I use
three four pieces of equipment that I've built myself. None are my own invention.
I made a capacitor discharger following the plans from Mr. Carlson's Lab (
https://www.patreon.com/MrCarlsonsLab ). I like to work on vacuum tube guitar amplifiers and voltages inside these can be several hundred volts DC. I use this tool to safely discharge the capacitors before starting work on an amp. Plans are super-double-secret for patrons only, but I'm pretty sure these devices have been made by hobbyists for decades. There are two LEDs on the top. One will light up until all of the angry pixies have calmed down. There are two probes for the capacitors and two banana plugs for plugging into a DMM if one is motivated to watch the voltage go down. You can tell that I built this before I bought Kapton/polyimide tape. I should modify it to include a buzzer so that I can keep my eyes on the probes.
I made a tool to measure the plate voltage or plate current on a vacuum tube (can be reconfigured for either) in an amplifier without taking the amp apart. The tube is unplugged and then this device is plugged into the amp tube socket and then the tube is plugged into the socket on the other side of the plug (should be obvious from the picture). The banana plugs are plugged into a DMM to measure mA or DCV. This is used (mainly) to help me set the bias on power amplifier output tubes. I really need to build a second one of these so I can measure two tubes at once. This is based off of a schematic created by YouTube's Uncle Doug. Here's the episode:
Finally, I made a light bulb current limiter. I have a selection of incandescent bulbs of various wattages that I use depending on what I'm testing. I use the limiter along with a variac and a KILL-A-WATT (A/C current meter) when I'm powering up an amp for the first time after messing with its guts. This can help prevent stupid mistakes from becoming expensive mistakes. Plans are pretty much obvious and also very easy to find via a web search.
Lastly, I built the Elliott Sound Products (Australia!) P86 Miniature Audio Oscillator. I built it from the PCB. I did a hack job but it works very well. It's super useful for testing audio equipment away from the bench. BTW, Rod Elliott responds quickly to emails (from customers, at least). His PCBs are INCREDIBLY well documented and supported. If you are looking for very neat audio projects I HIGHLY recommend ESP. No relation, just a happy customer.
https://sound-au.com/project86.htm