actually ,it is the case on a guitar.common ground is grounded to input jack,which is then transferred to the amp ground,which is grounded to your house electrical system,which is grounded to the 6 foot(usually) copper rod or copper cables which are buried in the ground.
if you don'tbelieve me,go outside to your electric meter and look for a copper wire running from the meter to the ground and look at what it is clamped to
capische?