Sporting the wildest mutton chop sideburns since Ambrose Burnside, flat picker Larry Keel plays bluegrass Americana that exhibits the kind of technical mastery of acoustic guitar only time can provide. Born in the Blue Ridge Mountains of Virginia, the veteran player and songwriter — alongside his band, Natural Bridge — has remained a much-respected fixture of the Appalachian musical tradition. With the kind of gruff vocal delivery that could take Pearl Jam’s Eddie Vedder by surprise, Larry Keel and Natural Bridge perform at Capital Ale House on Friday, at 10 p.m. $10. 643-2537.