Vacuum tube tester - photo

[Crusty]

Tube testers were in the supermarkets up til the mid 60's. I remember my dad taking all the tubes out of the tv, checking them on the tester at the grocery store and buying replacements for the ones that were gone or about to go.

That was know as Hollow State technology (the remnants of a bygone civilization of which there is no longer anyone alive that understands them).