Svetlana 6L6 Tube Soft/Red Duet Classic Sylvania STR sound The Svetlana 6L6 Medium tubes have the classic Sylvania STR sound 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