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The holy grail of microphones....

U47 (click for larger image)

..Oh yes. The King of the Condensers, the great, the famous, the best vocal mic. They say, it makes music.

So, what is inside to make this microphone so special?


The VF14 was build by TELEFUNKEN and mainly used in domestic AC/DC radio receivers ("Allstrom Empfaenger") in Germany after World War II. At the time when Georg Neumann decided to use this tube in the new developed U47 from 1949, the market for AC/DC radio receivers had already started to dry up and in the late 50's  Telefunken decided to stop production of the entire steel tube family for cost reasons.

VF14's had to be selected to be used in microphones, because they are used outside their normal specifications and quite different to standard applications. As a result a high percentage of the standard VF14 production proved to be unsuitable to be used in the U47.
Neumann had an arrangement with Telefunken Berlin to get a "first shot" on the produced VF14. They received tubes from the production, tested them in their own lab for suitability, stamped the 'good' ones with the famous "M" (for 'Mikrofon') and returned the unsuitable ones back to Telefunken (where they were packed and sold to be used in domestic radio receivers). However when Telefunken reduced and finally stopped the production of the VF14 it became clear that the U47 area has ended.
The book "Neumann -The Microphone Company" reads:
We had an agreement with Telefunken, whereby we could send back tubes that were not good for our purposes. After a while, though, we found we were rejecting a an even higher percentage, so we began marking discretely the tubes we returned to Telefunken. Sure enough, back they came a few months later.."

As a result the number of VF14M's available for service became very limited over the years and in the 70's this tube totally vanished from the market. From about 1980 it became nearly impossible to find a good VF14 for sale unless you want to trade in your car for it. There are just a handful of unused spare ones world-wide but who is keen to sell something that's in demand but not manufactured anymore? Also it becomes clear  from the history of the VF14 that if you find one for sale somewhere today, it is very  likely that this is a Neumann reject from the 50's, unless it a selected "M" version.

Some modification alternatives came up to replace the original tube with other types without changing the power supply and the microphone's circuit (remember there is only a single supply voltage from the standard U47 power supply). The first suggestion came from Neumann in June 1968, viz. a Nuvistor tube 13CW4. This and most of the other ideas have never really caught on, because the output transformer in our good old U47 was designed for the output impedance of the VF14. So most of these "quick" replacements seriously compromised the performance of the U47 resulting in significant loss of bass amongst other things. These mods very often turn the classic U47 into a decorative item instead of a great microphone.. (If you are interested in the Nuvistor circuit, I can provide the original instruction, but I would not recommend this solution).
There are however some good working replacement designs, involving other similar tubes from the old Telefunken steel tube-family, but non of them are plug-and-play, since they involve some changes in the mic and usually a new power supply. Because strictly speaking the U47 will not be original any longer after such a hefty modification and not compatible to the original power supply, many users are reluctant to go this way.



(click for larger image)

The capsule in the early U47 is the M7 type, which was also used in the U48 and the M49. It looks (and sounds) slightly different from the K47 as Neumann used in later models of the U47. Neumann's later K47 capsules have a flat ring with many tiny screws to mount the diaphragm, the M7 capsule does not have this because the diaphragms are glued directly onto the body.
The body of the capsule is made of brass and it contains more than 100 tiny holes per side, located with absolute precision. Even if you take the grille off, you probably want see them because they are hidden behind the diaphragms. (Do not touch the diaphragms, otherwise read: "How to get the diaphragms repaired"...). The diaphragms are made of polyester (PVC in very early models) and fit over the body with just a few microns clearance. The diaphragms have a thin circular layer of gold evaporated onto the surface to create the "moving" conductive part of the condenser.
With the passing of years the tension and strength of the polyester changes. So does the sound. In worst cases the diaphragms become perforated or the thin film cracks, losing all flexibility. This results in reduced bottom and top end, crackling noises and sometimes even total cut-outs at high sound pressure levels.
Now, how to get the capsule repaired? Who to trust?? There are only a few people worldwide who can rediaphragm condenser mic capsules, and if it comes to do this job to original Neumann specs, the field looks very poor...  in case this job is not done right, the capsule is just as a piece of brass with some plasic film on it....
I am specialized in Neumann vintage microphone repair for many years and an former Neumann Engineer in Germany (who has been specialised in Neumann capsules for decades) is doing all capsule service for us.  He is able to rediaphragm even old vintage capsules like the M7 or the K47 capsule to original specs. Please contact me if you need help with your capsule.
Capsule & diaphragm service 


OTHER U47 HINTS:

 


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Last modified: August 2006

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