Hey guys

Lately at rehearsals my ac15 get muddy at some points for no reason. It goes back to normal after a while and starts being muddy again when it feels like it.
I've read that it can be the pre-amp tubes getting loose in their sockets. It always goes to gigs with me in the car. Could this be the problem ? I have it for almost 6 months now and i play it only in rehearsals 2-3 time a week ( 2h each time ) and gigs.

Need some help pls