Unlike many advocates of American music who operate as if nothing of value was written here before 1950, except for the works of Ives, Richard Bales regularly explores and performs works from the whole range of American musical history. He is as much a fan of the voluptuousness of the mid-19th century as he is of the austere awkwardness of the 18th and the intricate techniques of the 20th.

For last night's American Music Festival concert at the National Gallery, Bales and the National Gallery Orchestra painted a romantic musical landscape. Charles Hommann's Overture No. 2 in D Major, written in 1840, set the stage. It is a piece rooted in traditional European soil, full of Old World charm and a certain amount of grandiose pretention. Bales tempered the posturing with a touch of lightness, and the orchestra, in fine form, played it delightfully.

Robert Evett, a Washington composer who died in 1975, contributed much to the Gallery concerts. A week ago the National Shrine Choir performed his choral set, "A Mask of Cain," and last night the orchestra played his Third Symphony. The orchestration here is thick and lush. Lines emerge from the texture and then recede. There is, throughout, a conscious nationalistic flavor to the shape of the tunes and to the harmonies, and a nice sense of proportion. The orchestra performed with considerable restraint and a fine sense of balance--something not always easy to achieve with the Gallery's problem acoustics.

Contralto Beverly Benso joined the orchestra after intermission for splendid performances of Shelley's "Ozymandias" and a marvelous cycle called "A Set of Jade" on old Chinese texts. The weight of Benso's voice never gets in the way of the movement or the shape of the vocal line. It is a lovely instrument and Benso uses it intelligently. In collaboration with Bales' colorful and skillful orchestration, she sounded marvelous.

The concert ended with a spritely setting by John Powell of three Virginia country dances.