The question comes up often: Now that VF14 prices are in the stratosphere, how can I protect myself from getting taken to the cleaners?
There is little any tube dealer can promise you when it comes to a high impedance converter tube's functionality, life expectancy, or hours it already has been in operation.
I know of no tube dealer who owns the expensive specialty testing equipment that would be able to weed out noisy or microphonic microphone-grade tubes. And there is no easy way to tell how close to disintegration the VF14's critical heater filament may be.
As underheating and consequent deposits on the cathode are yet more aspects that affect the functionality and life span of a VF14, do not feel reassured when the dealer rattles of transconductance figures or quotes you impressive sounding stats like "tests 15.5ma on a Funke W19". While such numbers may be impressive for an amplifier tube, none of them will reveal a microphone tube's vital statistics when designed for and operated in a high impedance, no-gain circuit.
What can you do?
There is but one reassuring way to buy a VF14. Get a 100% money-back guarantee from the seller in writing, before you send him money. (And do not mistake a store credit for money back!) Then immediately start testing the tube in your mic, under real-world conditions.
If the seller comes back with the argument: "I don't know whether your mic and power supply are set up correctly, and I don't want a dead tube coming back to me", you offer him a third-party check-out by a reputable microphone specialist. If that suggestion is refused, walk away.
Once you have received the VF14, peruse this forum for lots of pictures of real and fake VF14; then plug in the tube into your U47/48 after you have carefully calibrated the NG power supply to put out 105 VDC nominal, not more than 107 VDC max, under load (i.e. mic connected).
Then let the tube 'cook' for at least two days without interruption in your mic. Only then put your headphones on and listen for noise, preferably comparing it against another known VF14-equipped mic. ( another well-working Neumann tube mic will work in a pinch.) Also listen for any gross abnormalities in frequency response: Are the high and low frequencies well represented? Does the midrange dominate?
When the tube has passed these tests, check for microphonics: knock your index-finger's knuckle with the force you would use to knock at a door against the mic's housing tube, about midway up, and listen how the tube rings out: every VF14 has some filament ringing, but it should stop within a second or two, and should not be triggered by very lightly tapping on the housing tube. Some abused (or non-selected) VF14 will continue ringing in a very high pitched tone for more than four seconds.
If you encounter such tube, return it, because the ringing will superimpose itself on the audio the mic picks up through the capsule.
Finally, please remember: condition statements from the seller as: "like new" or: "never used" are only believable if the tube came with its original carton which bears the same serial number as printed on the tube itself. Otherwise, disregard these claims, but look at any such VF14 as 'well used, with an unpredictable life span', then price it accordingly.