If you’re old enough you might remember seeing one of these in a drug store, grocery store or even Radio Shack. You could pull all the tubes from your radio or TV, put them in a paper bag, and take them down to your local store to test. And hopefully you wrote down which one came out of which socket. Once you found the bad tube or tubes, the store proprietor would unlock the bottom and find new replacement tubes. And the price list is taped right inside the door.
I have an old (probably '60s) hifi amp. It’s awesome. Replaced the selenium rectifier with silicon, replaced a few caps, and put fresh tubes in it.
It sounds…basically the same as modern solid state stuff to my untrained ear. It’s pretty cool that in a sense we “solved” the problem of amplification back then. Most of the speakers of the day were probably complete crap by today’s standards (unless you had something upscale like a pair of AR-3s), but a well designed amplifier from the era holds up well.