When an orchestra feels good about its music director, it is bound to play better than when rancor riddles the ranks. And better playing was most noticeable at the New York Philharmonic last week , just after it had been announced that Zubin Mehta had renewed his contract as music director through 1990.
It put an end to the ridiculous speculation that he might leave New York to return to his old home, the Los Angeles Philharmonic. It evidently was just what the Philharmonic players wanted to hear.
Many of them had been dismayed when reports of in-house dissatisfaction with Mehta began circulating in the press this past summer. Evidently, they wanted everyone to know the orchestra is behind its music director. Unexpectedly, in Richard Strauss's technically demanding ''Sinfonia Domestica,'' the Philharmonic was playing carefully and attentively.
It was not just that Mr. Mehta was back in town, after having spent the opening weeks of the season with the Israel Philharmonic. In fact, Rafael Kubelik had opened the Philharmonic year in September and led the opening-weeks concerts. The orchestra had played in a routine fashion for him. And Mr. Kubelik is not a routine conductor; his reading of Beethoven's ''Eroica'' symphony was clear, precise, dramatic, stirring. His account of William Schuman's 10th Symphony (''American Muse'') was equally trenchant. (How nice to find a European conductor of Kubelik's esteem conducting American music.) Schuman's symphony is a pretty, affecting work, which Kubelik unfolded affectionately.
I first heard Mr. Mehta this season in a pension fund benefit concert that featured Mstislav Rostropovich. The opening work on that program had been Stravinsky's ''Petrushka'' and had been marked by particularly raucous, unruly playing. At a later concert, Debussy's ''Jeux,'' the playing all but fell apart for the technical indifference.