“Widowmaker” refers to amps that can kill you because they are not grounded and can, in fact, put 117 volts AC through your body.
Typically these are amps that do not have a power transformer. The tube heaters on these are wired in series to effectively reduce the wall voltage (it’s spread over their resistances), and the tubes are not your typical Fender-type tubes, which cannot easily operate this way. For a schematic example of this type of amp, see the Harmony H-400: In this example they used a small transformer for the heater of the 12AU6, but the 50C5 and 35W4 heaters have no transformer isolating them from the wall outlet voltage. 50xx and 35xx tubes are clues that you may have a “widowmaker” on your hands. These were used in hundreds of thousands, if not millions, of “All American Five” tabletop radios dating back to the 1930′s or 1940′s. Those circuits were adapted by many low-cost guitar amp makers. The radios did not require you to plug in or touch metal! Here is what Wikipedia says about the potential shock hazards of the design:
Many early examples of the ‘All-American Five’ posed a shock hazard to their owners. Lacking a mains transformer, the chassis of the AA5 radio was directly connected to one side of the mains electric supply. The hazard was made worse because the on/off switch was often in the wire of the mains supply which was connected to the chassis, meaning that the chassis could be “hot” when the set was either ‘on’ or ‘off’ – depending on which way the plug was inserted in the power outlet. The metal chassis securing screws were often accessible from the outside of the Bakelite or wood case, and there were many examples of owners receiving a shock by making contact with these screws while handling a set.
After public outcry and several sensational press accounts of the problem, the hazard was eliminated from later sets by the use of an internal ground bus connected to the chassis by an isolation network. Underwriters Laboratories required the adoption of the floating chassis, as isolation from the mains (the exact circuit and component values were not specified although the leakage current allowed was specified) to limit the shock to a “safe” current level. The chassis was maintained at RF ground (for shielding) by a bypass capacitor (typically 0.05 µF to 0.2 µF) usually with a resistor connected across it (typically 220 kΩ to 470 kΩ , although values as small as 22 kΩ were sometimes used or the resistor was simply omitted). See here for a typical schematic of a 1948 model AC/DC radio with a 220K isolation resistor.
Over the years, these paper capacitors often become leaky, and may allow sufficient current flow to give the user a shock.
If you follow the Harmony H-400 schematic from the power outlet plug through the on-off switch (red line in schematic) you will see it eventually connects to ground (through the “floating chassis” resistor and cap as described above), as do the instrument input plugs. Therefore your guitar strings will also connect to this circuit. I can personally attest that the resistor and capacitor will not protect you from a truly nasty shock. My first amp was a brand new H-400, 1966. If it wasn’t plugged in in the “right” direction, i.e., if “ground” was 117 volts) then when your lips touched a grounded microphone it hurt. It hurt bad. I suppose if you didn’t jerk back or let go of the guitar, and kept your lips on the microphone, and you were married, and especially if the capacitor was bad, it could produce a widow.
As is typical with guitar amplifiers there is a lot of myth going around, sometimes surrounding a nucleus of fact. Many people believe that if they do not see two transformers sitting on the chassis, then it is a Widowmaker. This is not always true. In the olden days many manufacturers put smaller transformers (usually output transformers) INSIDE the chassis.
Lectrolabs are often accused of being widowmakers, and some people have gone to great trouble to add isolation transformers to these amps. I have searched the internet for every available Lectrolab schematic and have not found one without a power transformer. However there are some schematics I have not yet found. To see what I have for any model, click on the link to that model on the right side of this page, or in the pull-down menu under Amp Models near the top of the page.
For an example of an amp that LOOKS like a widowmaker see this on the R200 page:
But if you look at the schematic for that same amp:
…you will see that there IS an power transformer.
Having said all that, ANY AMP WITHOUT A PROPERLY GROUNDED CHASSIS COULD POTENTIALLY KILL YOU. If in any doubt, and for any old amp you want to use, take your amp to a qualified tech. The widowmakers were a particularly egregious design, but no amps came with a grounded plug before the 1960′s. If you have one, and use it, modify it to be safe. By the way, it wouldn’t kill you to add a fuse to the R200B, but it might kill you if you don’t!
I have yet to see evidence that Lectrolab made a “widowmaker”. But I do not have schematics for the R203 and the R204D, both of which have the suspect 50C5 and 35W4 tubes. So the jury is out on these until someone sends me a schematic!