Construction of a dim-bulb tester
August 14, 2025Click an image to display full-scale
5 months ago the moon and stars aligned, which resulted in me walking past a Sansui 1000A on the curbside. 44 pounds of Japanese tube sound, out for the taking. 100 Watts. People in my 'hood surely love to toss things away.
I have since gotten around to restoring it. One invaluable tool for the safe verification of antique and vintage audio equipment is the dim-bulb tester.
A dim-bulb tester is simply a lightbulb wired in series between the Hot wire of your AC mains outlet and the equipment to be powered, e.g. a receiver or amplifier. It is effectively a current limiter. In the event of a short circuit, the bulb will shine bright instead of releasing magic smoke from your equipment.
The following diagram illustrates how I wired my dim-bulb tester:
* Ground connection omitted for clarity. It is wired straight across. DO NOT reverse Hot and Neutral!
A dim-bulb tester can be assembled cheaply and easily from parts found at your local hardware store. Below is a photograph of the ingredients I chose. The mains connectors and associated bits/bobs pictured are specific to the Americas, although this information can be applied to electric systems in other countries.
I started by screwing the 2-gang junction box into a plank of wood, then placed the switch and sockets in position.
Next I wired everything together with an old 3-prong electrical cord.
After that was done I screwed the lightbulb socket into the wood, screwed the switch and duplex outlet into the junction box, and hid it all behind a faceplate. I was very pleased with the result.
You may wish to read the same things I've just explained to you, but from people who are smarter than me:
- https://antiqueradio.org/dimbulb.htm
- https://pa3fwm.nl/technotes/tn29a-dim-bulb-tester.html
- https://audiokarma.org/forums/index.php?threads/dim-bulb-tester-build-and-how-to.808399/
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