generator hum coming through Fender amp

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?


  • ESBlondeESBlonde Posts: 961Member
    If the voltage is within acceptable tolorance there will be no problems. Generators tend not to have a classic ground in the domestic sense of the word.
    The Big gen sets used at festivals are fine, but typically the little petrol jobbies used for camping etc. will have plenty of unregulated current and unless someone attaches a separate earth peg there won't be one. Also unregulated power supplies can confuse digital devices whereas a valve amp or any analogue device won't mind at all.
    People like Furman make a selection of rack devices that will clean and condition power from gennys and if you do much of this kind of thing are worth getting for the modern delicate electronics.

    Heres one that conditions power for a reasonable cost

    I've got a phonic PPC8000 which does it for me.
  • DaveBassDaveBass Posts: 3,315Member
    You can get petrol generators which produce a regulated sine wave output, in fact I have an earlier version of this one:

    If money's no object, you could buy your own generator to power your amp.
  • ESBlondeESBlonde Posts: 961Member
    You are right @DaveBass but be very very very careful having multiple power sources on stage, the remote danger of you being between a 240v and another 240v supply are not worth contemplating, not least because it will fry the electronics too........
    In professional circles it's just not allowed for this reason.

    Also small gennys will struggle over time because of bass pulses in the PA or Backline gear. All that repetitive near max current draw is not a good thing. You can do a quite garden party once in a while, but for anything serious you need to oversize by a factor of at least 3 (the more the merrier), Who said size isn't important?

    Thats not a dig at you Dave, just a general heads up for people that might search afterwards and draw the wrong conclusions. I worry about bad advice on outdoor power. I know a little but will defer to properly qualified sparkies everytime.

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