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.
Memory Unlocked. We had moved into a house that had an old 60s tube radio built into the wall, it was broken, but my dad went through steps to sort out the issues and pulles a few tubes out. I recall the door with the chart of valve types.
The radio/amp sounded great once fixed. It had these two luninous stripes that you watched as you dialed in a radio station. It was a tuning aid. you tried to get the lines as close as possible to know you had matched a frequency, but if you over dialed the stripes dissapated, and you had to scroll back a bit.