Help me to repair my Guitar

[arnoldclab]

Why would a guitar amplifier emit a loud humming sound whenever you touch your thumb to the tip of the phone plug?

[frosty]

static electricity in your body??? should it really matter??

[Veracohr]

Don't worry about it, it's completely normal.

[Veracohr]

OK, I got an answer I should have known myself without having to ask other people:

 

Your body is acting as an antenna for ambient electromagnetic energy and noise, like 60-cycle hum (I don't know where you are but in the US electricity is transmitted at 60Hz so we call that supply-induced hum "60-cycle hum"). So that noise you are picking up goes through you into the cord and then amplified and pushed out the speaker.