Dirt simple and very useful.
When it comes time to test an amplifier, be it a hi-fi or guitar amp, you don't always want it hooked up to a speaker. If it's broken, it could damage or destroy the speaker and if you're testing max output power, you certainly don't want to listen to test waves at incredibly high volumes.
I already had some high power resistors I had been using for testing, but I was just using jumper leads and it was less than ideal. So I hopped on the Internet and ordered a small aluminum box for a few dollars. I had the rest of the parts, a few mini switches and banana jacks.
Concept is simple, 2 sides, or channels, and a switch that selects 4 or 8 ohm for the load. You can test both sides of a stereo hi-fidelity amplifier this way at the same time. But if you work on guitar amps, especially vintage amps, you also know that 2 ohm loads and 16 ohm loads are quite normal. This box handles them all.
The mono/stereo switch in mono position links the 2 channels together, so you then can have 12 and 16 ohm options. Of course, 16 is probably what you will need. Now to get 2 ohms for old Fender amps and the like, you leave the switch in stereo position and simply wire the 2 sides in parallel set at 4 ohms each.
Here's all the combinations you can have:
Connections are normal banana plugs typical on hi-fi equipment. And for guitar amps, I made a 1/4" to banana plug cable to make hooking up a breeze.