Interesting. Makes sense for the 12AY7, but for the 6C4? The Collins console bus following the 6Q (from memory) is a 600/600 T feeding a 620 series resistor to the bus. I remember trying to cypher out the bus Z years ago, and being somewhat puzzled. I haven't thought about it in awhile, and my scribblings are buried away, but I remember something in the 200 ohm range, if I was anywhere near right (I easily wasn't). The program amps are resistively loaded to have a 2K4 input Z to the bus. So.....the paralleling of input channels into the bus presents a pretty low source Z to the program amps, and it's really a bridging input relationship with the program amp. Anyway, I'm riffin' here and easily missing something obvious. Suffice to say the 6Q when wide open into the bus is driving a pretty low load, given all the parallel channels. There's some blah blah for ya!