Shocked by mic, when touching strings. Amp ground?

[Searing]

How would I check to make sure that my amp is properly grounded?  I want to eliminate it from the reasons as to why I am getting shocked by the mic when touching guitar strings.  Yes, I did plug everything into one socket, and the ground is good at the practice space.  Thanks!

Chris

[sluckey]

A 3 prong power cord with the green wire connected to chassis on your amp AND on the PA should prevent that shock.

[eleventeen]

Use the benchtop lamp limiter! 

Just kidding, and it's a serious subject.

There are shocks and there are shocks. A nine volt battery can sting your tongue pretty good. (Hey, that's how we test 'em!) A line voltage shock can obviously kill you or scramble your brain if it happens when you least expect it which is when it happens.

What *I* do is to 1: Be very, very certain that your items are grounded and that connections etc; are tight and beefy, on an ongoing maintenance basis. 2: Before I move mouth close to mic, I grab with my left hand the neck of my guitar (like I normally would) and stretch out a few fingers and touch the mic. This way, if there is a shock potential, it goes across my hand, instead of any other vital machinery. I just kind of do this as a ritual, having done it for so many years. I think when you go to a new venue, even if you are super confident in the grounding integrity of your OWN gear you may be encountering a house PA and you should not rely upon anyone or anything else. It is also possible to have very very long cable runs which are apparently properly grounded but have been run parallel to higher-current lines (bad practice, but there you go) and get a few volts of ground offset. Probably won't kill you, but in your mouth, with the moisture of your saliva, can zap you with as little as 10 volts, esp if you are caught by surprise.

Do the outstretched finger test.

[Willabe]

Do the outstretched finger test.

I know your not saying this but to be clear.....

If you grab the mic with the palm side of your hand towards the mic with your other hand on your guitars strings the shock could cause your hand to close on the mic and not let go.

But if you touch the mic with the back of your hand the shock, if bad, will make your arms muscles pull/kick your hand/arm away from the mic.

You can use a meter to check for voltage on the chassis. I seem to remember touching the meters probes 1 to each chassis set for acv on 2 amps. With an amp and a mic go from the guitar strings with 1 prob and the other prob to the mic, set for acv.  


             Brad          

[sluckey]

He's saying touch the mic with outstretched fingers of the same hand that's holding the guitar neck. That way no current can possibly flow thru your arms and chest. I like this idea a lot.

[Willabe]

He's saying touch the mic with outstretched fingers of the same hand that's holding the guitar neck. That way no current can possibly flow thru your arms and chest. I like this idea a lot.

Oh, that is a good idea.


             Brad       

[eleventeen]

OMG, in case it wasn't clear, you are touching the mike with THE SAME HAND with which you are grabbing the guitar neck. Sheesh! Not across your heart!

You are not GRIPPING the mic...just touching it with the tip of your finger. Grip the GUITAR NECK like your four fingers are on the handle of a pistol and your index finger is stuck out like it would be as if a trigger finger. All on the same hand. Touch the mic with the index finger with a brushing motion, not a jab.

And for the most part...it is DC that is considered the type of current that contracts muscles, not AC. But rather than having that theoretical discussion, just do this ritual and DON'T find out!

[Willabe]

And for the most part...it is DC that is considered the type of current that contracts muscles, not AC.

That's interesting. I would think either would do it.


              Brad     

[ckilgore]

After checking your amps ground cord spend $5 on an outlet tester http://www.homedepot.com/p/Gardner-Bender-120-VAC-Outlet-Tester-1-clam-5-clams-master-GRT-3500/202867894 and make sure the outlet you are running the amp on is properly grounded.

[G._Hoffman]

The cheap 100% solution is to use a foam wind screen.

Or you can touch your guitar strings to your mic (don't touch anything metal on the guitar while you do it), and see if it sparks.  If it does, change stuff until it stops.

But the wind screen always works.



Gabriel

[Searing]

Thanks all, but I was hoping to figure out how to test that the amp is grounded?  How do I check the power cord?  I have a DMM.  Just don't know where to stick it 

[Willabe]

Do you have a 3 wire power chord installed on the amp?

If so I'd check for continuity between the 3rd round pin on the wall plug and the chassis.

After checking your amps ground cord spend $5 on an outlet tester http://www.homedepot.com/p/Gardner-Bender-120-VAC-Outlet-Tester-1-clam-5-clams-master-GRT-3500/202867894 and make sure the outlet you are running the amp on is properly grounded.

Our friend brings up a good point here. You'd be surprised how many houses and buildings even if new or redone are wired wrong.

IMO, all guitar/bass players should have a good 1 of these.


