The Cantab Lounge in Mass Ave, Cambridge was my regular port of call in Boston.

They have a good open mike on Monday nights hosted by Geoff Bartley (which clashes with your oldtime jam unfortunately).

They also have a really good bluegrass jam on Tuesday nights, as I remember. Strangely enough they were always looking for a guitar player, but there is (or was) a phenomenally good mandolin player called Adam who used to join in each week.