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.
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I’m pushing 40 and while I know about vacuum tubes I definitely don’t remember seeing these bad boys around
I’m pushing 60 and these were around in my childhood. I never saw anyone use it because everything was solid state by then.
