We've got a tube tv that's really not that old, but when we bought it, flat screens were still way more expensive.
Geez, I'm older than I thought.
When I read "tube TV" I wasn't thinking CRT "picture tube" but "vacuum tube."
In the old days, children, there was no microcircuitry as there is today. Televisions (and computers!) used to have things in them kind of like heavy-duty light bulbs, called vacuum tubes, that took the place of circuitry. (Don't ask me how they did this, because I don't know.) Like light bulbs, they burned out from time to time and had to be replaced. As you can imagine, this was a nuisance. You either called a repairman, or you opened up the TV, removed all the tubes but the picture tube, and took them down to the TV repair place, where you tested them one by one until you found out which one was bad. Then you bought a new one of the right size, took them home, and hoped that you were replacing them correctly. Often enough, you didn't, and incurred a repairman's visit anyway. (Alternatively, you could remove just the one that you thought was bad, and test that. If it was good, you got to go home, remove another one, and go back to the TV repair shop. Repeat until you hit on the right one.)
All the iterations of "you" here are aimed at the gentlemen, BTW. Ladies and children, being creatures of non-mechanical aptitude, were either excused or forbidden from monkeying with the television's innards.
The sheer nuisance of this is why, with the exception of my ILs, the rest of the world gladly leaped on solid-state circuitry televisions when they were introduced in the late 60's or early 70's.