What is a widowmaker amplifier?

A widowmaker amplifier has no power transformer or fuse. Instead, the two-prong, non-polar power cord is wired directly to the rectifier tube. It is very unsafe, because under even minor failure conditions (such as a shorted capacitor or even - in some models - plugging the power cord in backwards), the mains voltage might end up on the guitar strings and the user will be shocked/electrocuted.

On the plus(?) side, power transformers are expensive so widowmaker amps could be manufactured and sold at a lower price. This ensured that every aspiring guitarist in America could own a low-fi death trap if that’s what they really wanted.