Let\'s start out with the understanding that: 1. A good portion (if not all) of the radio interference that guitar players tune in, rides freely into the amp aboard the guitar cable\'s negative shield, and: 2. The input jack(s) at the amp need two distinct grounds, an AC ground, and a DC ground.
The metal Switchcraft type jacks (such as used on classic Fenders) get a tight, solid AC & DC ground connection (when properly tightened that is) to the chassis. This is desirable for shunting radio interference to ground, preventing it from getting into the amp, but not always desirable for controlling hum due to the potential formation of a ground loop.
The insulated plastic input jacks (typical of Marshall) have no connection to the chassis aside from what they\'re given, via a lead. This is desirable for controlling the DC return path (DC ground) for prevention of ground loop induced hum, but not effective at giving the incoming RF a swift, solid shunt to ground, and therein lies the problem. But alas, there is an elegant solution! A .01 ceramic cap soldered from the ground lug of the input jack directly to the chassis (with the leads kept as short as possible) will effectively shunt incoming RF straight to ground before it gets far enough into the chassis to make "music". Re-an (1-973-808-0063) and Cliff USA (1-800-806-6659) sell grounding rings that can be easily slipped on the collar of the jack before installation. They\'re basically star-washers with a lug for soldering onto. In the case of multiple input jacks, each jack requires to have this done if RF is to be completely blocked from entering the amp. '; 1;