There’s hype of global proportions surrounding the 50th anniversary of the Grateful Dead this year. But there’s another musical milestone closer to home. The band Dolly Varden is celebrating 20 years together. Sure, they haven’t been touring and recording non-stop for two decades. But 20 years is a long time for any musical endeavor to stay together, stay creative and stay friends. We hear some music and find out the secret to the band’s longevity as they prepare for a big anniversary concert May 2 at the Old Town School of Folk Music.