|www.ethanwiner.com - since 1997|
Telefunken U47 FET Retrofit
(Mini-article from Recording-engineer/producer magazine, June, 1980)
NEW: April 25, 2003 I added the exchange that occurred between me and Stephen Temmer, then-president of Gotham Audio, who took great exception to this article. The exchange begins below the schematic diagram.
NEWER: July 21, 2004 I added a note emailed to me by Peter Drefahl at the very end.
NEWER STILL: April 30, 2008 - Gotham Audio COO Lew Frisch recently expanded the Gotham web site and added a museum section devoted to the company's history. Click HERE to visit the Gotham site and read much fascinating information.
I have enjoyed the excellent series of articles about condenser mikes in R-e/p. The one on the Telefunken (Neumann) U47 was of particular interest to me as the recent failure of the VF-14 tube in my unit prompted me to look into an alternate preamplifier.
Trying to find a replacement tube seems extremely difficult because there is little available data (to my knowledge) on the characteristics of the original tube and also, of the few tubes being manufactured today, none are intended to operate at the unusually low plate voltage within the U47. And besides, tubes are such "messy" things. So it was an easy decision to go solid state. Though some may disagree, it is my contention that the sound is governed mostly by the capsule which, by the way, is not the same as the one supplied with newer FET versions of the U47.
The circuit shown will provide good noise and overload performance provided the metal film source resistor is optimized for the particular FET being used. Start with 4.7K and adjust up or down for approximately 0.3 mA. of drain current. Use a low noise N-channel FET such as the 2N5457 or TIS-58. In fact, it is advisable to purchase a half-dozen or so FETs and try each selecting the one that gives the lowest noise.
Very important: With the tube removed from the circuit the power supply output will rise to over 300 volts, so a load must be installed where the tube's filament had been. Use a 220-ohm, 2-watt resistor between the existing wirewound dropping resistor nested in the curve of the case and ground. Since the wirewound resistor is still active, the mike case will warm up as it always did so you can make believe the tube is still there. (This is undoubtedly why my U47 microphone has retained that "warm" sound that U47s have always been known for.)
Soon after the above article was printed, R-e/p magazine received the following angry letter from Stephen Temmer, president of Gotham Audio. (At that time Gotham was the US importer for Neumann products.) My reply follows, then another from him, and then a final comment from the magazine's editor. This was all back in 1980.
From Stephen F. Temmer, President, Gotham Audio Corp., New York City:
It is always unfortunate when statements are made in the press which simply are untrue. I specifically refer to the statement in Mr. Ethan Winer's U-47 Retrofit article that the capsule supplied as replacement in tube U47 Neumann microphones "is not the same as the one supplied with newer FET versions of the U47." I want to assure your readers that the statements which we have been making ever since the U47 FET was released are correct: Both the capsule and the head grille are identical to that used up to 1960 on the U47 models. Only the pattern switch, which allowed selection of cardioid/omni, and did no more than to connect the front and back membranes, was eliminated.
I would also like to comment that the long ago discontinued VF 14M tube which the U47s used, rarely if ever failed. I urge anyone who has such a tube in operation not to discard it even if he felt it had failed. Speak to Gotham instead. For anyone desiring the specifications for the VF 14M tube, they are available (in German) from us.
Gotham, as Neumann's U.S. representative is devoted to the knowledgeable maintenance of all Neumann products, regardless of age. Bear in mind that the last U47 (tube) was built 20 years ago!
Reply from Ethan Winer:
I can't understand why Mr. Temmer refuses to acknowledge that the original PVC capsules used in early versions of the U47 were discontinued in later years and replaced with units made of Mylar. If I may quote from literature supplied by Gotham Audio (sheet #0469591): "...yours is a microphone below a certain serial number ... made using a PVC plastic membrane which has hardened with time and now exhibits as much as 6 dB. loss at 100 Hz. referred to 1000 Hz. The new element ... has Mylar membranes and is not subject to this aging effect. We are sure that you will find a major difference in the microphone's response quality..." (italics mine)
Regarding the failure of VF 14M tubes, it is well known that Nuvistor conversions are performed on U47s when the tube fails and a new tube either cannot be located or is deemed too expensive. If I may again quote from Gotham's own literature (sheet #0868i103): "It is to be bourne in mind that this conversion is NOT an improvement for these microphones, but rather a last ditch effort at keeping them in operation. DO NOT discard your VF-14M tube as long as it is working!" This is then followed by instructions for performing the conversion with step #2 reading "...discard all six screws, 3 metal plates and the tube socket and tube."
I hope this clarifies any misunderstandings raised by my original article on performing an FET conversion to a tube U47.
As to item one: He correctly quotes from one of our bulletins, whose number he also correctly quotes, and which shows it to be dated in 1969 (0469 means April 69), long before we ever thought of a U47 FET Solid State Unit. The entire bulletin deals with the change in membrane material FOR THE TUBE U47 ONLY! His statement in his original article clearly stated that we were liars for claiming that the U47 FET had the same capsule as the U47 tube after 1965; the first U47 FET appeared in 1972.
As to two: Our statement stands, that in our recollection we have never known a VF 14M tube to have failed to the point of unusability. As was the custom during the tube era, people routinely replaced tubes in all equipment and many sent their mikes to us asking us to do just that. We had plenty of them and compiled. We ourselves never installed any Nuvistor kits, but simply sold them to the people who insisted that their VF 14M had failed. We have never seen such failure. They did occasionally become microphonic, but proper shock mounting made them perfectly workable.
ed-- Having spoken with Mr.Winer, R-e/p has been assured that there was absolutely no malicious intent in any of his publishings to this matter.
Postscript (written in April 2003): For those too young to remember Stephen Temmer (now deceased), he was a very angry and arrogant man. I recall a nasty letter he wrote (many years ago) to one of the pro audio magazines complaining about all the "students and youngsters" that attend the AES show. In that letter he suggested the show should be closed to all but active professionals. Aside from the stupidity of petitioning against the very people who might one day be his customers, denying AES attendance to someone dedicated to learning more about audio borders on evil. Mr. Temmer's company, Gotham Audio, also methodically defaced every Telefunken microphone received for repair by removing the original and beautiful Telefunken logo, and replacing it with a Neumann sticker. I was shocked and more than a little upset when they did this to my U47 described in this article.
I've just read your U47 retrofit page, which is quite interesting concerning final failure of VF14M tubes, which does never occur - to quote Temmer's words.
In my significantly shorter repair work history there were at least two VF14M which went definitely unusable:
#1 had an internal interruption, so that there was no electrical access to the plate any more.
#2 suffered from a serious parasitic current, which shifted the circuitry completely off its normal parameters. There was no cure possible by external means.
On the other hand, it is correct to say that a noisy or weak VF14 should never be discarded. In 90% of cases I succeed with a self-refined, "smooth" regeneration process, which makes crackles and noise disappear completely for another long time of operation. In one case, even a completely inactive VF14M could be restored to full gain and absolutely satisfactory noise figure.
Therefore, modifying a U47 to a FET design or different tube type should be regarded as the very last "desperate" step only.
Peter Drefahl - www.drefahlaudio.com
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