Building A Dim Bulb Tester
Since I am holed up in the biggest blizzard New England has seen in a while, it seemed like a good time to build something every amp builder and repairman needs on the bench, a dim bulb tester. Funny name, I know, but what does it do? It is a simple current limiting device that is very useful when powering up amplifiers either for the very first time, or when one comes in with a potential power supply issue. It’s pretty cool, actually.
It employs a standard household light bulb wired in series rather in it’s normal parallel configuration. The bulb is wired to the top of a standard household duplex outlet, and an amp is plugged into the bottom. When the tester is switched on the bulb is in series with the hot side of the feed to the amp. If the amp is OK, the bulb glows bright for a second until the amp starts to draw current, at which point the bulb dims. As the tubes in the amp heat up and start to draw current, the bulb, depending on the wattage vs. the amp wattage glows dimly. This means there is no short circuit in the amp.
If there is a short circuit in the amp, the bulb comes on brightly and stays bright. This would be the same if there was a piece of wire replacing the amp, and allowing the bulb to get the current it needs to glow brightly. It’s a great tool not only for servicing amps, but for powering up new builds for the first time. I built this one from home electrical parts from Lowes for about $11.