or on the inside back of radios.
I still have a tube tester and a heath voltmeter that uses tubes believe it or not.
I've been downloading and reading books regarding the design and manufacture of vacuum tubes. Damn, those were smart people. There is a lot of physics and chemistry packed into those things. Now I'm drooling over Welch fore-pumps and Varian diffusion pumps on eBay. Making tubes would keep me off the streets.