Is String Grounding "Safe"?

[Wasabi1264]

Hey, folks...

I just got an American Jazz Bass 75 Reissue on eBay, and I love it. The only problem aside from a few cosmetic things is that there is a ground buzz (not 60 cycle hum) that only goes away when I touch the control plate. It doesn't go away when I touch the strings.

The stock bridge had been replaced by the previous owner to a Hipshot A. My theory is that something wiring-wise was messed up when the bridge was replaced, and the repair guy I just talked to, who is very recommended agreed with that.

He said he will make sure to do a "string ground" on the bass. I had read somewhere that this may be unsafe, especially if there is a polarity issue.

Was there originally a string ground on the bass, and is this a safe thing to do? I currently do get a slight shock when hitting the mic with my lip, and that happens with all my basses, so I know there is something polarity-wise going on in our system.

Thanks!

 

[Trevorus]

It is generally safe with a proper electrical system. No matter what, I would get that shock thing figured out! Check for bad/improper grounds!

 

[202dy]

Hey, folks...

I just got an American Jazz Bass 75 Reissue on eBay, and I love it. The only problem aside from a few cosmetic things is that there is a ground buzz (not 60 cycle hum) that only goes away when I touch the control plate. It doesn't go away when I touch the strings.

The stock bridge had been replaced by the previous owner to a Hipshot A. My theory is that something wiring-wise was messed up when the bridge was replaced, and the repair guy I just talked to, who is very recommended agreed with that.

He said he will make sure to do a "string ground" on the bass. I had read somewhere that this may be unsafe, especially if there is a polarity issue.

Was there originally a string ground on the bass, and is this a safe thing to do? I currently do get a slight shock when hitting the mic with my lip, and that happens with all my basses, so I know there is something polarity-wise going on in our system.

Thanks!

Yes, the strings were originally grounded on your bass. For what ever reason someone removed the ground wire or it is no longer making good contact with the bridge. The latter is probably the reason. Most of the time you can just pull the bridge, reposition the wire and replace the bridge. It's also a good idea to rough up the bottom of the bridge with some 220 grit to make a better contact between the wire and bridge plate.

String grounds are generally safe. When you get shocked on the lip the problem is that the PA system and your amp do not share the same ground. Going direct into the PA will solve this problem. Failing that, make sure that your amp shares the same ground with the sound reinforcement.

If you are really concerned about getting nailed there are a couple of solutions. Remove the string ground will ensure that nothing gets to you. Most folks find that the hum is objectionable. Another way is to use the safety feature that Adrian Legg suggested in his book. A 220K ohm resistor and a .001 capacitor with a minimum voltage rating of 500 volts are wired in parallel between the strings and the ground. It is supposed to limit the shock to about 40 volts but isolates the strings only. The knobs, control plate, and anything else that is connected to the ground circuit will still get you.

 

[Wasabi1264]

That is very helpful. The shock mystery is a difficult one. I have gone direct before using a battery powered SansAmp Bass DI, and still had the problem, with all of my basses. It's not a terrible shock, but annoying, and it makes me worry. Could there be some kind of flaw in my personal mic that might be causing it?

Anyway, glad to know that there originally was a string ground. The repair guy doesn't seem too concerned about doing it, and did mention using the resistor, etc, but probably we won't need to go there.

 

[202dy]

That is very helpful. The shock mystery is a difficult one. I have gone direct before using a battery powered SansAmp Bass DI, and still had the problem, with all of my basses. It's not a terrible shock, but annoying, and it makes me worry. Could there be some kind of flaw in my personal mic that might be causing it?

Anyway, glad to know that there originally was a string ground. The repair guy doesn't seem too concerned about doing it, and did mention using the resistor, etc, but probably we won't need to go there.

When you have gone direct, did you Y out of your DI to a small amp that you used as a personal monitor?

 

[Wasabi1264]

Nope. We have a distributed Aviom system, so I use that with a pair of in-ears. I'd just go direct into the board. I finally started using an amp again because our PA wasn't delivering what we needed from the kick drum and bass alone. I use the DI off the amp into the board at this point. Again, I wonder if maybe my mic is screwed up...

 

[202dy]

It is possible that you have a reversed polarity in your mic. It is also possible that you have a cable that is wired out of whack. Continuity can be checked with a multimeter. Or the problem may be at your Sans Amp. There is a polarity switch on that unit. Try it.

