Tube amps—Ya love ‘em, ya hate ‘em. The crackling sound you describe sounds like it’s coming from one of three probable sources. The preamp tubes—little glass vacuum bottles with tiny metal bits inside you beat the shit out of playing “Poundcake.” Switch them with our spares. (Repeat after me: “I must carry spare tubes and fuses…I must carry spare tubes and fuses…I must carry spare tubes and fuses…”) If this doesn’t work, it’s off to the warranty center...