John Harbison's "Deep Potomac Bells," a piece of music commissioned to commemorate the 350th anniversary of the founding of Maryland, had its world premiere yesterday afternoon on the east steps of the Capitol. It will be performed again tonight in the Tawes Theatre at the University of Maryland, but after that, chances to hear it will be rare--except when 200 or more tuba players are gathered together. That happens only once a year, at the International Tuba and Euphonium Conference, which will have its gala closing concert tonight on the College Park campus.

Harbison is a major composer (one of America's best), but "Deep Potomac Bells" is hardly a major work. The most interesting thing about it is that Harbison--usually noted for exuberance of imagination, variety of forms and depth of expression--is taking a step toward the minimalism of such composers as Steve Reich and Philip Glass. On one level, "Deep Potomac Bells" is a a study in ostinato rhythms and the almost obsessive repetition of very short phrases in the Reich-Glass manner. On another level (a very deep one, naturally, given the instrumentation), it explores unusual sound-textures, which can be supplied abundantly by the tuba and euphonium (a higher-pitched relative of the tuba). Some of the material is drawn from the Maryland state song, but it is chopped up into motifs too small for easy recognition.

Yesterday's performance sounded more like a final rehearsal--partly, perhaps, because of the outdoor atmosphere of the Capitol during rush hour, with cars driving past the players, tourists wandering about and policemen with crackling intercoms directing traffic and controlling the crowd. The players were considerably more at home in such familiar items as the "Pilgrims' Chorus" from Wagner's "Tannha user," and tonight's performance should be tighter.