On Thursdays, starting at 19h - free entry. One can come and jam - beginner-friendly.
Website of Planet5 and organizers
of the jam (Resonance Jams)
Jams on Thursdays, starting at 21:30 - free entry. Usually first half-hour is a concert and then one can register and jam with others -
though the level expected seems to be higher than in Planet5.
On Tuesdays from 20h - first concert, then one can join the jamming session.
Website (see further events under "related")
On Wednesdays, every time new group plays, afterwards may be possible to join the jam.