The closing of Johnny D’s Uptown in Somerville left a big hole that needed to be filled: a place for blues musicians to jam on Sundays and support one another. That is the spirit The Porch fosters with its weekly People’s Jam. Bass amp and two guitar amps provided. DIs, vocal mics, horn mics provided. Just come on in and add your name to the signup sheet. Each group of jammers plays three songs and no one repeats until all the players have had a turn. Admission is free.