Forums / Archive / Thread
VF14 v/s UF14
Author: Posted by Francisco Bouteille
Date: 04/14/2000
making the change on the power supply and on the chassis of the U47 mic. What will they be the main difference on sound between VF14 and UF14 tubes? of course with 'nice' M7.  
Sorry, no info
Author: Posted by M. Schneider / Neumann
Date: 04/14/2000

Dear Mr. Bouteille,
sorry, we have no info on that alteration. It works, but if it does affect the sound in any way....
Best regards, Martin Schneider Neumann Mic. Development  
UF14: a questionable VF14 alternative (long)
Author: Posted by Klaus Heyne
Date: 04/14/2000

Francisco, If I were you, I would not convert a U47 from a VF 14 to a UF14 tube, for three reasons:
1. The conversion to UF14 (some use the EF14) is intrusive and alters the 'originality' of both, mic amp and power supply circuitry. In addition to component and circuitry changes inside the mic and power supply, you might need to replace cabling and pinouts on the connectors. These alterations reduce the collectible value of the mic by at least as much as you thought you were saving by not buying a VF14 tube in the first place.
2. If you go with the existing one-voltage-only power supply system, (this conversion does not require power supply/cable alterations) your new voltage divider resistor inside the mic will get very hot, and various components which were not designed for such heat will deteriorate or fail much faster than with the original design, because of the thermal instability and lack of cooling. The UF 14 tube also will not run at its optimal operating parameters, because of this heat build up inside the housing tube.
3. The U47 has a sound which is hard to quantify objectively beyond the few, rather primitive, ways modern science has come up with so far. Frequency response and harmonic behavior is only a small part of the mic's magic. My experience has been that the whole dynamic character of the mic -its sizzle- goes flat with the use of UF14, EF 14, or, as was another fad in the seventies, 6072s. Others have described the change as lacking in liveliness, more akin to the sound of a cathode follower circuit (AKG's 12A, as example). Of course, those who want to sell you such conversions always describe the sound as being 'the same as a VF14'. Fine, then.
Even if prompted, I don't perform UF or EF 14 conversions anymore. My ears tell me the mic no longer sounds like a U47 through this modification; it therefore ceases to be a U47, in my mind. I am also not convinced that tube alternatives to VF14s make financial sense- you gain a little by buying the cheaper tube, but you lose a lot of the U47's collectible value in the process of removing the 'heart' of the original system.
If I were you, I would go out, search the world market for a used VF14, pay the outrageous price, have it independently tested as part of the purchase, and enjoy one of the finest microphones that has ever been made- the way it was supposed to sound: with a VF14.
Kind regards, Klaus Heyne  
Re: UF14: a questionable VF14 alternative (even longer)
Author: Posted by Uwe Sattler, Neumann|USA
Date: 04/18/2000

It iss apparent that there is still a lot of myth about the VF14 and subsequently its influence on the performance of the U47 microphone. According to my Telefunken and other data books, the only difference between EF14, UF14 and VF14 (or any other possible variant of xF14) was the filament construction! The only function of the filament is to heat the cathode sufficiently to serve as the source of free electrons for the tube. It makes no more difference to this function how exactly the proper operating operating temperature is generated that it would be for a soldering iron to achieve its set temperature from a 24V, 115V or 220V supply. From the cathode sleeve to the anode all other elements were identical in all versions of xF14. Thus, at any given cathode temperature any xF14 version will have identical operating parameters. The nominal filament current of the VF14 was given as 50mA, but in the U47 it is operated at 40mA. Since the power (thus the heat) is related to the current by square law, the tube in the U47 is used only at approximately 2/3 of its nominal filament power, or 1.4Watt (35V x .04A). Whether or not this may be the secret to its legendary sound shall remain part of its mystique, but it certainly goes a long way toward the tubes extreme longevity. To create the same conditions with the UF14 would require to run the filament at 80mA instead of the nominal 100mA. This in turn would require substantial recalculation of the resistor values in the power supply and inside the microphone. For the plate voltage to remain unaffected with a current draw of 80mA, the four 2Wt/1kOhm resistors in the power supply need to be changed to 500Ohm/4W. To keep the UF14 cathode at the same temperature, the filament has to produce 1.4W at 80mA. This means that the original 1.78kOhm wirewound resistor dissipating 2.85W would have to be changed to 1.03kOhm dissipating nearly 7W of heat, while the cathode resistor will need to change to 15Ohm to keep the proper bias. With proper respect and understanding of basic vacuum tube function and design there will be no difference in the amplifier characteristics. But it can be clearly shown that the major issue will be heat management in the microphone. It is for this reason only, that I agree with Klaus NOT to follow this route. Any use of a tube other than the VF14 has to be accompanied by separating the filamant from the plate supply, affecting power supply, connectors and the cable between them. Good Luck! Uwe Sattler, Neumann|USA  
Re: VF14 v/s UF14
Author: Posted by Eric Weber
Date: 04/15/2000

