6L6GC

You do anything with audio or music and you will immediately recognize that number. I was exchanging emails with someone who commented on the Jim Marshall obit and the subject of these delightful bottles of magic came up. Turns out that there is a nice post on Wikipedia:

6L6
6L6 is the designator for a vacuum tube introduced by Radio Corporation of America in July 1936. At the time Philips had already developed and patented power pentode designs, which were fast replacing power triodes due to their greater efficiency. The beam tetrode design of the 6L6 allowed RCA to circumvent Philips' pentode patent.

These tubes are still being manufactured today (five factories located in Russia, China and Slovakia) and are still are very actively used in new designs. This is not a bad track record for an electronic device that has been manufactured for 76 years.