Vacuum tubes: Radio tubes, TV tubes, Audio tubes

Many of the tubes from radio and TV are used in audio components. Designers through experimentation found these tubes had the pleasant qualities needed to play music. There are so many TV tubes that became the central design of high-end audio circuits.

In the days of tube designed FM radio and TV, there were millions of tubes manufactured to service these components. Now, in modern times these original tubes are coveted due to the small supply of NOS tubes that test well.
NIB (new in box) is what every audiophile wants. It means the tube was originally manufactured in a specialized factory with date codes and has never been used.
NOS (new old stock) is the next gem quality tube. It is an original period tube, but it’s use or non-use can’t be verified. The tube is tested to see if it measures as new. This is why a trustworthy dealer is important.

Re; radio tubes on Ebay. There are still vintage FM radios in collectors and audiophile's homes. Not to mention Ham radio operators and collectors. They need these tubes to keep their gear up and running.