I’m a skeptic when it comes to period-instrument performances, especially when they indiscriminately subject works by Mozart, Beethoven, and other post-Baroque masters to the treatment. I admit curiosity might get me to listen–once–to late-18th-century music performed as it might have been on instruments of the period (or reasonable replicas). And it’s true that some such performances have shed light on orchestral practices and limitations faced by forward-looking composers. But Mozart and Beethoven, to name two, were constantly on the lookout for louder and more versatile instruments in their experiments with denser and fuller orchestral sound. The snobbism that equates scholarship and “authenticity” with quality is just what has salvaged the careers of opportunist hacks like Roger Norrington. What makes period-instrument performance good is the same as what makes any performance good: musicality, personality, conviction, and coherence. These traits have been demonstrated consistently by the local period-instrument ensemble Basically Bach in its Baroque concerts. And for its Mozart bicentennial salute, conductor Daneil Robinson has prudently chosen lightly scored early works–Piano Concerto no. 9, Divertumento K. 251, and Symphony no. 29–not the great symphonies and concerti. The concerto soloist is Penelope Crawford, renowned for her delicate touch; she will perform on a fortepiano copied from a late 18th-century model. Robinson and his crew may yet make a convert of me. Tonight, 8 PM, Scottish Rite Cathedral, 935 N. Dearborn. Saturday, 8 PM, First Congregational Church, 1445 Hinman, Evanston. Sunday, 7 PM, First United Church of Oak Park, 848 Lake, Oak Park; 334-2800.