Thanks Boswell, In the JLM mode he removed the Memory module and replaced it with this...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.
I managed to get 6386 with no issue,maybe the supply chain is better now.I'm not surprised it had a 5670 instead of the 6386, as 6386s tested for matching halves have not been available for many years. The 5670 is about as close a replacement as can be found these days, but apparently does sound different at higher compression levels.
Here's an interesting piece on the Manley site about the 6386 availability problem, and the lengths that some will go to overcome it!
I don't where the polarity was swapped on the JLM mod schematic, but it could be easily done via the J2 adapter.1) Bypassing the input pentodes changes the overall signal polarity from inverting to non-inverting, so either the wiring on the input or the output transformer will need to be changed to correct the input-output polarity. It's probably part of the JLM mods anyway.
That's a good thought, I am sure I can figure out a way to buffer the meter impedance from the cathode voltage.... Thanks for that!2) The resistance values are strange in the R14, R15 meter adjustment. The other circuit half has 240R as the cathode resistor, but the meter feed section has 300R paralleled with 1K2 to 2K2 plus the meter resistance. This can only get down to 240R (to match the other half) when the pot is shorted out and if the meter has zero resistance. I would have thought that resistive balance at the 6386 cathodes was critical, as it determines the transconductance matching of the two sections. Prior to this schematic, I had not before seen the self-bias voltage of a valve being used to set the 0dB compression level on the meter. I think you should squeeze in a FET to buffer the cathode voltage to the meter and then change the cathode resistor to match the other half. Not.