Klaus Tennstedt was stricken with a throat ailment yesterday, and a replacement for him had to be found within a few hours for last night's Philadelphia Orchestra concert at the Kennedy Center. Fortunately, the orchestra's associate conductor, William Smith, was able to step in and conduct on short notice without any program changes. It was a rare opportunity for Washingtonians to hear this musician, who has been the associate conductor in Philadelphia for 34 years but has only conducted here once before (in Mahler's Seventh Symphony), at least in the past quarter-century.

On the evidence of this program, Smith is highly capable, as one would expect of anyone who had held his position for so long. The program, including Barber's Adagio for Strings, Prokofiev's Third Piano Concerto and Shostakovich's Fifth Symphony, was well-conducted without making any allowances for the circumstances.

The Barber, composed by a Philadelphian and ideally suited to the orchestra's magnificent string section, might have gone almost as well without a conductor; the players became something like a giant string quartet for this music (which was originally written for quartet), and produced a rich, luminous tone, which is virtually all the music needs to enchant an audience. But Smith surely deserves some credit for the ideally chosen tempo and some of the subtler pianissimo effects.

The concerto, with a brilliant solo by Soviet emigre' pianist Alexander Toradze, was considerably more of a challenge for the conductor. The balance between soloist and orchestra could occasionally have been improved, but the problems were barely worth mentioning. The coordination of orchestra and soloist was first-class throughout, and the orchestra sounded splendid in Prokofiev's glittering, powerful orchestration.

Much of the sound in this concerto is sketched in bold primary colors, with brilliance in the brass to match the thunder coming from the keyboard. But there are also moments of fine subtlety, and unearthly calm is sometimes invoked to contrast with the energetic, driving rhythms that provide the music's strongest impact. In the contrast between savage modernism and lyric romanticism (particularly notable in the theme-and-variations slow movement), Smith was a master of both idioms and the orchestra, which always plays superbly, seemed to be making an extra effort for its long-standing associate.

For Washingtonians, there would have been a special interest in Tennstedt's Shostakovich Fifth, which has almost become a theme song for the National Symphony under Mstislav Rostropovich. We can only wonder how it would have been handled by Tennstedt, who is almost completely identified with music in the Central European tradition. Smith handled it capably, directing an orchestra that is still far superior to the National Symphony, but he generated only a fraction of the tension and excitement found in NSO performances of this work. The slow movement, for example, often sounded rather like the Barber Adagio -- exquisitely played but perhaps ignoring the music's subtext. The last movement was exciting, as it has to be when played well, and the others were superbly performed but lacked a sense of deep involvement.

Two conclusions may be reached from this performance:

The Philadelphia Orchestra will rarely sacrifice beauty of tone for intensity of expression -- and, of course, seldom has to.

Part of the excitement in the NSO's performances seems to come from the sense that the orchestra is being pushed up to and beyond its limits -- something that probably never happens in Philadelphia.