You are getting the light to illuminate, so your connections are correct. No need to fuss with it further. Uwe (!) explained the mathematical calculation of the slow rise to nominal voltage. I concur from my experience.
Let's not focus on B+ or the heater supply alone (by the way, 107VDC will give you almost exactly 36V on the heater in many cases of NG + U47 + VF14 I have observed): a good portion of the health of the tube can be deduced from the plate voltage generated. Anything above 32VDC is fine, anything much below this will be audible as tube noise. Observe* that voltage 5 minutes after start-up, then again after a few hours. After a long time on the shelf, sometimes a lower than nominal plate voltage of VF14 will come back up, once the cathode is freed of deposits.
* Don't do it if you are not familiar with the mic's circuit. The voltage inside the mic can be deadly. Even experienced technicians will keep one hand behind their back when measuring tube mic voltages!