NOTE: THESE ARE ALMOST 4" TALL SO CHECK TO BE SURE YOU HAVE ENOUGH SPACE.
These have indirectly heated cathodes like 5AR4s and 5V4s. This means that it takes a minute or so for this tube to supply B+ voltage to the rest of the tubes in your amp, thereby allowing them to preheat and prolonging their life. Prefect in all amps without standby switches. The few extra bucks you spend on one of these will save you lots of money by extending the life of the other tubes in your amp.