The filament may be hard to see. Set a multimeter to the ohm (Omega symbol) setting, then touch one tester probe to each of the pins at the end of the bulb. Plug the tube into an electrical device to check its glow. If the heated filament inside the tube glows orange like a setting sun, it's usually a sign that the tube is healthy.

The first test is a voltage test, which you perform with the multimeter set to measure AC voltage in the 200-volt range. Turn on the machine to activate the tubes, and look for an orange, red, or purple glow.

There are two procedures to check a light socket with a multimeter, one of which requires the power to be on and another you can do with the power off. If the tester shows a reading between 0.5 … If you don't have another fixture for this simple test, you can test the bulb for continuity with a multimeter or continuity tester. Put the tube into your guitar amplifier, tester, or other tube-using machine.