If your rig has more than one amp in it and you are experiencing excessive hum or buzzing then you may have a ground loop. A ground loop is an unwanted current in a conductor connecting two points that are supposed to be at the same potential, often ground, but are actually at different potentials. Ground loops can be detrimental to the intended operation of the electrical system. Translated into guitar amp terms means that if each one of your amps in your rig has its own ground then you will have a ground loop and will get all that nasty hum. In a multi-amp set-up you only want to have one of your amps grounded. How do you only ground one of them? Simple, by using ground lift adaptors on all but one of them. This also applies to any external reverb unit like the vintage Fender units. Doing this will eliminate the hum and keep you safe from any chance of electrical shock. Ground lift adaptors are available at harware stores and even at supermarkets.