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Today I noticed that if I touch both the guitar strings (while connected to the amplifier) and the metallic radiator in my room, I can feel a current flowing (nothing too strong).

Does this mean the amp might not be properly grounded?

It's a 15 watt Marshal 3 wire plug amp.

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    The question may be more appropriate for Electrical Engineering, but do you measure a voltage between metal parts of guitar and radiator? I find sometimes the sensation comes from the metal just being cold. – Ye Dawg Nov 5 '17 at 22:10
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    It certainly means that you shouldn't use that setup again until it's been checked by someone who DOES understand such things. – Laurence Payne Nov 5 '17 at 22:38
  • @YeDawg I'm borrowing a voltmeter tomorrow to see how much it is exactly. At first I thought the radiator was shaking or something, but no, I checked multiple times, (touching the radiator without touching anything, etc) and it definitively is a current passing through – Chapi Nov 6 '17 at 0:27
  • It is also entirely possible that your 3 plug wall outlet isn't properly grounded. – Paul Nov 6 '17 at 15:48
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If you can use a multimeter, don't bother plugging the amp in. It sounds like there is a problem. To check earth continuity, switch the meter to continuity and touch one probe to the earth pin, other to chassis - or your guitar strings. No buzz or quiet buzz means a bad earth. You could also use the resistance part of the meter if you know what you're doing.

But it would be best to take it to a repairer before using it again.

It would also be worth checking the socket used, partially by plugging into another socket, preferably in a different room or level from the one normally used.

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