In old amps, it is common to find a "phase switch". This switch connects one of the wires of the AC power cord to ground through a capacitor. When the wire connected is the "neutral", this grounding helps to make the amp quieter. Problem is that if the ground connected is the "live" wire, then the amp chassis will not be at ground potential (it will be alive), and if you touch something grounded (like a mic or someone playing another amp where the phase switch is opposite to yours) you get a shock! Never saw anybody hurt by this, but if you have heart problems it can be very harmful. Anyway it's very annoying.

The most safe and recommended way to deal with this is to install a cord with a 3 prong plug. If you only play through 3-prong outlets, do this and forget about this mod. Besides, if your AC outlet voltage is 220V, then you probably have two live wires and no neutral, so you'd better REMOVE the phase switch and cap.

In some places however, like where I live, it is very UNLIKELY that you will find 3 prong outlets. In fact, it's not that hard to find places where there's not even outlets for plugs with flat pins, just the rounded ones. In this case, 3 prong plugs are useless (you will have to use an adapter) but there's one quick and dirty method to avoid shocks (without removing the phase cap, which does make the amp quieter):

• 1) hardwire the phase cap to one of the conductors, eliminating the switch;
  2) use the continuity test of your multimeter to determine which plug pin is connected to the phase capacitor and mark it (with a paint dot near it) as your "neutral" pin;
  3) get a neon circuit tester (found at Radio Shack or any hardware store - some even double as screwdrivers, really handy);
  4) hold one of the probes of the circuit tester with your hands and put the other probe in each of the outlet's holes (one at a time): the one which DOESN'T causes the tester to shine is the "neutral" hole (the neon will not light very brightly - the current is small in order to avoid a shock);
  5) plug the cord such that the neutral pin coincides with the neutral hole.
  

This will work fine, but if you arrive late to a gig and has to "phase align" two guitar amps and a bass amp... You definitely can make something wrong. And Murphy's law says that the paint mark you made will disappear when you most need it.

The "Phase Light" mod is a built-in neon circuit tester that is coupled to the phase switch. When the phase switch is at the right position, the light will turn OFF. So you just plug your amp and find the position of the switch where the neon will NOT light. Now you can turn your amp on and play. If every amp in your band is correctly grounded (with its own built-in Phase Light or with the aid of an external circuit tester or with a 3-prong cord plugged in a 3-prong outlet) then you will never get shocks again!

Here's what you will need:

• a neon lamp assembly with resistor (the kind used in amps to show they are "ON") - you will have to drill a hole in the chassis for it (the better place is near the phase switch, at the back panel of the amp);
• if the phase switch in your amp is not already DPDT, a DPDT switch that fits where the phase switch is mounted;
• if the ON/OFF switch in your amp is not already DPDT, a DPDT switch that fits where the ON/OFF switch is mounted;

In my amp, both switches were DPDT, although not all poles were used, so I just need to add the neon assembly. Here is how it's wired:

The ON/OFF switch will open BOTH wires so that the rest of the amp will not interfere with the phase testing circuitry. The phase switch will connect the cap to the same wire that the lamp is connected, but only when the ON/OFF switch is turned ON. If it was not isolated from the lamp by the ON/OFF switch, the cap would short the lamp and it would never light, and if you touched the amp while you were testing the phase, you could get a shock. The lamp only lights if its connected to the "live" wire. The current path from the live wire to ground is as follows: through the lamp, its resistor and then through the capacitance to ground of the amp chassis. If the "phase light" is lighting too dim, you can touch the amp to make it a little brighter - the resistance and capacitance of your body will be in parallel with the capacitance of the chassis.

IMPORTANT: Make a habit to only turn the amp on AFTER you have the phase switch in the right position (phase light OFF). This way your amp chassis will never be connected to the "live" wire.

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