Earth hum from a J bass...ideas?

[NickT]

I have recently installed a pair of DiMarzio Ultra Js and they are giving off an earth hum. The hum has occured so far in three places, in my dining room and my living room (it stopped when I turned the lights off) and in tech's tech's workshop. I took the bass on a gig (as a spare) and in a large function room with lots of lights it was whisper quiet. The hum also stops when I hold the bridge or machine heads.

 

I'm assuming it's some form of earthing issue.

 

I installed the pickups myself, but the bass has been set up and the electronics have been checked since then, so all the earths seem to be ok. The problem is a little better, now that one of the volumes has been swapped for a 500k pot (the other will be swapped next week due to stock problems).

 

The tech reckons it's because the pick ups themselves aren't shielded internally and that the wires to the pots aren't shielded as well as they might be.

 

Does anyone else have any ideas about what the problem might be, or a possible solution. Short of identifying the offending type of light bulb and making sure that there aren't any on in the room. I will be taking the bass to another tech shortly for a second opinion.

 

When this buzz isn't happening the pickups sound great.

[g.]

My Ultra 5 in bridge position is not totally quiet (the 5 is a special issue perhaps since it most likely has three stings on one coil, and two on the other and they may not be exact mirror images electrically), but it is much quieter than single coils, less succeptible to external influences. My computer monitor, and close proximity facing toward my amp are the only times it is even noticable.

 

If mine was a problem FOR ME I would (1) shield the routed cavity it sits in, (2) shield the cable run into the fantastically shielded Carvin control cavity.

 

Sorry I can't be of more help in your instance.

[NickT]

Maybe I should have said that whilst replacing the pickups I sheilded the control cavity.

[pumpcat]

Originally posted by NickT:

The hum also stops when I hold the bridge or machine heads.

Does the hum stop when you touch the strings? If so, then there is nothing wrong. Nearly all electric guitars/basses exhibit this behavior. The ground (earth) is usually connected to the bridge, which obviously connects with the strings and tuning pegs. When you touch any of these, your body completes the circuit to ground and drains away the hum.

 

check out these threads:

 

http://www.musicplayer.com/cgi-bin/ultimatebb.cgi?ubb=get_topic;f=1;t=014629

 

http://www.musicplayer.com/cgi-bin/ultimatebb.cgi?ubb=get_topic;f=5;t=002643

[NickT]

It has been suggested to me that the problem may be related to dimmer switches.

[Ajax]

NickT wrote

It has been suggested to me that the problem may be related to dimmer switches.

I have the DiMarzio P/J set on my bass, and have expierienced a hum when close to a dimmer. For example, our side stage light bars have a dimmer attached, and I'm often wearing my bass when I adjust it. When I get within 4-5 feet, the hum begins. When I back off, the hum abates.

So... STAY AWAY FROM THE LIGHT! DO NOT GO INTO THE LIGHT!!!

[g.]

NickT, if you think you need to shield the wires coming out of the pickup, you can just take a piece of cavity foil and wrap it around the two wires, and another thin uninsulated wire that is long enough to ground at the cavity end (and at the routed pickup hole if that is already shielded). I've never found it neccesary.

 

I have to think something else is amiss here.

 

Note to those discussing the issue: these Ultra Jazz pickups are not the same as TRUE single coils though they have the same outer form factor. They are quieter at idle and if everything else is wired right reject most noise almost as well as fully-shielded side-by-side humbuckers. But with CRTs, the inverse square law still applies, as time goes by.