Rather than hijack the other thread started in response to the column on whether playing electric guitars is dangerous, I decided to start this one.
I was in a band over a decade ago. Our guitarist bought an old Marshall amp from the late fifties or early sixties because he liked the sound (it may have also be the type Townsend used, but I can’t remember). It had no grounding wire in the cord. Instead, it had a switch, labelled, IIRC, “polarization”. You ahd to periodically flip this toggle switch to keep from getting zapped, which happened to him several times in concert, when he had forgotten to flip it, and let his lips brush the mic.
We must remember that standardization has not been around forever, that something led to the need for it, and that old dangerous things are still for sale.
Very true. A good tool to have might be a non-contact voltage sensor. These high-sensitivity devices can quickly determine if near line voltage is present where it shouldn’t be, such as mic and guitar cables.
What the “polarity” or “ground” switch on an old guitar amp does is to connect a small (0.1 uF or so) capacitor between the amp chassis and one side of the AC line.
The purpose of doing this is to provide a bypass from the amp chassis to the grounded circuit conductor (the “neutral”) at RF frequencies to reduce RF interference. The idea is that the reactance of the capacitor (its impedance) is low at radio frequencies, thus grounding the amp chassis, but high enough at the power line frequency to avoid excessive leakage current. Ideally, the chassis should be bypassed to the grounded side of the line, but with a two-prong plug, this is a 50-50 chance- so they give you a switch to select which side of the line the bypass cap connects to.
The trouble with this scheme is that if the bypass cap is connected to the “hot” side of the line, a small leakage current will flow between the amp chassis and any grounded object it, or you, might come in contact with.
Assuming a 120VAC 60 Hz line and a 0.1 uF bypass cap, the leakage current will amount to about 4.5 mA- more than enough to feel, and in some cases, to injure. And, if the bypass cap should short, you’re @#$%ed.
The best thing to do with an old amp that has a two-wire power cord and a “ground” switch is to get a technician to install a new three-wire power cord, with the grounding conductor solidly connected to the amp chassis.