At a jazz concert, impromptu musical collaborations don't take place but at informal sessions they do.
At a folk club, impromptu musical collaborations don't generally take place but at informal sessions they do.
I write "generally" for folk clubs though I can remember quite a number of occasions where they have done. For example, at our folk club we presented the first ever performance outside America of a remarkable multi-instrumentalist called Dave Ruch and he was joined on some numbers by a local guitarist called Mike Ainscough.