The Death Cap and Ground Switch
The original purpose of the "death cap" was to create an AC ground for the chassis to provide radio frequency shielding. In the days of non-polarized two prong power cords that could plug into the wall socket upside down or right-side up and with no chassis safety ground, the death cap connected the power cord neutral (ground) wire to the chassis to reduce hum picked up from outside the amplifier.
In the USA a power cord's black hot wire has 125 volts AC on it, and the neutral white wire is connected to ground at the service entrance so "neutral" = ground for a two prong cord. Assume a two prong power cord in the following discussion.
Some amps like the Fender 5F1 Champ had a death cap but no ground switch. The death cap was hardwired between one of the power cord wires and the chassis. With this setup the amp user would try the two prong power cord plugged in both ways in the wall socket. One way would connect the death cap to the neutral wire, the other way would connect the death cap to the hot wire. Normally the least hum resulted from having the death cap connected to the neutral (ground) wire.
Death Cap With No Ground Switch
Death Cap in the Fender 5F1 Champ connects power cord wire to the chassis.
The "death cap" got its name because if it shorts out it can electrify the chassis with wall power, which is 15 amps of 125v in the USA and plenty of power to kill you. With a shorted death cap and a two prong power cord there are two possible outcomes: the amp will be normal with the amp plugged in so the neutral wire is connected to the death cap, but flip the power plug around in the socket and the chassis will have 15 amps of 125 volts on it because the shorted death cap will connect the hot wire to the chassis. This represents an extreme shock hazard. A guitar connected to this amp will also have it's bridge and strings electrified at 125 volts.
Even with a perfectly functioning amp when the death cap is connected to the hot wire the chassis will be electrified at 125v AC because the death cap will allow a small AC current to pass through it. The typical .05uF 600v death cap will pass only 2.4 milliamps of AC current so any shock received by touching the chassis or guitar will be mild, but of course in modern times no shock is acceptable.
A .05uF death cap has 53k ohms of reactance at 60Hz. With the death cap connected to the hot wire a max of 2.4 milliamps of AC current will flow through the cap, onto the chassis and through you if you ground the amp or guitar.
Reactance of a capacitor = 1/(2πFC) with F=frequency and C=capacitance
1 / (2*3.1415*60 Hertz * .00000005 Farads) = 53,000 ohms
125 volts / 53,000 ohms = .0024 amps (2.4 milliamps)
With a two prong power cord and no death cap the chassis voltage will float since the power transformer primary is not connected to the chassis. A normally functioning amp's chassis will not be electrified by 120v no matter the orientation of the power plug in the wall socket.
It is true the DC side of the amp circuit is grounded to the chassis but there is a complete loop from the power transformer secondaries, through the amp circuit, back to the transformer so with a normally functioning amp there is no shock hazard. But the problem is there is no protection from a hot AC or DC wire coming loose and electrifying the chassis to create a shock hazard. The now mandatory third power cord wire, the safety ground to chassis, mitigates this risk by shunting any stray voltage and current on the chassis straight to ground.
The death cap serves no safety purpose. This is a myth I have seen spread around the Internet. The death cap provides only a signal ground, not a safety ground. With the ground switch in its usual quietist position with the chassis and death cap connected to neutral (ground), if a loose wire electrifies the chassis the death cap will limit AC flowing through it to ground to 2.4ma, the rest of the current will flow through anyone that touches the chassis. Of course no DC can flow through the death cap to ground so again, it will flow through anyone that touches the chassis.
I strongly recommend replacing all two wire power cords with three wire cords. I also recommend disconnecting or removing all death caps because they can be an extreme shock hazard if a three prong power cord is plugged into an improperly grounded power receptacle.
Some amps have a ground switch to select which power cord wire the death cap and chassis are connected to. With this switch you don't need to flip the power plug at the outlet, you just try all positions of the ground switch and use the quietest. Some ground switches have a third position that completely disconnects the death cap.
Ground Switch With Death Cap
Ground Switch in the Fender 5E3 Deluxe. The death cap can be connected to either power cord wire.
Normally the quietest setting of the ground switch will have the death cap and chassis connected to the neutral (ground) wire so the chassis becomes a shield from radio frequency interference (RFI).
Three Wire Power Cord and the Death Cap
If you put a three prong power cord on an amp with a death cap there's no risk of shock as long as the cord's safety ground wire is secured to the chassis and the amp is plugged into a properly wired power receptacle.
If the ground switch is connected to the hot wire the 2.4ma of AC current will be shunted to ground through the ground safety wire. If the death cap shorts out the current will flow through the ground safety wire and an amp fuse will blow or a circuit breaker will pop.
As anyone that carries a power receptacle tester to gigs can tell you many venues do have improperly wired power receptacles so I still recommend disconnecting or removing the death cap from the ground switch.
3-Way Ground Switch
Note the 1970 era Bassman 100's grounded 3-prong power cord and 3-way ground switch. With the chassis always grounded by the power cord's green safety ground wire the quietest ground switch position is usually with the black hot lead connected to the death cap to act as a power line filter.
A ground switch in an amp with a three prong power cord actually becomes a noise filter switch. Selecting the hot wire with the ground switch will sink AC line noise to the grounded chassis so the ground switch can be useful but I still recommend removing the death cap because it is a shock hazard if the amp is plugged into an improperly wired power receptacle.
By Rob Robinette
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