Yes, it is also an issue with the U67. The voltage resistance of tube heaters and certain power supply components in most tube mic systems goes down, as these components age or dry up. Consequently, the supply voltage goes up. It is therefore vital for the preservation of any tube mic's tube and capsule, that you:
1. calibrate the supply voltage to the mic/tube that is used with it, when you first power up the mic (after purchase)
2. periodically, but at least once a year, recheck the supply voltages, and, if necessary, have them adjusted by a competent microphone technician.