When did television become known as the 'tube'?

Solution 1:

To answer the when part of your question, it looks like the nickname took hold in the early 1960s. The earliest use of it in print I could find is from a 1962 television trade publication:

For such of the faithful who do care, Mike Dann, CBS-TV's vice president of network programs, has some happy thoughts. 'I think the boys are about to have their turn on the tube,' Dann cheerfully predicts.

^{Television magazine, Volume 19, Issue 3}

It appears the term was used widely by advertisements for televisions promoting the latest technology behind their "color picture tubes." The term's popularity increased greatly in the 70s, peaked in the 80s, and has been in decline since then. (Unless, of course, you count the use of YouTube.)

Solution 2:

The CRT or Cathode Ray Tube is the vacuum tube/electron gun combination that (before plasma and LCD televisions) was the basis of all televisions and computer monitors.