I saw one of these tube testers in the 1970s, maybe at a TV repair shop. I think that my dad simply bought a replacement vacuum tube for one that had gone bad in a TV. He was good at fixing things.
Vacuum tubes work like transistors in that they allow a small current to control a much larger one. This is necessary for the amplification of radio signals that are very weak. In the case of the vacuum tube, electrons on a grid block the flow of the larger current.
Given how primitive this seems, I am impressed that something as complex as a cathode ray television could be made with these tubes. The response time to controlling the current flow would have to be in nanoseconds to deal with VHF signals. Furthermore, they have to control the path of the electron beam on the cathode ray tube.
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