Yes, they're talking about https://en.wikipedia.org/wiki/Vacuum_tube s

I think so. In some languages electron tubes are called lamps.

My guess would be that those letters are an even more shortened form of linear amp(lifier). That term does not really imply anything about transistors vs tubes, but the way GP uses it suggested some jargon exists where it's exclusively used for the tube incarnation of the concept.

thermionic valves I would guess.