I’ve been told the 6CG7 is a direct replacement for the 12AU7.
Can the 6CG7 be used in the M1200?
Regards
Paul
When RCA launched that tube in 1954 it was pitched as a direct replacement for a 6SN7 in a small bottle. As I understand it, its heater element wiring differs from the 12au7. I don’t ever recall anyone ever advising use of this tube in a 12au7 circuit.
It draws 2x the heater current and would require circuit modification to accommodate different pinout
I would certainly check with PSA before I altered the circuit in anyway. Lots of killer 12AU7’s out there, do you have a stash of 6CG7’s or just looking for something different?
Best,
-JP
6CG7 is compatible with 6DJ6/6922 but not 12AU7.
I’m not a tube expert by any means but I know the ‘6’ and the ‘12’ are the voltages of the tubes. For that reason, I don’t think they would be interchangeable. 
In BHK Pre, there is a switch to make some of the “6” tubes work (do not ask me which ones). But there is no such switch in M1200 to my knowledge.