Oli, thanks for this info, but can you please clarify: when you say "getting higher," do you mean the grid leak itself, or the value of the resistor in that position? And should we refer to this resistor then as a "grid leak resistor"?
Hello J.J,
getting higher refers to a higher value of R2, during the time the U47 was made there was nothing higher available that 200M and then super expensive.
R2 was chosen at 60M because during the 1950s 40Hz was the norm for recording mics. Changing this resistor to somewere from 500M to 1G will open up the lowest frequency of the mic, but also depending on the state of the tube might increase the tube noise.
Grid leak might not the proper name, it should be bias/ground reference resistor, but the same as the Buffalo is actually a Bison, it might be not correct but it sounds proper after all those years of mis-use.
All resistor should be updated to metal film or carbon composition for better noise specs.
Best regards,