Yes, I read about the supposed origins of the CBS gain control units in the groupdiy threads. My guess is that the two different compressors (Audimax and Volumax, for operation in series) were originally designed and built for the US AM radio market. It's then possible that a military comms engineer spotted the products and wondered if they would work in the appalling acoustic conditions found inside a tank. He approached CBS Labs, who responded by adapting the threshold levels and time constants for those conditions. I still shudder at valve (tube) products being used in critical applications inside the first large passenger aircraft, let alone a tank.
Regarding the 6AL5, it's a rectifier, so, in signal terms, a lot of the time the cathode will indeed be more positive than the anode, and this will only reverse for input peaks greater than the threshold. In d.c. terms, the polarity is all down to what's inside the "memory unit" plugin. If that is unchanged from an original CBS potted unit, pins 3 and 9 are not shown with any connection inside the compressor, so it's hard to guess at how the wired mod is supposed to work.
I find the CBS circuit diagram hard to follow. It was probably drawn by a graphic artist who made it his job to get everything on one page without any understanding of the way signals should be represented flowing across a schematic.
The Physics and Radio Electronics diagram is about as misleading as you could find. The purple arrows flowing into the anode (plate) and round into the +ve terminal of the power supply are clearly meant to show electron movement, but labelling it "Electric current flows" is opposite to what is normally meant by a current.
Looks like a nice build so far..
