Q: “I have an old Fender guitar amp I sometimes like to use at live gigs. I’ve noticed that I sometimes get a shock when I touch my guitar and my vocal mic (or another object on stage), but it only happens when I use this particular amp. What can I do to prevent this?”
A: Older guitar (and bass) amps didn’t do as good of a job of shielding the end user from the voltages within as more modern designs. Some of this simplicity is what helps them sound as great as they do, but it can be a hazard. Your guitar strings are electrically connected to your amp to provide a ground reference. However, if the polarity of that reference gets reversed from other components in your system you can find a sizeable voltage differential between your strings and other elements on stage. The assumption here is that the PA and other items on stage are wired correctly (not always the case) so you may or may not get shocked depending upon the polarity of your amp’s ground reference. Most of the time you can correct the problem by flipping the plug over in the electrical outlet (which reverses the electrical polarity feeding the power supply) or, in some cases, by flipping the three-position toggle switch for power in the opposite direction. On some amps these switches are wired so they can perform the same task as flipping the electrical plug. The center position is off. The other two positions are both on, but with opposite electrical polarity.
More background – Since most of these old amps don’t have a true ground wire they rely on the neutral lead at the electrical outlet to provide a ground reference. This is not uncommon – many devices don’t have (or require) ground pins on their electrical cords to safely operate. Additionally these older designs don’t care which side of the power supply gets this ground reference. In other words, you can supply voltage of either polarity to the power supply and the amp will work the same. Again, this is true of most devices. It all gets converted to DC at the power supply anyway. The problem arises from how the external components in the amp derive their ground reference. In some cases they are connected, more or less directly, right to the power supply input, which is of course connected directly to the electrical outlet. If you plug the device into the outlet “backwards” you have provided the external parts the hot lead from your electrical supply as a zero, or ground voltage reference. Since the other devices generally have a true (correct) ground reference you are in effect walking around holding the hot lead from your electrical service anytime you’re touching the strings of your guitar. All you have to do is touch something correctly grounded (or connected to neutral) and the current will flow through you to ground. The severity of the shock you will get relates in part to how the amp is wired and how direct the electrical path is. This is one reason why modern appliances that do not have ground pins are required to have a polarized electrical plug. These are the type where one of the two leads is larger, which prevents you from plugging it in backwards, thereby keeping the ground reference the same on all of them.