RCA released the first 6L6 vacuum tube in 1936. Nearly 80 years later it remains the most used power output tube. It dominates the electric guitar tube amp world. Fender has been using them since the 1940s and since then there have been many others who have designed hugely popular tube amplifiers that rely on the 6L6 tube types for their tone. Unlike the EL34 power tube, 6L6 tubes generally sound a little cleaner and tend to have a bit more beef in the low end as well as more bite on the top. Some players prefer the extra clarity and punch of the 6L6 tube types.
As one would expect from a tube that has been around so long, there have been many revisions over the years. There are 6L6G, 6L6GA, 6L6GB, and 6L6GC tube types. As well when used by the US military the "W" was added to the suffix: 6L6WGA, 6L6WGB, and 6L6WGC. Other common names are 5881 and 7581. The 5881 power tube is a 23 watt version while 6L6GC and 7581 vacuum tubes are 30 watt. These different variations also have different plate voltage limits. If you're not certain of which options are suitable for your 6L6 based tube amp, ask us.
Be sure to click the “Additional Info” tab when viewing a specific 6L6GC / 5881 / 7581 tube. There you will find related articles and downloads such as datasheets, pin-outs, ratings, and matching information when available. Other excellent links for technical information can be found at 6L6 in Wikipedia, 6L6 tubes in Duncan Amps, and 6L6 tubes in The Tube Directory.