When Jim Marshall released his JTM45 and Blues Breaker tube guitar amplifiers there was nothing else quite like them. After a few revisions, one of the unique traits was the somewhat unusual KT66 power output tubes. Unlike EL34 tubes, the KT66 does not get overly saturated, but offers more bite. It has a solid and clear low end. The highs sing and are very smooth. One thing the KT66 tube does have in common with EL34 tube is fantastic crunch when overdriven. In the stereo tube audio world, it was Quad who first brought the KT66 output tube to prominence with their popular Quad II mono amplifiers. Current production versions of these monoblocks are still being produced today.
One thing worth noting is that many folks consider the KT66 tube to be a substitute for the 6L6 tube types. Although this is acceptable in some circuits they are not direct substitutes. The 6L6 tube type has a heater filament that draws approximately 0.9A of current. The KT66 draws about 50% more current, 1.3A. Many 6L6 based tube amplifiers are not equiped with power transformers that can handle this extra current. This is more the case in amps that use four or more 6L6 tubes. The one exception is the Tung-Sol KT66 tube. This power tube is unlike other KT66 tubes in that its heater filament draws 0.9A of current, just like a 6L6. Because of this they are perfectly safe to use in any 6L6 amp where they will fit, with no risk of over-heating the power transformer.
Be sure to click the “Additional Info” tab when viewing a specific KT66 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 KT66 in Wikipedia, KT66 in Duncan Amps, and KT66 in The Tube Directory.