BTW, the way to check if you still have a problem at the mic is to bring your bass up to the mic and let your stings touch the windscreen. If there is a polarity mismatch you will see a small spark jump across the gap as you near the target. This method saves your front teeth on a bad night.

 

[theshadow2001]

Could someone explain what they mean by reverse polarity and how that affects the whole grounding situation.

 

[202dy]

There is negative grounding and positive grounding. Sometimes the grounding can be different in the same room. The electrical circuits in most clubs and bars are always suspect. If you look in the kitchen or the mechanical room you will usually find what appears to be the history of electricity in the form of panel boxes and fuse boxes, many fabricated by companies that have been in the defunct since the first half of the last century, hanging on the wall. Many electricians of varying talent and expertise have worked on these systems. It is almost a lock that at least one of them did not bother to check the grounding of the rest of the wiring before they added circuits. This is compounded by the fact that some installations going back seventy five years did not have grounded outlets.

If Wasabi's techs do not check for ground compatibility, or use ground lifts because the only outlets available are of the two prong variety there are sure to be problems.

 

[Turnaround]

Every electric instrument player should be aware of the hazards. Good info to be found here:

http://www.guitarnuts.com/technical/electrical/safety/index.php

 

[Baleen]

String ground:
I always leave mine grounded, but I've read in TB a good description of star grouning and shielding that suggest if all else is done well, then you don't need to. However, it was mentioned above that the control plate can still get you. I think the best thing is to check your environment when you set up to avoid getting big, accidental shocks.

My old Ampegs used to shock me if I had the polarity switch in the wrong position. At first I tested by holding the strings and quickly tapping a grounded screw on an outlet (I was in high school and felt indestructible). Later I arrived at this method: -turn the amp on
-put a patch cord in the input (with nothing on the other end of it)
-turn up the volume
-try the polarity switch in both positions. The amp always buzzed louder in the "bad" position. No more shocks.

The shocks from the mic:
I agree with above comments that it very well could be the PA, not your bass/amp that's causing that in this case. You might be able to try the same thing that worked for me on my Ampeg with the PA to make sure sure your polarity is good before you grab the mic.

FYI, my amp had a specifically marked polarity switch, but often amps/PAs have a switch with two On positions. These two positions select the different polarities.

 

[bigbeefdog]

I won't downplay the importance of grounding. However, when it comes to safety, a GFCI (ground fault circuit interrupter) outlet is your best protection. A GFCI's sole purpose is to prevent electrocution, by detecting "leakage" in the circuit (such as current "leaking" through your body to ground), and cutting off the juice within milliseconds.

This may be a bit unpleasant if the GFCI cuts off your amp during a gig.... but even then, it's better than the alternative. Considering the issues you've already identified, I'd strongly suggest you use one at least until you find the culprit, and get the issues settled.

They sell GFCI "pigtails" (basically a 2' - 3' 3-prong extension cord with a GFCI interrupter in the line) at Home Depot, Lowe's, etc. Roughly $30 - $40. I use 'em all the time.

 

[theshadow2001]

Reading the linked article there. Im amazed that the situation of ground being lifted and live and neutra switched around arising. This shouldn't happen. Ever. I mean this set of circumstance is truley truley mind boggling. How anyone could in good conscience leave wiring like that is beyond me.

The other two circumstances seem to arise from a grounded piece of equipment becoming live again thats really shouldn't happen. Under exceptional circuistances like something gets into the amp and shorts a connection or something. Or you or someone else did a half arsed re wiring job on it

Just wow.

Are those GFCI dealys things that check what goes through the live comes back through neutral and if there's an imbalance (ie an earth leakage) it trips? We call them RCBO's here and don't you have them on every mains circuit there?

 

[PilbaraBass]

if all the other wiring codes are adhered to on your amp AND sound system, AND club wiring, then yes it is safe...

however, too many times this is just not the case...especially in the area of club wiring.

I have some ideas about earthing bridges...one way is to use a 10 mega-ohm resistor in line between the bridge and the cavity shield....this is a much safer way than just a straight wire...and it really doesn't take much conductivity at all to drain off noise.

 

[bigbeefdog]

Are those GFCI dealys things that check what goes through the live comes back through neutral and if there's an imbalance (ie an earth leakage) it trips? We call them RCBO's here and don't you have them on every mains circuit there?

Yep, same sort of thing, at least with respect to the monitoring for leakage and circuit interruption functions.

Nope, they're not required on all circuits; they're only required on outdoor and/or bathroom circuits, and many older installations don't have them even there. I have yet to see a power feeder at a gig that's GFCI protected.

