Before transistors there were vacuum tubes. Anyone who grew up in the middle years of the the 20th century were familiar with the old radios and TVs that took a few minutes to turn on. This is because the vacuum tubes needed to heat up before the would operate. And how many remember opening the back of the TV and wiggling the tubes to get the set to work.
Shep liked to use the phrase "6SJ7GT" whenever he seemed to need a number designation for something he was describing. It did not necessarily have to be for a radio tube, it could have been a serial number, model number, or driver's license number. Anything he needed to put a number to for effect, he used "6SJ7GT".
Shep was a Ham Radio enthusiast from an early age and since most ham radio enthusiasts built their own sets, they were familiar with electronics and the components. The 6SJ7GT was a fairly popular tube and is still somewhat available today.