My amp hums like crazy- grounding issue? (non-mandolin-specific)

[SincereCorgi]

My new Vox AC15 hums very loudly and irritatingly when I plug in my Nash jazzmaster or Earnest tenorcaster (with humbuckers). The hum almost entirely vanishes as soon as I touch the strings or metal of the bridge. I was tucking a spare cable behind the bridge with one end stuck in my waistband to ameliorate the problem, but this seems like a ridiculous long-term solution. The amp doesn't do this when I plug in my venerable Korean strat clone or an Epiphone Casino.

I took my guitars into my local luthier to see if maybe some grounding wire had come loose, as I'm pretty novice about guitar electronics, and they assured me that this was normal. We could reproduce the problem on their shop amps, but it was considerably less pronounced. I was advised to stand farther from the amp and rotate; this did nothing.

What should I do? Live with it?

[MediumMando5722]

If it does it with some guitars but not others, the problem is in the guitars, not the amp. Despite what you were told, your buzzing guitars are not grounded properly. Single coil pickups, like in a Jazzmaster, will always have 60 cycle hum, but what you are describing is indicative of faulty grounds. Ideally, all the grounds are connected to a single point. Fixing it will not be difficult or expensive, provided you own a soldering iron and some decent shielded wire.

[multidon]

The fact that it goes away when you touch metal, that is the dead giveaway. You ground the guitar when you do that. There is a ground fault somewhere and just seeing if the ground wire is connected is not enough. I have a Les Paul that used to be like that. I took it to a Gibson Authorized Repair Center. They traced the ground fault and corrected it. A competent luthier should be able to do this. Additionally, I paid extra to have all the cavities lined with conductive foil. The guitar wiring is grounded to that, and the foil forms a Faraday cage that shields everything from outside interference. Guitar is whisper quiet now. It was the best money I ever spent.

[Steve Ostrander]

I took it to a Gibson Authorized Repair Center. They traced the ground fault and corrected it.

Ironic that your LP is fitted with "Humbucking" pickups....

[multidon]

Steve- true enough, but even humbuckers will make noise like crazy if they aren't grounded properly. Adding to the problem was that my Historic R8 has pickups that are not wax potted, just like they were made in the bad old days, so they were acting somewhat microphonically. Thus the decision to foil line all the cavities. Worked like a charm. But it must have been a real challenge to get a quiet signal in the 50's.

[pops1]

If you look in the cavity for the volume and tone controls look and see if there is a bare or any wire from the backs of all the pots and the ground of the jack. It should also go the the frame of the switch. If any one of these is missing or loose, it will cause your hum. Also look to see if there is a wire going from somewhere in that ground bunch of wires that goes underneath the bridge. It sounds like the bridge is grounded as your wire going to your waistband helped as it made you the ground, but if the bridge was grounded then just touching the strings should do the same thing and stop the hum. All the pickups should also have a wire going to ground too.

[MediumMando5722]

It's possible you have a cold solder joint. A good starting point with this if your knowledge is limited would be to touch a hot iron to all the ground solder joints. Lining the cavities with conductive foil (I use foil furnace tape on mine) would be a nice addition.

[pops1]

Chris has a good point. When you use the hot iron to all the soldier joints be sure to have the tip of the iron with some soldier on it to hasten the melting of the old soldier. I usually want a little ball of soldier on the tip before I soldier anything, it makes it faster and doesn't heat the surroundings up as much.

[David Lewis]

and, just in case I didn't miss it, flourescent lights off, computers off, tvs off, and microwave ovens off. That might just kill the hum too. (Although if it's only happening on some guitars, then everyone up above is pretty correct)

[Ivan Kelsall]

As Don said,if you touch the metal parts on your guitar & the hum goes away,there's possibly something wrong with the guitar wiring.
Here's an article which might throw some light on what's happening & maybe a possible solution :-
http://music.stackexchange.com/questions/7491/noise-while-not-touching-guitar-strings-or-metal-parts
Ivan

[allenhopkins]

Maybe it hums 'cause it doesn't know the words....?

[MediumMando5722]

If you're unsure of your soldering abilities, look up a video on basic soldering.

My money's on a cold solder joint. If you didn't know, that means only the solder was heated initially. Soldering properly involves hearing the parts being soldered, and melting the solder with the parts, not the iron. A cold joint will still look ok, and I'm betting that's why the tech said everything was good.

[mandroid]

Done a Ground check on the outlet you plugged into?

[SincereCorgi]

Done a Ground check on the outlet you plugged into?

I think this was it, embarrassingly. I didn't realize the outlet I was using was ungrounded, ran an extension cord to one I knew was grounded and it seemed to clear up the hum. Still, strange that it only showed up on some guitars. Thanks to everyone.

[pops1]

If your amp has a ground switch you should have been able to flip it on any two prong outlet and stop the hum if it was the outlet.

[vegas]

Maybe it hums 'cause it doesn't know the words....?

Not trying to make light of the issue, but that is pretty funny. :confused:

[mandroid]

Signal to noise ratio is a Thing the spec data may mention.. example: FWIW,

I look @ my Etek Note Mix PA amp users manual, it says "> 92dB"