A dim bulb tester is an old trick that is essential for testing old tube radios

A dim bulb tester is a current-limiting device. You wire it in series with the load (radio) to prevent too much current from going to the radio.

Think of the bulb as a giant resistor that doubles as a light!

How Does it Work?

Can you use an LED bulb?
No! You MUST use an incandescent bulb. LED may not light properly when used in a tester like this.

What size bulb should I use?
That depends on your radio. Think of the bulb like a fuse. You want to use the correct one for the job. Either on your radio or in the schematic, it will tell you how many watts the radio will consume. Most AA5 radios take 30-35 watts.