Yes, there are some vintage amplifiers out there that want to kill you.
“Widowmaker” amps are inexpensive amplifiers from the 50s -70s that use line voltage and no isolation transformer. These amps typically utilize a 35W4, a 12AU6, and a 50C5 tube.
These amps are not safe. Under certain conditions they will often have 120 VAC on the chassis, waiting to shock you. Sometimes a widowmaker will have a capacitor or a capacitor/resistor between the 120 VAC power and the chassis ( the capacitor is often called a “Death Cap”).
This capacitor is not for safety! It is an RFI filter to reduce line noise to the amp.
There are lots of these little amps out there and technicians are always being asked to make them safe.
The only certain way to safe these amps is with an isolation transformer, as in this article by gmoon: https://www.instructables.com/Isolation-transformer-upgrade-for-old-guitar-amps/
There are drawbacks to this method: it is expensive, it takes up a lot of room, and it requires circuit modification and (gasp!) math. You can read about it on this website under “Taming a Widowmaker Part II”
A simpler, less expensive (and perhaps somewhat less safe) method is shown below. I call it “Taming a Widowmaker Stage 1”.
I show the stage 1 method on a 1958 Danelectro Cadet amplifier.
First, remove the 2-prong plug and replace it with a polarized 2-conductor plug (most electronic stores carry these – the neutral “prong” is larger than the line prong and thus can only be inserted one way into a modern wall socket.
Now, unlike in the original design, the line and neutral are always fixed; it is not possible for them to be reversed* (the main problem with Widowmakers). In the case of this Danelectro, there is an RFI filter consisting of a 0.1 uF capacitor and a parallel 68K current return resistor.
If the resistor is in good shape, you can leave it alone, but replace the 0.1 capacitor with a 0.01 Safety “Y” capacitor. The “Y” safety capacitor is designed to only fail open. Other caps can fail closed, resulting in full voltage and current to the chassis (and you).
That’s it. The widowmaker is now very reasonably safe. Not as safe as employing an isolation transformer – * If the wall outlet is wired incorrectly (reversed polarity) the line and neutral are reversed and voltage will be present on the chassis.
In the case of reversed line polarity, the new RFI filter will limit current to 2.3 mA. The probable effect on the human body is a faint tingle (at 5 mA, a slight shock is felt).
Also, importantly, the original Cadet circuit had the switch on the neutral leg and the fuse on the line leg. This is not a good idea, since there is a possibility of line voltage being on the chassis even when the amp is turned off. I changed the switch to be on the line leg and after the fuse.
