This is SLIGHTLY off topic, but I am curious how you tell whether a Variac is isolated or not? I was told that there are both types. Can I just measure to see if there's DC continuity between the primary and secondary? The ones I've taken apart had wipers like a regular rheostat. So I've always made a habit of using an isoformer even when I'm on a Variac.
***It's true that the isolation is only needed in transformerless sets, but it's a GREAT habit to use it on all sets. Case in point: I knocked myself across the room once on a 50's RCA set with a transformer power supply. I had my elbow on a file cabinet and I went to adjust the vertical size, and WHAM! One of the AC line filter mica caps was shorted, effectively connecting one side of the line to the chassis, and with my luck it happened to be the hot side
***
Charles