I had to learn a Grateful Dead tune for a wedding early this year. I found this lick while transcribing, and getting a feel for the tune. It's a very simple bluesy/jazzy, scale built of the Major Pentatonic. The b3rd is added (like a blue note), as well as the b7 & major7, to build chromaticism. It reminds me of a scale that Charlie Christian or Chuck Berry would use, though when you string it in a straight line like the Grateful Dead did, it sounds totally Bluegrass. Scale: 1, 2, b3, 3, 5, 6, b7, 7