This one currently sits at ~ 5.3V. (depending were the mains voltage wanders off too) I suspect Sony ran it at lower volts to reduce noise but this seems a bit too low, I vaguely remember other mics running them at 5.8V. Replacing the vintage wire wound series resistors and mounting new ones is a bit of a pain.
Khron, thanks for the article!
Ian, I will take those 6AU6's off you. PM me the details. Thanks.