                 Brad      

[sluckey]

Thanks all, but I was hoping to figure out how to test that the amp is grounded?  How do I check the power cord?  I have a DMM.  Just don't know where to stick it 

Set your DMM to measure ohms. Connect one lead to the round pin in the plug at the end of the power cord. Connect the other lead to chassis. Zero ohms is good.

[Willabe]

Thanks all, but I was hoping to figure out how to test that the amp is grounded?  How do I check the power cord?  I have a DMM.  Just don't know where to stick it  

Set your DMM to measure ohms. Zero ohms is good.

Sluckeys right and I should know better by now. Continuity is not good enough as a test.


          Sorry,    Brad    

[Searing]

Sluckey,

It reads 0.3ohms.  Is that ok?  I have played in this rehearsal space other times, and never had this problem?  I have a socket checker, but, again, I have played there before with no issue.  Could it be the guitar?  I am getting no buzz, as is usual when the ground is bad on the guitar.  The P.A. also has a three prong cord.  Is there a way to check the guitar with a DMM?  Thanks all.  This is very strange.

Chris

[eleventeen]

What's even better is to get one of those yellow plug deals with the LEDs (on neon lamps) that indicate whether there is any kind of fault inthe AC supply. Open ground, missing neutral, reversed.



This one is $4.42 w/free shipping, look up "outlet tester" on ebay. This little sucker can save your life, it is far more rugged than dragging a meter with leads around, you can plug it in and it will stay.

If your amp has a 3-wire accessory outlet, that would be about the best test you could have to plug one of these things into it. Otherwise the ohms test, checking continuity of your amp's ground to the ground pin on the AC plug is good, but you cannot always run that test on every other piece of gear that might be around and in operation.

[labb]

try a different connecting cord between the guitar and the amp and see if the problem goes away..The 0.3 ohm reading is probably the resistance of the DMM leads and the connection.

[PRR]

> It reads 0.3ohms.

Fine.

Now do the same on your _PA_ system. Wall-cord 3rd pin to mike-jack shell. Wall-cord 3rd pin to _mike_.

Are you sure both units are the SAME ground?? If you can reach both outlets with the meter, first check for dead-zero volts (AC or DC) between the 3rd-holes. Then check for <1 ohm between them.

The guitar and cord ARE good; else it wouldn't play.

[Platefire]

I've had some awful experiances with this in the days before three conductor cords. I've seen fire fly from my lip to the mic and is was plumb distracting regarding words and concentration on the remainder of the song;>).  This will surely make you mic shy! In those days you had to remember not to wear leather sole shoes when playing on concrete--had to wear rubber sole shoes. Never had a problem in 30 years---you got a bad connection somewhere. Platefire 

[G._Hoffman]

There was a guy I used to work with who liked to have about 12V between the mic and the guitar - he said it made his playing and singing more "electric."

And I should probably clarify; that is not a joke. 

He rarely got what he wanted, though, since I made sure everything (inside the amp, and on the stage) was wired properly. 


Gabriel

[RickRS]

Thanks all, but I was hoping to figure out how to test that the amp is grounded?  How do I check the power cord?  I have a DMM.  Just don't know where to stick it 

Hopefully you're still alive and haven't electrocuted yourself just yet. 

Since you have a DMM, stop shocking yourself to determine if there's voltage between your guitar and the mic.  Set that DMM to AC Volts and measure from metal barrel of plug of your guitar cord to the metal body of the mic.  You want zero volts, or at least something way less than ~120 volts.  If you have 50 volts or greater, OSHA said you have a workplace shock hazard. You want much lower than that to reduce the potential for shock to nil. 

I'm not familiar with mic phantom power, but if AC reads ok, check the same guitar to mic points with the meter set on DC volts just to be sure.  Again, you want to see zero volts or at least very low levels.

While eleventeen has some good points about using a three prong outlet tester, if you check the outlet you're plugged into and it's fine, you're still at risk of a shock from the mic if the PA system has a ground problem.   I would prefer checking voltage between grounded surfaces of all the electrical equipment in the room or on the stage to be certain you wouldn't get shocked if you touch something else while holding your guitar. 

Concerning your amp:  if you measure resistance between the amplifier's ground pin on the cord and your  amp chassis at 0.3 ohm, you have a decent ground connection for your amp.  Why not zero ohms?  The wires in the meter leads has resistance.  Try measuring the resistance by just touching the two meter leads together firmly.  It won't likely read zero, but instead some low resistance less than a ohm.  But that does nothing to ensure the outlet you're plugged into is wired correctly with a good ground for your amp.

[Ritchie200]

As has been said, check the outlet(s) - first.  Do the easy stuff first.  It is very likely that one of the outlets you are using has been wired reverse polarity.  Be it at your buddy's basement practice area that he "professionally wired" or the local pub, I see it everywhere.

Jim