Left: Wagner U47w                                         Right:  Old original U47

The Tube:

Most people have heard about the old VF14 in the U47 and a lot of speculation and hokus pokus continues to surround this tube and its possible replacements.

The main reason why it is difficult to replace a broken VF14 in an old U47 is the fact that there is only one single supply voltage (105V) coming from the power supply. Neumann came up with a 'quick-and-easy' solution using a Nuvistor as a replacement to the VF14. This tiny tube also works with the high voltage to heat it up, so it could be used with very little change in the old U47 design. Unfortunately this never worked out very well. 
You can read about this in the "U47 history section" here

Most VF14 appearing today are rejects from the time when Neumann had an agreement with Telefunken to get the first shot on all new VF14 from production. The tubes were tested for suitability in microphones and tubes good enough were kept and stamped with the famous "M", the rest was returned to Telefunken for use in domestic post WW2 uni-voltage radio receivers (German: Allstromempfaenger)m for which this tube was originally developed. When there was no demand for this tube for radio receivers any longer, Telefunken decided to end  the series and Neumann was forced to  either (A) use another tube or (B) end the series. Other - more modern - mics were ready in the drawer or even already in production (M49, U67), so Neumann went for the second choice. (Source: Neumann, The Microphone Company, 2003)

Since Neumann could not find any VF14 within specs in the 60's, and even had to go for the unpopular Nuvistor solution to service their existing mics, how can we assume that we can find some now, 45 years later?

But there are some tubes in the Telefunken "steeltube" family which are perfectly suitable to replace the old VF14, however only under the condition that an independent filament supply is provided. But the tube selection is critical, as many of these tubes are not recommended for DC heating (which is essential for microphones). Also some tubes (like the popular EF14) draw high heater current, so the temperature in a mic using this tube could easily raise to a point where the capsule and other components can be severely damaged in the long term. A popular solution is to "underheat" the tube to overcome this problem. Although it is true that a similar method was used for the original VF14, this method is not recommended for most tubes because if tubes are not designed for underheating, the cathode is only heated partially. As a result the emission of the tube is unsettled and unreliable over time. Also it is known that the tube will become susceptible to toxications and their cathode will become "numb" over time.

For many years we are using a special selected Telefunken steel-tube, which is a very close ancestor of the original tube as used in the old U47. Like the VF14 it has been designed to be used with DC heating. Also it draws low current (100mA) for the filament, which gives the tube the characteristic like the underheated VF14 and preserves the capsule and other components in the U47 from overheating and increases life of the tube.

The circuit design in the Wagner U47w is as simple as in the original U47. It does not need any tricks like "dummy loads" etc  for the tube to perform like the VF14 in the Neumann U47

Our selection procedures and parameters to select the tube are the same or better as Neumann used from 1949. A wide selection of ultramodern and historical test equipment is used to select the tubes and finetune the completed microphones.
Once a new U47w is completed, it is run on a logged 5 days endurance test were final tweaks and adjustments are done. 

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