If you know the Boston roots rock and folk scene, you surely know Ry Cavanaugh. And if you don’t, it’s not too late. He’s one of the mainstays of the heralded Session Americana collective/band that’ll be playing at the Windham Ballroom on Thursday, April 9. We had him on the phone for a bit. Session Americana has been described as a rock band in a tea cup or a folk band in a whiskey bottle. It doesn't strike me as an either/or in this case, and that’s not a bad thing. Their newest CD, "Pack Up The Circus," didn't officially come out until the day after our conversation, but we played some tracks anyway.