Barry Manilow's Nissan Pavilion concert Friday reflected both the ambition and the self-deprecating humor that have long endeared him to his fans. This particular tour finds Manilow fronting a 30-piece orchestra, which turns out to be a good thing, as well as a bad thing: good in the sense that some songs benefited from added coloration and expansiveness; bad in the sense that other selections were undermined for the same exact reason.

For instance, "Tryin' to Get the Feeling Again," "Weekend in New England," "Mandy" and "When October Goes" (a Johnny Mercer lyric to which Manilow added music) are all built on simple, elegant melodies that suggest musical theater rather than pop, and their charms were most apparent when Manilow sketched them out at the piano with minimal support. Later, when the orchestra piped in, it almost always overwhelmed the material, Las Vegas showmanship trumping Tin Pan Alley craft.

Manilow has actually written a new musical, "Harmony," about a vocal ensemble in pre-Nazi Germany (it's headed for Broadway next year), and one could easily construct a convincing theatrical revue around such songs as "Even Now," which pledges devotion both despite and because of difficult circumstances; wistful ballads like "When October Goes" and "This One's for You"; the life- and love-affirming "Forever and a Day"; and even the jocular, rhythmically spry "Copacabana," which set Manilow's hips to swiveling . . . sort of. "Eat your heart out, Ricky Martin!" Manilow gushed at song's end.

Manilow, a gregarious entertainer, is not above poking fun at himself and his image. For instance, he did a surprisingly funny routine built around his first instrument, the accordion, including idiomatic polka versions of Madonna's "Like a Virgin" and Martin's "Livin' la Vida Loca," and then whipped out a kazoo for a spirited "Flight of the Bumblebee," complete with bug-spray solo.

Manilow also plucked out an audience volunteer--Amy from Gaithersburg--for a wonderfully sloppy, hopelessly off-key duet on "I Can't Smile Without You," a song whose sentiment is wrapped around a ridiculously simple sing-along melody.

Not all of the concert was as infectious. Manilow's a fan of the big-band era but he wasn't particularly convincing on hard-charging, rhythmically driven songs like "Bandstand Boogie" and "Singing With the Big Bands," much less the sophisticated swing of Frank Sinatra. The singer's latest album is "Manilow Sings Sinatra" (or at least songs made famous by Sinatra), and while the sentiment itself is unimpeachable and Friday's extended homage well intentioned, the results were strangely uninspired. This was particularly true on the Sinatra-Sammy Cahn-Jimmy Van Heusen chestnut "All the Way," which Manilow sang in Sinatra's sweet slight voice from the '40s, not the weary, wounded '60s voice of the singer burdened but unbowed by romantic experience. As respectful as this turn was, Manilow does better with his own material sung his way.

CAPTION: Manilow fronted a 30-piece orchestra, which worked better on some songs than on others.