Neumann (Vintage) Tube Microphones
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 TUBE.
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:
- Mains Voltage: The U47/U48 was designed for exact 220V or 110V mains supply. If you use one in the UK, AUSTRALIA or in one of the countries with different mains voltage, make sure that your power supply has been properly modified. Overvoltage for the V14 tube is like high cholesterol for us: It reduces the life expectancy.
- Capsule Cleaning: It is true that the garbage you will probably see on the surface of the diaphragms from all the years of whisky breathing singers in combination with dust is affecting the sound of your U47. It is not true that this can be easily removed with a brush and alcohol etc. Scratching around on the diaphragm with a brush and a solvent is like using a belt sander to wash your face: It cleans - but it also damages the surface. This is especially true if your diaphragms are still originals, around 40 or so years old. There is very little chance that they will survive your cleaning efforts. In case the capsule is in top condition, (but only then!) the diaphragms can be cleaned with an extremely soft brush and distilled water. (If the capsule was not in top condition, it is very likely that you will have to send me your capsule for rediaphragming afterwards ....) So my message is: BETTER DON'T TOUCH IT!!!
Here is a photo how a capsule looks after a fatal cleaning effort:
- Hum Problems: There are 5 Siemens electrolytic caps in the power supply. Over the years it is very likely, that they have dried out and have lost capacitance. I discovered a source for this original type, so if you want to replace the caps in your PSU with the originals, I can provide them.
- Hiss & boiling noises: If you are lucky, it is just the 100MOhm polarizing resistor which sometimes creates noise. (Not easy to get but I found out that RS stock them). Otherwise (if you are not lucky) it can be caused by the tube or the capsule. See above paragraphs for explanation and solutions.
- Does not work at all: Check the cable and the power supply or get somebody technical to do it for you. If this seems ok., send me the mic for a free test and I will tell you what would be involved to bring the U47 back to original life. Send me an email to arrange details.
- Circuit diagram of the U47: Click here for a U47' s circuit diagram (1024x768)
Any Neumann microphones available?... Check out the "FOR SALE" section
Last modified: August 2006
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