Beam-power tetrodes I have known
Two of my Friday Lunch companions play guitar -- one acoustic and one electric. Guitar pickups, amps, and other things luthier-ish come up in our free-ranging discussions over British ales and bangers and mashed or margaritas and molé. I found this fun 2014 BBC documentary on Marshall Amps -- Play It Loud: The Story of Marshall Amplifiers -- yes, some go up to eleven, but no further spoilers here; watch it yourself. I noticed that Marshall were using KT-88 "valves," which are similar to the 6550 I used in the 25W bootleg AM transmitter I built in 10th grade. KT-88s are also in the 75W McIntosh amp I rescued from the dumpster when SRI's Sound and Vibration lab was being shut down. My mom's old tube amp used 6V6GTs and the PA amp my Uncle Norman used for Bingo Night at their shul used 6L6GTs. All these are beam power tetrodes... so are the Eimac 4CX250B and 4CX5000A used in WRPI's old FM transmitters. Poking around for info on the KT-88...