I used to use a variac ( I have a really beefy one, with and isolation transformer built in,I think they all are?) anyways, with an ammeter needle gauge in series to get an actual load, I already knew what the drop off was, so it was easy calculate a circuit. This was at the birth of the internet when a repair/specifications manual usually cost $40 bucks. I was doing repair though, not design. There was nothing worse than losing an ammeter from running too many amps through it. Now, it's pretty easy to get the specs for anything.
I still have a couple of 100 Megahertz O'scopes, but I don't do much with electronics anymore. I made some electrostatic speakers, and a pair of tapered 1/4 wave tubes, but that's about it. I enjoy vintage audio. I have more fun in my barn working my milling machine, lathe, and Mig/Tig, welders. I'm behind the times with electronics. I only know what I need to, to fix my car,TV etc. Otherwise, I'm lost!