When I was a little boy, back in the early 1950s, I remember going to the drugstore with my dad whenever our television set quit working. He would have a paper sack full of vacuum tubes; he pulled out of our television. He would test each tube on the drug store's tube tester. The tube tester had a lot of sockets configured to accept the different pins on the bottom of each vacuum tube. Some tubes required the use of a clip, attached to a cord, which clipped to a nodal at the top of the tube. The tubes ranged in size from very large to tiny. The tubes that registered as "bad" my dad would set aside. He would buy new tubes, the seller would remove from the locked case underneath the tube tester, to replace the bad ones and we would go home; to fix the television. Since everything went to transistors, then computer chips, you don't see the old radio and television vacuum tube testers anymore.