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Why does an amp need an earth?

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Andrew5000
(@andrew5000)
New Member
Joined: 12 years ago
Posts: 1
Topic starter  

Hi,

I've got an interesting problem you might be able to help with...

I've got a Marshall Valvestate AVT150 amp, which I love. I bought it second hand with a normal, 3 prong, kettle plug, power cable.
Played by itself it sounds awesome. When played with the rest of the band, it picks up an annoying hum.

When we swop the power cable for a 2 prong plug (i.e. not earthed), the hum goes away and it sounds great.

My question is: Why does the amp need to be earthed? I can't have it earthed when playing with the rest of the band, but is that such a bad thing?

Anyway, your thoughts would be appreciated.

Thanks,
Andrew


   
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Moonrider
(@moonrider)
Noble Member
Joined: 19 years ago
Posts: 1305
 

My question is: Why does the amp need to be earthed?
Andrew

Because if it's not earthed then you're gambling with dying like this . . .

Leslie "Les" Harvey: Age 27
Stone The Crows
(b. 1946, d. 3 May 1972).

Leslie Harvey younger brother of Alex first recorded work was on an Alex Harvey LP called " The blues" in 1964 when Leslie was 16 years old. He joined his brothers band and from then until the formation of Stone The Crows in 1966 . The quintet's early blues-based albums were notable for both Maggie Bell and Jimmy Dewar's expressive vocals and Leslie Harvey's textured, economic guitar work. However, an inability to match their live popularity with record sales led to disaffection and both John McGuinness and Dewar left on completing ODE TO JOHN LAW. On 3 May 1972, Leslie Harvey died after being electrocuted onstage at Swansea's Top Rank Ballroom. Although the group completed a fourth album with Jimmy McCulloch from Thunderclap Newman, they lacked the heart to continue and broke up the following year.

Or this . . .

Keith Relf: Age 32
The Yardbirds
Rennaissance
1944, Richmond, Surrey, England, d. 14 May 1976; vocals/harmonica).

Keith Relf did not die in the bathtub.
He had fitted out the basement of his house as a recording studio and this is where he was found on the floor by his son. His guitar was not properly earthed, and although the shock was fairly mild Keith had been ill and was not fit enough to withstand it.

WHEN YOU USE IMPROPERLY GROUNDED EQUIPMENT, YOU'RE PLAYING ANOTHER FORM OF RUSSIAN ROULETTE. THE MORE YOU DO IT, THE GREATER PROBABILITY THAT YOU WILL DIE.

Playing guitar and never playing for others is like studying medicine and never working in a clinic.

Moondawgs on Reverbnation


   
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Nick Torres
(@nicktorres)
Illustrious Member
Joined: 15 years ago
Posts: 5381
 

Or me....touched a metal window sill while playing guitar. The shock surged through me, sticking my hand to the window until a band mate kicked out the plug. I got better. Mostly.


   
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Ande
 Ande
(@ande)
Prominent Member
Joined: 15 years ago
Posts: 652
 

Yeah-

If the ground you're connecting to has other stuff running through it, that MAY interfere with your sound, as the OP observes. (I'm not enough of an engineer to know exactly how that works. Anyone??) It doesn't happen all the time, but it can.

If, however, your equipment isn't properly grounded, there's a chance that it may, at some point, go to earth through your body. This will interfere with your sound. At best, you won't sound good while you're shouting. At worst, it can be your last shout.

The levels of current running through a lot of musical equipment is just NOT to be messed with. A couple of years ago, I saw a friend take a good shock from his amp (through his guitar, grounding out into the mike stand.) Scariest thing I've seen in a while. Conveniently, fuses blew before he did, so there was no further damage beyond some really unpleasant burns to his hands and lips.

Sure could have stopped his heart though.

If you're getting interference off your ground, can you go into a socket in another room/ on another circuit?

Best,
Ande


   
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TRGuitar
(@trguitar)
Famed Member
Joined: 16 years ago
Posts: 3709
 

Thats not even something you want to mess with. I used to have cheater plugs to use in older buildings without the 3rd prong. I threw them out. I knew better than to use them so I didn't. Not sure why I got them in the first place. In the old days it was just 2 wires. Amazing more people didn't fry playing electric guitar.

Here is a link to a good atricle I found.

http://proaudioland.com/news/ground/

"Work hard, rock hard, eat hard, sleep hard,
grow big, wear glasses if you need 'em."
-- The Webb Wilder Credo --


   
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kent_eh
(@kent_eh)
Noble Member
Joined: 17 years ago
Posts: 1882
 

My guess is that one (or more) power outlets in that room aren't wired properly.
Get one of these and use it every time you plug into a strange power source:

http://lh3.googleusercontent.com/public/GDxm7-NlJUy6wJMjVmap8y76XU871yWf_oDKCEdgTxVXq3R8PmP_i2IGLsw_4ptYDGaR2en7t1MsUFE8GoYW-QRotMNRaapx9Ur42dFqqVQdS0cDuJsjachjmGrbsHoZz70_EUg77E6rfABNOItqSpv_K4NoAunxf3PqRg
(or the equivalent device for your country's power type)

I wrapped a newspaper ’round my head
So I looked like I was deep


   
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493dart
(@493dart)
Active Member
Joined: 12 years ago
Posts: 7
 

Already mentioned -- but always use a grounded AC cable = 3 prong.

If you have a rack and a bunch of noise you might have a ground loop. One way to eliminate that is a product called Hum-frees . They are plastic isolators that wrap around the rack screws on your rack gear . They isolate your rack units from the rack case and each other.

BUT--- if you are running a wireless and you have ground loop noise, you can lift the ground on one or more units in the chain to kill the hum/buzz .


   
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imalone
(@imalone)
Reputable Member
Joined: 13 years ago
Posts: 267
 

Meant to post this a while ago:
Your amp will always have an earth: make sure it's not you.


   
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