Not to mention, playing an outdoor gig off a generator is an even WORSE scenario for potential shocks (more often than not, VERY poorly grounded)...... generator gigs were what got us started using the GFCI pigtails in the first place; now I don't leave home without 'em....

 

[Beej]

Star grounding has solved many a hum problem for me over the years. I don't know the theory behind why ground loops should be avoided, but avoiding them certainly can kill the hum. Good cavity shielding and star grounding for me have helped me avoid having to ground the bridge (string grounding)...

 

[Subculture13]

BTW, the way to check if you still have a problem at the mic is to bring your bass up to the mic and let your stings touch the windscreen. If there is a polarity mismatch you will see a small spark jump across the gap as you near the target. This method saves your front teeth on a bad night.

This works, but can be hard to determine sometimes.

What I find to be foolproof is to grab the mic, while NOT touching the strings. Then slowly bring your exposed forearm close to the strings. If there is any electrical charge across them, the hairs on your arm will get drawn to the strings, and you will feel a slight warmth.

Instant solution (assuming it is not the mic anyway) is to get an isolation box, like the ART Cleanbox II or such thing.

It is a passive transformer that electrically isolates the signal from all ground issues. It's a straight signal in = signal out device, just puts a transformer between you and the amp to stop any shorts from grounding and what not.

 

[Baleen]

Reading the linked article there. Im amazed that the situation of ground being lifted and live and neutra switched around arising. This shouldn't happen. Ever. I mean this set of circumstance is truley truley mind boggling. How anyone could in good conscience leave wiring like that is beyond me.

The other two circumstances seem to arise from a grounded piece of equipment becoming live again thats really shouldn't happen. Under exceptional circuistances like something gets into the amp and shorts a connection or something. Or you or someone else did a half arsed re wiring job on it

Part of the problem arises from using old gear that doesn't have a grounded or polarized plug. Even if the wall outlet is properly grounded and wired with the correct polarity, you can still plug in the old amp's power cord both ways which leads to the polarity problem. And while it is full wall voltage that makes through on the signal ground to your strings or control plate, it's at a reduced current judging by the numerous shocks I received. I don't think (not sure) that it would kill you under almost all conditions.

When I had my 60's Ampeg serviced a few years ago, it was modified very easily to have a grounded plug. The polarity switch now has no significance, and the amp is properly grounded.

I reckon the polarity switch was added before polarized and grounded plugs, and the outlets that accept them, were common. It was the equipment manufacturer's way to make it possible to always have the chassis grounded, or at least neutral, no matter which way you inserted the plug. Also, the chassis is not directly tied to one of the A/C legs, rather the polarity switch directs either leg through a capacitor on its way to the chassis. I guess this makes the ground good enough to blow a fuse if there is a direct short, but not too, too dangerous if you don't set the polarity switch correctly.

 

[PilbaraBass]

Yep, same sort of thing, at least with respect to the monitoring for leakage and circuit interruption functions.

Nope, they're not required on all circuits; they're only required on outdoor and/or bathroom circuits, and many older installations don't have them even there. I have yet to see a power feeder at a gig that's GFCI protected.

Not to mention, playing an outdoor gig off a generator is an even WORSE scenario for potential shocks (more often than not, VERY poorly grounded)...... generator gigs were what got us started using the GFCI pigtails in the first place; now I don't leave home without 'em....

if the circuit your plugged into isn't grounded properly, a GFCI will not work properly. It's also a good practice to get one of those power outlet testers, and check them before you plug in your gear.

It's almost to the point where you need to take a licensed electrician to a gig with you.

America really needs to update their NEC requirements...

in australia, all homes built in the last 10 years have a GFCI "safety switch" installed in the main power box that protects all of the power outlets...(lighting circuits are not protected)...

 

[PilbaraBass]

This works, but can be hard to determine sometimes.

What I find to be foolproof is to grab the mic, while NOT touching the strings. Then slowly bring your exposed forearm close to the strings. If there is any electrical charge across them, the hairs on your arm will get drawn to the strings, and you will feel a slight warmth.

Instant solution (assuming it is not the mic anyway) is to get an isolation box, like the ART Cleanbox II or such thing.

It is a passive transformer that electrically isolates the signal from all ground issues. It's a straight signal in = signal out device, just puts a transformer between you and the amp to stop any shorts from grounding and what not.

test-by-touch is NOT a recommended practice...DANGEROUS...

check out your power situation, BEFORE you plug in!