Before putting a polarized wall plug on a vintage amp's power cord, you must know which wire to connect to the plug's wide prong and which to the narrow.

This video finds the safe orientation of a non-polarized wall plug in a polarized socket by testing the voltage between the amp chassis and a grounded lamp.

At the end of the video, the chassis is free of risky AC voltage.

NOTE:  The amp need not be turned on in this test, just plugged into the wall.

NOTE:  If more convenient, you could plug a guitar into the amp and probe its strings instead of the amp chassis.

Once you find the safe orientation for the AC plug, pull out the plug and mark the prong that was in the narrow slot of the wall socket.

NOTE:  The electrical tester used in this story is from a bygone era.  A modern neon tester is available in our store:

This photo shows the Vibro Champ^® chassis removed from its cabinet.

A multimeter set to ohms (Ω) shows that the prong we marked is the black wire of the power cord.

Of course, if you just want to change the wall plug, you can simply cut it off the cord, strip the two wires, and check which one goes to the marked prong without removing the amplifier chassis.

Now you can screw the power cord wires to the prongs of the new, polarized plug.

The black wire goes to the narrow prong because the black wire went to the narrow slot when the amp was safe.

Twist together the strands of a wire before wrapping it around a screw.

And always wrap a wire clockwise around a screw, the same direction as the screw tightens.

Here, both the black and the white wires have been connected to their prongs.

NOTE:  2-prong, polarized wall plugs are available in our store:

Here, the plug's prongs are pushed back into its housing and the yellow "REMOVE" piece is reinserted.