“Widowmaker” amplifiers were cheap, small amps that were designed without a power transformer, used tubes powered directly from AC wall current, and the chassis was often connected to one of the two power cord wires. Lots of nameplated amps used this circuit: Harmony, Penncrest, Silvertone, National, Supro, and others. The power cord was a “2-prong” and could be inserted one of two ways into the wall outlet. You have a 50/50 chance of plugging it in the wrong way, and the chassis would be at full line voltage. Thus the nickname “Widowmaker”. Plugged in the “right” way, with line voltage to the rectifier plate, and the neutral (ground) wire to the chassis, the amp is relatively safe.
(See also Windowmaker Part II, here: https://hardway-vintage.com/2021/11/taming-a-widowmaker-part-ii/)
I recently worked on a 1963 Penncrest 833 amplifier, a typical Widowmaker. It is identical to the Kay 703-C circuit (both were made by Valco). Here is the Kay schematic:
The power cord is not connected directly to the amp chassis. There is no fuse – thereby saving 20¢ in keeping with the “cheap” design goal. The chassis ground is on the other side of the AC ground cap, (C5), or “Death Cap”. It is connected in parallel with the Current Return Resistor (R7); which provides a return path for the V1 cathode current to the main’s neutral wire. Plugged in correctly, there is no voltage on the chassis. But flip over the power cord plug, and there will be 120VAC on the chassis.
R7 and C5 are there to limit the current to the chassis – usually to about 4 mA. The user would experience a “tingle” shock if grounding the chassis in this case, but (probably) not lethal. Probably. Times have changed. In the early days of electronic radios and amplifiers, and washing machines and etc., it was considered acceptable if the consumer experienced a mild shock from the equipment from time to time. That is, of course, no longer the case. But musicians love their old vintage guitar amplifiers, and many, if not most, were designed to the old, dangerous standard.
This is the same theory of Fender and other amplifier designers of the “Blackface” era of two-conductor power cords. The Death Cap got its name because if it fails short, lethal voltage and current could be on the amp chassis. Several musicians have died from electric shock from this design. The death cap is not a safety capacitor. It is in the circuit to reduce AC line noise, not protect you from a shock.
The amp will exhibit less noise and hum when plugged in the “right” way. Fender and others often incorporated a ground switch to reverse the mains polarity by just flipping the switch, instead of turning the power cord plug over 180 degrees. The inherent danger is that if your amp is plugged in one way, and the PA is plugged in the other way, the player can receive a nasty shock while holding his guitar and touching the microphone plugged into the PA.
If you do not know who Rob Robinette is, you should. He is an expert on all things amplifier, and he published an excellent article on Widowmakers – https://robrobinette.com/Widowmakers.htm.
The best way to solve the Widowmaker problem is to add an isolation transformer between the incoming power and the amp circuit. The problem here is that the properly sized isolation transformer is big, and there is no good place to mount it in these little amps, and it is also expensive.
You can make the Widowmaker safer (but not completely safe), by installing a 3-conductor power cord with ground to chassis, and utilize the death cap and current return resistor of the original design. (You should absolutely replace the “Death Cap” with a Class Y Safety Capacitor. The Class Y cap is designed to fail open). Here is a Class Y capacitor:
This modification does two things: first, it fixes the polarity of the line cord, so the neutral is always incorporated into the chassis ground, and second, it keeps the functionality of the AC capacitor and current return resistor, in the event of a line-to-ground fault, limiting the current just as it was designed to do in the original circuit. If the Class Y capacitor should fail, it will fail open, keeping full AC off the chassis in the event of reversed polarity.
Here are the modifications to the Kay 703-C:
Note the added fuse assembly. Line power (“Hot”), goes through the fuse and then to the switch, which is on the tone control. The neutral connects to the chassis side of the circuit, and the ground wire is fixed to the chassis. I replaced C5 and R7 with new components for safety. There is only one way to plug in the power cord, so the neutral is always grounded.
The only potential problem arises if the wall outlet itself is wired with reverse polarity, and this is more common than you might think, so it pays to always have an outlet polarity checker, especially when playing at a new venue with unknown power quality. A polarity checker is an invaluable $10 insurance policy.
