A tube is a tube from the simplest diode to the most complex multi element combination tube used in TV's. Since its first inception in the early 1900's two element diode used to detect radio signals (via Morse code in ships), to the three element Audion (triode) used for signal and sound amplification to the latest tubes manufactured, the theory of tube technology has not changed much. Early radio brought us the triodes WD11's, WD12's and 01A's used in battery sets in the early 1920's. Later sets used the 80 rectifier and 71A triodes in early AC sets from the late 1920's. The early 1930's brought us the 2A3 triode output tubes in Stromberg Carlson consoles capable of an amazing 8 watts of power in push pull configuration. Western Electric brought us the 300B first used in long line telephone amplification and used in early movie theaters amplifiers before Altec. The mid-late 1930's bought out the first power pentode multi element tube (with cathodes enveloping the filament) as the 6L6 (and derivatives 6V6, 6K6) tubes and when used in quad sets could put out 15-18 watts with THD distortion levels less that 2%. The famous E.H. Scott, McMurdo and Lincoln long wave, standard broadcast and short wave radio receivers (many with thirty tube dual chassis and Jensen or Electrovoice multi array speakers including tweeters) plus the high end receivers from Zenith (Stratosphere) and Philco were truly near or at high fidelity without being advertised as such. Early FM on the now defunct 42-50 mHz band also required near high fidelity to bring out the true beauty of noiseless radio. WW2 brought a stop to FM and early experimental TV but after the war technology and the use of multi element and specialty tubes was needed for both the high frequencies required (up to 300 mHz) in TV tuners and IF circuits.
The big difference in tubes is in their application. TV's and high frequencies require a different tubes than those in audio frequencies. Originally most radios and TV's used tubes with 6 volt filaments powered by a power transformer. Later radios and then TV's used various filament voltages and when used in series would add up to 115 volts AC line voltage to save the expense of a power transformer. An example would be a five tube superhet AM radio (millions made) that used a 35W4, 50C5, and three 12 volt filament tube in series (combined total of 111 volts) so that the filaments could be run directly by 115 volt AC line voltage without the use of an expensive power transformer. For Hi Fi the first digits indicate the filament voltage (6CA7 6 volt filament, 12AX7 12 volt filament).
Since most radios and then TV's went solid state in the late 1960's and most of these earlier tube sets are obsolete there is little call for the tubes needed in these sets except for restoration. The most popular tubes are those for early Hi Fi from the 1950's on up and new amps using tubes. The most popular numbers are the 6CA7 power pentode (Dynaco and others), 12AX7 (dual hi mu triode all), 12AU7 (dual medium mu triode all) 7195 power pentode (Fisher, Scott), KT88, 6550 (all) and others.
There were literally over 1000 tube types listed in the RCA Receiving Tube catalog of the mid 1960's. Except for the types needed for Hi Fi they have become obsolete and have little value since most of the sets that required them are no longer around.
If shopping for replacement tubes try to find NOS RCA, GE, Sylvania, Westinghouse, Raytheon, Tungsol or other American brands. JAN tubes (Joint Army/Navy) tubes are also of high quality since the specs were often higher than those of the consumer market. They were made under contract by the above manufacturers mostly GE and Westinghouse. If you go used make sure that the tube has been check on a good Hickock (not Hitchcock) tube analyzer and get the readouts to make sure that they have at least 85% of their rating when new. Beware if some on E bay and when you find a good vendor with a high rating, stick with them. Another good source for used tubes is a local radio/Ham club. They generally have meetings a few times a year with flea markets and most collectors have overlapping interests which include Hi FI. I would trust this group the most way over E bay for honesty and reasonable prices. Hopefully this answers your questions about tubes and sorry about getting so wordy. In response to an earlier answer. I do enjoy watching Twilight Zone, Alfred Hitchcock Presents and Leave it to Beaver episodes on my restored 1948 32 tube DuMont "Meadowbrook" console TV with 12" B/W screen and 12" speaker.
