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Between the years 1939 and 1975, radio and television manufacturers had gone through a number of changes in what one might call a common tube. For instance, in 1951 you could pop the back off of nearly any TV set and pull, 5U4s, 6SN7s, 6CB6s, 6BG6s all day long. Most radios of the 50s had 50L6, 35W4, and 6AU6, but by 1961 most all of these tubes had been replaced by other tubes and semiconductors. In the late 60s and into the 70s smaller size had become the trend so you saw a lot of Compactrons come into play, and most tubes at that point had no keyed base like an Octal or a Loctal. Speaking of Loctal tubes, their use is centered around and after WW2 as the design was to compliment military equipment. Have you ever seen a loctal tube that was loose in its socket? I doubt it! In my mind and due to the time frame of my interests, the 5U4 diode tube was the most common as it was the diode of choice for most manufacturers from the 40s through the early 60s.
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