loud hum in amp--like ground hum(?)

Discussion in 'Amps/Cabs Tech Corner: Amplifier, Cab & Speakers' started by Paul S, Oct 19, 2004.


  1. Paul S

    Paul S Member

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    Hi,

    just fired up an amp that only gets used every few weeks (a Carvin Belair--2 12", 4-el84's--nice amp, if not boutique). It sounds good, except for a pretty audible hum. It almost sounds like a guitar w/ single coils would hum--except it was humbuckers. I tried different patch cords, different humbucker guitars, and even switching tubes around--nothing did any good. The hum would get louder or softer depending on which way the guitar was turned--again, like a single coil guitar would.

    I'm wondering what to try next? it soulds like a simple problem, just not sure what to try now. Thanks for any help!

    Paul
     
  2. Sir Ricardo

    Sir Ricardo Member

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    3 ideas:

    1) hook up a mic / keyboard etc., see if the hum continues.

    2) does it hum with no cord?

    3) does it hum with the cord, but no instrument?

    Cheers,

    Richard
     
  3. Paul S

    Paul S Member

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    Hi, Richard

    no mic or keyboard to try it with, but I did use 3 different humbucker guitars, and 3 different cords, none of which hummed before, or in another amp plugged into the same wall socket as the amp in question. It hums with cord and no guitar, and hums slightly with no cord.

    Thanks,

    Paul
     
  4. Sir Ricardo

    Sir Ricardo Member

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    Hello Paul:

    Well, here are a few more ideas.

    It sounds like the amp hums even without a cord in it, and until recently was working fine... so...


    With no cord in it...

    why don't you turn the volume up quite high, to really bring out the hum. That way, if any of these following ideas do anything, hopefully the lack of hum/increase in hum should be very obvious.

    And then......

    1) What happens when you turn the reverb off via the footswitch? Contrast the hum with the reverb on 10, then, with the reverb cut off via the footswitch.

    2) What happens when the reverb is on, and you turn the reverb all the way up, then all the way down?

    3) what happens when you switch the "ground" switch on the amp?

    4) What happens when you plug it into a different outlet (grounded outlet) in another room?

    5) Are you using an extension cord for the amp? If so, try it without it.

    6) In general, assuming the volume is all the way up, are there any combination of knobs & controls that either make the hum considerably worse, or considerably better?

    Let us know, Paul, maybe one of these, or something you come across, really boosts/cuts the hum. Then, it will be easier to zero in the underlying problem.

    Cheers,

    Richard
     
  5. drbob1

    drbob1 Silver Supporting Member

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    All that said, if an amp suddenly starts to hum when it didn't before there's just a few things that need checking:

    1. Pots-if it crackles when you twist them, clean with pot cleaner
    2. Jacks-spray pot cleaner on a 1/4" end and work in and out a few times (not on the speaker jack, but that won't cause hum anyway) on each pot, especially if it has an effects loop.
    3. Filter caps-when these die, you get the loud 110 or 220 cycle hum, simple fix for your tech...
     
  6. Kappy

    Kappy Member

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    So is it safe to say that lots of hum happens when you plug an amp into an ungrounded outlet in the wall? I have a house with an upgraded circuit, but very few upgraded outlets/wiring. I'm noticing insane hum from my SC guitars since moving here. I guess I just need to try it in one of the known grounded sockets to find out for sure.

    Also, is there a safe way to use an ungrounded wall outlet (i.e. something I can put in between, like a conditioner)?

    Thanks for any ideas.

    Dave
     
  7. slider313

    slider313 Silver Supporting Member

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    When you say,"switching tubes around" did you try new preamp or power tubes?
     
  8. Curt

    Curt Member

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    110 or 220 cycle hum? I think you mean 60 cycles (in the US).
     

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