An AC line ammeter and a variac.
Back in the bad old days when I owned the TV and comm shop, I had a couple of these--one for 0-5 ACA, and another for 0-25 ACA. This is when Simpson meters were easy to find and fairly cheap. Any set or rig with PS issues was always hooked up to this and brought up slowly to observe the draw. Amazing how many fuses, regulators, transformers and other components this saved...
I had wanted another for years. When I was at Dayton in 2008, there was a seller that was dumping NIB Simpson panel meters for $5 each. Jesus... I picked up a half dozen voltage and current varieties in values that I felt would come in handy later--including a 0-5 A for this particular project. Seems that it just takes a bit of time to get to things.
Anyway, all of the parts came together in the last month. A nice Budd aluminum enclosure, the meter, a couple 2 3/4" rack handles, a grounded extension cord, and a couple Heyco 7P-2 strain relief's. So here it is, and a good diagnostic tool for bringing old gear slowly back to life--or making sure that something is not drawing too much current if it has had some issues. Works with both vacuum tube and solid state gear.
I still need to add some low profile feet, and about 6-8 ounces of lead sheet on the inside bottom to give it real solidity.
Note the step bit in the picture. The people who make these things believe that they are made of precious metal given the cost they sell them for. This one, which goes up to 7/8" was $45 with tax. But they sure make a better hole than a regular drill bit, and covers about everything encountered for plugs, jacks, pots, switches, etcetera for panel and chassis builders--with a clean hole. Get one. For bigger stuff that you don't want to fiddle all day with a nibbler--bite for a couple common Greenlee large diameter knockout punches. These really seem to be made of gold these days... However, good tools are important. Spend wisely--but get the best and widest variety you can get across time. Such amenities (and essentials) add greatly to the overall bench experience.
Be sure and write down what you spend and what things are worth so that your widow doesn't get screwed when the clowns show up to buy your estate.