I have often heard/read people mention a "death cap" when talking about older Fender amps.
Is this a capacitor ?
What does it do ?
Why is it called the death cap ?
On older, two prong power cord Fenders, there's a 0.047 uF cap that you can switch in across the two power leads. It filters out some high frequency noise on the line. If the cap fails it can result in your amp's chasis, and your guitar, being connected directly to the mains. Possibly resulting in death.
Here's a diagram for converting a 2-prong Fender to a 3-prong setup with a fuse, getting rid of the cap in the process.