My Blues Junior sound fine when plugged into a domestic mains socket but yesterday at an outside gig, there was a low frequency hum/crackle which the stage hand said came from the generator. When I started playing, I never noticed the hum at all but I remember at a previous gig, again outside, hearing this hum/crackle and thinking that my amp was faulty! I assume that the generator used in both cases was producing say, a square wave output, rather than the expected sinusoidal output from the domestic mains. I appreciate that the hum/crackle is caused by harmonics inherent in the square wave, but can they damage the amplifier components, like valves and speakers? For future outside gigs with power supplied from a non-mains source, should I be investing in a mains filter? Or should I not be bothered about all this?