Ad for a tube tester. These were found in drug stores, grocery stores, and other venues. You would rock up with some tubes, and plug them into the right socket on top. Then the tester would tell you if the tube was good or bad. If bad, it had drawers full of new ones, which you could purchase to replace your bad tube.
When transistors took over, there was no longer a need for tube testers, and they faded away by the beginning of the 1970s.
Here is an RCA branded one in the flesh.
On one hand, things were progressing amazingly. On the other hand, it made it really easy to take away the ability to repair things instead of buying a new one.
I toyed with the idea of putting together an electronics repair shop, for things like tvs, but it's just not economical. I could make more money going back to being a garbage man than i could resoldering components on $1000 tvs, since I'd have to charge much less than the tv is worth for the repair, and they're deliberately difficult to get into.