Downtown dinner theaters have failed for years, from F and G streets to Dupont Circle, but 23-year-old John L. Figel is convinced that K street NW, between 13th and 14th, is the place for success in '78.

With 27-year-old Nathan Hull as his partner-director, producer Figel's Encore! Dinner Theater will open Friday night in the Almas Shrine ballroom with "Fiddler on the Roof." Tony DiDio, who recently acted "The Merchant of Venice" at the Hartke, will have the lead of Tevye.

The tables will seat 200, with the stage on the wider side of the big room. The Shriners' chef will do the catering. Performances will be Wednesday through Saturday nights, with dinner at 6, curtain at 8. The $15 food-and-show ticket will include parking.

A general rule for dinner theater success has been to stake out space near a thruway accessible to thousands of potential driving customer. In some cities in town dinner theater places have worked out but, so far, not here.

Last winter, while both were working on Adventure Theater's "Oklahoma!" in Glen Echo Park, Figel met Hull, a computer expert who had studied at Catholic University's speech and drama department. They heard that the Shriners wanted night-time action in Almas Temple.

What inspires the faith that Franklin Park can break the downtown jinx?

"Firstly, we'll have good productions," promises Figel. "Nathan, who has acted and directed, is staging 'Fiddler." You'd be amazed at the quality of performers who turned out for our auditions.

"Secondly, our location is working for us. We're within walking distance of a score of hotels. Not just tourists come to Washington, you know. There are business people who come here for a night or two. And, working through the hotels and travel agencies, we're in contract with school groups.

"Above all, we're looking at the many people who are coming back to live in town, midtown. The papers have stories about them all the time and thousands of these returnees can easily walk to us. Economically, we're a bargain. Our hours mean that office workers won't have to go home and then out again."

How Figel's theory works out in hard practice, this summer will tell. With a taped 13-piece orchestra supplying accompaniment, the cast of 30 expects to be doing "Fiddler" through the summer. Income for most of these non-Equity performers will come from tips earned waiting on the tables.

To allow time for getting the kinks out of a new operation, the press is not being invited until June 24, though performances start Friday, June 16.