The Telefunken Elektroakustik VF14k vacuum tube is a veritable replacement for U47 and U48 tube microphone systems having an original VF14M tube issued by Telefunken or Neumann manufacturers.
The VF14k vacuum tube operates with the same power supply voltage of 105 volts, installs exactly in the vintage U47/U48's VF14M vacuum tube socket, and requires no modifications or fitting difficulties whatsoever for use either in Telefunken Elektroakustik microphones or those issued by Telefunken or Neumann.
The VF14k vacuum tube is military specification grade, has lower self noise, maintains performance within par of the operating curves of the VF14M, and operates comparatively and in the majority of instances quite indistinguishably from the characteristics of the original vintage VF14M (the "M" suffix merely means that, at the time, the VF14 vacuum tubes were listened to and hand selected for fitment into the factory issued U47 and U48 tube microphones). No worries, you will be happy with the sonic performance of this tube in your U47 or U48 microphone...
In any case, purchasing this product will enable your vintage U47 or U48 to remain active!