I own two pairs of 6bg6ga Sylvania's. Got them in an ebay lot of various power tubes. They came with adapters to 6l6 sockets. They are without a doube a MAJOR secret weapon guitar amp tube, warm, sweet as can be, spectacular clean and blues tones fo sho. You can get the goodies to make your own adaptor sockets at www.tubesandmore.com AKA Antique electronic supply. So if you get the goodies and make the adaptors you can get the Syl 6l6gc sound (str415??? help!) at practically pennies on the dollar compared to what the NOS Syl 6l6gc's go for today. 6bg6ga can be found 7-20 bucks each on ebay. They have a great build quality and last a long time like NOS Sylvania tubes are supposed to.
There are quite as few wonderful sounding output tubes that most guitar players never hear. For instance, by running a wire from pin 3 to a plate cap one can run a Mullard EL38 in a socket wired for el34, the el38 is nearly identical to the the famous Mullard el37 but it has a plate cap. The el38 is a smoother sounding tube, they go for about a quarter to fifth of what their el37 brothers go for. Then there's the 7868...