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We use the short check lamp (series 60 or 100 watt lamp) to check for shorts but we generally replace electrolytics and paper caps before even plugging in something more than 40 years old. Haven't really used the variac for this
purpose...the lamp will usually alert you i f there is a short (which is usually shorted caps).
Some have reported success with cap rejuvination with the variac but we always just replace. I have tested some of my own tube type equipment using original electrolytic caps (just as a test) that were
working OK and they gradually began to develop problems with use so we find that replacement is best over reforming and using old electrolytics.
Variac is a trade name of General Radio for a variable autotransformer. Also there was an Ohmite "V.T.", and a Superior Powerstat among other brands...I think Superior is the only one still making them...you just need to make sure it can handle the current draw of the load.
Also a Variac (or other such transformer) is not an isolation transformer and you have to watch as on some types you can step up the line voltage to 140 volts by turning the knob all the way up.
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