Telefunken VF14: How to Buy With Confidence
Now that VF14 prices are in the stratosphere, how can you protect yourself from getting taken to the cleaners when you need to buy one? There is little any tube dealer or private seller can promise you when it comes to a microphone tube's functionality, noise, life expectancy, or hours it already has clocked. I know of no tube dealer who owns the rare, very expensive specialty testing equipment that is capable of weeding out noisy or microphonic microphone-grade tubes. And there is no easy way to tell how close to disintegration a used VF14's critical heater filament may be.
As underheating and subsequent deposits on the cathode are yet additional criteria that affect functionality and noise floor of a VF14, do not feel reassured when the seller rattles off transconductance figures or quotes you impressive sounding stats like "tests 15.5mA on a Funke W19". While such numbers may be impressive for an amplification tube, none of them will reveal whether a microphone tube is suitable to work, free of noise or microphonics, in a high impedance, no-gain circuit.
What can you do?
There is but one reassuring way to buy and test a VF14. Get a 100% money-back guarantee from the seller in writing, before you send payment (and do not equate store credit for money back)! Then immediately start testing the tube in your mic, under real-world conditions (see below).
If the seller raises the (reasonable) objection "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 Internet forums, their archives and Google Images for lots of pictures of real and fake VF14; then make an initial test of the resistance between the two heater pins, F1 and F2 (you will find the appropriate pin positions on a Google search for "U47 schematic"); genuine VF14 will read between 150 and 170 ohms between the two pins. (Also read reply #40 of this thread: http://repforums.prosoundweb.com/index.php?topic=24046.30)
Then plug in the tube into your U47/48 and immediately calibrate the power supply's output to settle around 105 VDC, but no more than 107 VDC max, under load (i.e. mic connected). Then let the tube 'cook' in your mic for at least 48hrs without interruption. Only then put your headphones on and listen for noise, ideally comparing it against another well-working, VF14-equipped U47; (but any well-working tube mic will do in a pinch as comparator). Also listen for any gross abnormalities in frequency response: are the high and low frequencies well represented? Does the midrange dominate?
After the tube has passed these tests, check for microphonics: knock with your index-finger's knuckle against the mic's housing tube, about midway, with the force you would use to knock at a door (i.e. quite hard), 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 light tapping on the housing tube. Some abused (and some non-selected) VF14 will continue ringing in a high pitched tone for more than four seconds.
If you encounter such tube, return it, because the ringing will superimpose itself on the audio through body transfer of sound pressure. (https://repforums.prosoundweb.com/index.php/topic,37339.0.html)
Please remember: statements from the seller as: "NOS", "like new" or "never used" are only believable if the tube came in its original white, red and blue Telefunken carton, bearing the same production stanp and hand-written serial number as printed on the tube itself. Otherwise, disregard these claims, and assume any such VF14 is 'well used', with an unpredictable history. But if you still want to take the risk, price it accordingly.
A bit more on sellers' claims of "NOS" VF14. There are a few things I consider when looking at VF14 which are claimed to be New Old Stock (whether with 'M' stamp -for low micophonics- or not):
1. Even if the tube's serial number matches that on the box, unless pictures are included that also show the unopened box with paper seals intact, I always assume the tube has been used in a mic for an unknown amount of time, and under unknown circumstances. So if you are a seller of a genuine NOS VF14, make sure to snap a picture of the unopened box to prove your claim. Afterwards, you can safely open the box to show what's inside without losing your claim.
2. With an open box, unless the serial number on the tube matches the serial number on the box, the tube is no longer credible as 'new'.
3. The Telefunken box that comes with the VF14 for sale does not show a serial number? Telefunken VF14 boxes always show the actual tube's serial number in blue ink on the space market "Kenn-Nr."
4. If the connection between tube and tube box cannot be established through an intact seal and identical serial numbers, a claim of NOS is impossible to prove.
What about VF14 vs. VF14 with white 'M' lacquer stamp?
Initially, Neumann returned VF14 that were not up to snuff (mainly with regards to microphonics) to Telefunken for credit (Telefunken would send these rejects off to radio hobby shops and small specialty manufacturers*), and would keep the ones that passed Neumann's rigorous tests, put them inside U47/48 and keep additional specimens as spare parts.
But Telefunken ceased manufacturing VF14s before Neumann ceased making U47/48. That meant, even spare-parts VF14 'M' versions ran out in the 1960s, and the Neumann company and its owners had to increasingly resort to alternatives. These first consisted of using non-'M' tubes, then of retrofitting U47/48 with the questionable "AR47" Nuvistor retrofit kit, starting in 1968. Some owners went to transistorizing the mic. A few now also use glass tube substitutes-like a U.S.-made version where one or two miniature tubes are hidden under a VF14-looking metal jacket.
In my opinion, the best alternative to the VF14 with an 'M' stamp was and still is the one without the stamp: Neumann's yardstick for tube quality was so high that, at least in my experience, enough non-'M' tubes have low enough microphonics, and work as long and reliably in a U47 as 'M' versions do. There clearly are more true "rejects" among non-'M' VF14 than among 'M'. But I have seen enough used and some new 'M' tubes with questionable performance that these days I recommend to buy any VF14 tube, regardless of classification, and rigorously test, as outlined above, before committing to the purchase.
*the latter is unconfirmed