Svetlana 6L6GC Power Tube
The Svetlana 6L6GC produces a tight clean sound, and has a nice, smooth breakup. This tube is every bit the equal of tone to the winged C.
Soft, medium, and hard style descriptions refer to break up modes of the tube for an overdriven guitar signal. Soft-rated tubes break up easier and have the least headroom, yielding a heavier, harder distortion. In comparison, hard-rated tubes must be driven harder to yield a distortion which typically ends up being thinner and warmer sounding distortion, while medium-rated tubes fall in between.
A bit about the 6L6
Radio Corporation of America (RCA) introduced the 6L6 vacuum tube in mid 1936, it was widely used in many applications, military included. It quickly found popularity in guitar amplifiers. The original 6L6 valve was contained within a black metal tube or casing.
Later the 6L6G had an all glass envelope which improved the radiation cooling of the anode.
Other versions of the 6L6 valve included the 6L6G, 6L6GA, 6L6GB, and the final version which was the 6L6GC – these all had glass envelopes.
For more technical information, read here.