6L6GC STR Matched Power Tubes Hard Duet. Built to the same special tube request specs of leading amp manufacturers of the 1960s, Tungsol 6L6 GC STR Power Tubes are rugged and reliable power tubes that excel in the most demanding guitar amp circuits. Choose from soft, medium, or hard in the drop down menu to get the right tube for your needs. Soft, medium, and hard style descriptions refer to break up modes of the tube for an overdriven guitar signal.Soft tubes have lower plate current and transductance matching numbers and reach saturation and break-up quicker. They are perfect for blues guitarists for the because of the tonal break-up and sustain they give you. Hard tubes on the other hand have a much higher amount of clean headroom before they give you that break-up and distortion. With a high plate current and transconductance matching numbers, they take longer to reach full saturation and are the preferred tubes for those into jazz, country, or bass, and guitarists who use various effects pedals for creating distortion. Medium tubes, as you'd think, fall in between these two, as far as break-up, headroom, saturation, etc. and are a more versatile option if you play a little bit of everything.