to provide music for the connoisseur," he expects, "not that we're going to be sloppy, but our music will be geared for audience enjoyment - for people who are beginning to like classical music.

And if the indomitable Maria Fisher has her way, a lot of people in Washington may be doing that before long.