[Phil G4SPZ]

Once I was coming to the end of a long restoration job on a 1939 Ever Ready all-dry battery portable. I'd duly identified the four battery connections with labels showing "HT" and "LT" and was preparing to make the connections and switch on.

Then I got interrupted.

When the interruption was over, I realised I was running short on time. Red was HT positive, I assumed. You can guess the rest. There was a brief bright flash from all four valves, followed immediately by a sickening feeling and some muttered profanity. That cost me around £25 for a set of scarce valves.

So in addition to not assuming anything, I'd advise avoiding interruptions!