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.
But if you plug the amp in "backwards", mains or wall AC voltage (125v in the USA) will pass through the death cap and on to the amp chassis and guitar strings. The current is limited to about 2.3 milliamps but at 125 volts it's still enough to give you a good shock if you touch a grounded object such as a microphone while touching your guitar strings. The cap will behave this way even when working properly but if the death cap fails as a short it can pass full wall voltage and current--15 amps at 125 volts--to the guitar strings which can kill. That's why it's called a "death cap" although no electrocution has been traced back to a failed death cap but the bottom line is if a death cap fails as a short it can kill. That's why Class Y safety capacitors were invented.
Maybe we should rename it the "f*%k that hurt cap".
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 and use the way with less hum. 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.
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. With 125v 60Hz mains 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 it can be enough to startle you and cause injury. With 220v 50Hz mains the .05uF cap will pass 3.5ma which is still a safe level. 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)
Back in the 1960's we had a washing machine in the basement that would shock you when in bare feet. I didn't know that all I needed to do was turn the wall plug around to keep that from happening.
The "death cap" got its name because if it fails as a short it can electrify the chassis with full mains power, which is 15 amps at 125 volts 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.
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 floating chassis will not be electrified by 125v no matter the orientation of the power plug in the wall socket. Floating chassis are not used in tube amps for two main reasons: A floating chassis will provide no RFI noise protection and there is no protection from a hot AC power transformer primary wire coming loose and electrifying the chassis to create an extreme 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 safety ground also provides RFI noise protection.
The death cap serves no safety purpose. The death cap provides only AC RFI 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 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 with a chassis ground. 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.
If you are dead set on keeping a death cap then replace it with a Class Y safety capacitor that is designed to fail open. This $1 Kemet 900 Series .01uF 400VDC 250VAC Class Y safety capacitor is what you want for mains voltage up to 250 volts AC.
Class X safety caps are designed to connect across the mains from hot to neutral and are not safe to connect line to ground like a death cap. Class X caps are unsafe as death caps and should not be used. Class Y is the correct type cap for a death cap replacement.
I highly recommend everyone keep one of these $7 outlet testers in their guitar case, especially if you play shady venues. It can keep you and your band from getting shocked from miswired electrical outlets.
I'm a fan of Uncle Doug but his video on the Death Cap comes to incorrect and unsafe conclusions. Near the end of the video he measures voltage from chassis to ground but fails to try the cord plug in both orientations. Had he spun the plug around he would have measured full wall voltage on the chassis!
Infamous Uncle Doug Death Cap Video
Near the end of the video if Uncle Doug would have reversed the plug in the socket he would have measured 125 volts on the chassis. Because he failed to reverse the plug he came to the conclusion that it might be ok to leave a death cap in place. This conclusion is wrong.
Ground Switch
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 wire so the chassis becomes a shield from radio frequency interference (RFI).
Three Wire Power Cord and the Death Cap
There was a short transition period between 2 and 3 prong power cord electrical code in the United States where amp makers began using 3 prong cords but continued to use ground switches and death caps. The Fender Bassman 100 shown below is an example.
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. Typical GFCI (ground fault current interrupt) receptacles typically won't trip until 4 or 5 milliamps flow through them so a death cap will usually not set them off.
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 have improperly wired power receptacles so I still recommend removing, disconnecting or upgrading the death cap to a Class Y safety capacitor.
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.
The Bassman 100 3-way ground switch shown above is useful if you plug into a wall socket that is not grounded--which is surprisingly common. With no ground the amp will usually have more hum and noise because the chassis will not act as an RFI shield. If you move the ground switch so that the neutral wire is connected to the death cap and chassis the chassis will be RFI shielded and reduce hum and noise.
A ground switch in an amp with a three prong power cord can also be used as 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 or replacing it with a Class Y safety cap because it is a shock hazard if the amp is plugged into an improperly wired power receptacle.
If you are dead set on keeping the death cap then replace it with a Class Y safety capacitor. This $1 Kemet 900 Series .01uF 400VDC 250VAC Class Y safety capacitor is what you want for wall voltage up to 250 volts AC.
See the Widowmaker Amp page for similar information.
By Rob Robinette
References
RCA Corporation, RCA Receiving Tube Manual, RC30.
Merlin Blencowe, Designing Tube Preamps for Guitar and Bass, 2nd Edition.
Merlin Blencowe, Designing High-Fidelity Tube Preamps
Morgan Jones, Valve Amplifiers, 4th Edition.
Richard Kuehnel, Circuit Analysis of a Legendary Tube Amplifier: The Fender Bassman 5F6-A, 3rd Edition.
Richard Kuehnel, Vacuum Tube Circuit Design: Guitar Amplifier Preamps, 2nd Edition.
Richard Kuehnel, Vacuum Tube Circuit Design: Guitar Amplifier Power Amps
Robert C. Megantz, Design and Construction of Tube Guitar Amplifiers
Neumann & Irving, Guitar Amplifier Overdrive, A Visual Tour It's fairly technical but it's the only book written specifically about guitar amplifier overdrive. It includes many graphs to help make the material easier to understand.
T.E. Rutt, Vacuum Tube Triode Nonlinearity as Part of The Electric Guitar Sound
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