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.
> My guess would be that those letters are an even more shortened form of linear amp(lifier).
No, they were colloquially called (void) lamps in at least French and Slavic languages, because they looked like incandescent bulbs, that were called lamps (lampe/лампа) as well.
