"Once Upon A Classic" almost becomes "Once Upon A Classic Dreary," while we ponder, weak and weary, the choices made in bringing Mark Twain's "A Connecticut Yankee in King Arthur's Court" to television. Previous entries in the "Classic" series on public TV were British-made; this one, at 7 tonight on Channel 26, was done by WQED in Pittsburgh.

A child could notic the difference. An adult won't be able to help noticing the difference.

Partly it is simply a matter of experience. The British are able to produce such programs quickly and competently, and they've m astered the technologies involved, so that they can mix film and tape within the same program and still keep things flowing.

The outdoor shots in "Yankee" are strikingly sunny and handsome, but the jar with the compartively clunky stuff done in the studio, and director David Tapper can hardly be said to have minimized the incongruities.

In abridging Twain for TV, writer Stephen Dick starts right off with the bop on the head that sends our hero back in time; no character is established for the fellow in the opening scenes, so that the bop seems arbitrary and the story that follows coy. Perhaps Dick thought the attention of the young audience would wander from any sort of introductory exposition. He has underestimated his audience.

Paul Rudd still manages to make something of the character of Hank Morgan, the inventive Yankee with the good memory for ancient eclispes, although Rudd's curly-top haircut makes him look like Cecil Bill, a perpetually startled Kuklapolitan whose vocabulary consisted entirely of variations on the sounds "doo" and "dee."

Tovah Feldshuh, who played the young Czech woman married by Joseph Bottoms in "Holocaust," certainly has a warming influence in her Lady Sandy role, but the script doesn't give her much to do besides smiles wistfully.

"Yankee" is not Twain's best work, and children accustomed to science fiction characters who meander through time as nonchalantly as if driving to the supermarket, will not be impressed with the frailty of this conceit - a bop on the head, indeed. The hour is by no means a waste,since there are intervals of consideration charm, but it's all a tad too cuted up. The passage of days, for instance, is represented with "headlines" from a Camelot "newspaper."