The following information might be of help if you do not allready have it...
fillament voltage / current
EF14 6.3V / 0.47A UF14 25.0V / 0.1A VF14 55.0V / 0.05A
You will find that the vf14 is operated at reduced fillament voltage in the U47.
Modification for UF14's will require changeing the values of the 30 ohm and 1780 ohm resistors in the fillament circuit to maintain the 1.1 volt cathode bias (the dissapation will be the same as before so there is no additional heat as suggested by others) In addition a new power supply design will be required to provde the proper bias and filtering at the 100ma fillament current.
As mentioned by others changing the resistors may have a effect on it's resale value...
Re: VF14 v/s UF14
Author: Posted by Klaus Heyne
Date: 04/15/2000

Eric, Sorry to disagree. My experience has been that replacing the original flatboard wire wound resistor with a new commercially available resistor of correct value and wattage to work with the UF14 develops significantly more heat inside the U47's body cavity. That's why most conversions were done by feeding in separate heater and high voltages through new cabling and power supply. You are correct- the current draw stays the same, but the total lack of heatsink ability inside the mic cooks the body cavity beyond what I consider safe and stable. Neumann's ingenious design consisted of perfectly sandwiching a wire resistor between a curved metal plate and the body's housing bell, and distributing its heat throughout the mic's housing surface for cooling. Most heat felt when touching a stock U47 with VF14 tube after a few hours is that of the 1780 ohm sandwiched resistor, not the tube. As you might have observed, even with the optimal heat sinking the tight space allows, that heat is substantial. If you have found a way to heat sink and properly mount a super hot 6 watt dividing resistor so that it does not get the mic any hotter than the wire wound stock one after 10 hours of operation, please share it with those who still think of the UF14 as a viable alternative to the original tube.
Kind regards, Klaus Heyne
P.S.: It's not changing a resistor here or there that kills the resale value of a U47, as you suggest- it's not having a $1500.- tube inside!  
Re: VF14 v/s UF14
Author: Posted by F. Bouteille.
Date: 04/16/2000

But, if you have a new UF14, and making the rigth changes inside the body, without replacing the flat resistor of and so on.... (this can be done, I know it)
Do you prefer an old VF14 instead of a new UF14 ?, I mean what if you can enjoy of the best mic ever made. Even if you change the tube it sound better than many. What if who is inside. That why my question 'what about the sound?'.
Kind regards, F.Bouteille.  
Re: VF14 v/s UF14
Author: Posted by Klaus Heyne
Date: 04/16/2000

When using a UF14 you somehow need to bring down your heater voltage from 105 V to the neighborhood of 18 V (unless you are not interested in a long life of the tube) INSIDE of your microphone. As I said, I don't know of a way to bypass the existing flatwound resistor in your U47 without creating a lot of heat from a new resistor which is hard to heat sink. Short of using a new power supply feeding a separate heater voltage into the mic, I don't know how you can solve this problem.
How does the UF14 sound? Please reread item No. 3 of my original post.
Would I prefer an old microphone-select VF14 to a UF14 which was never intended to work in the U47? You bet!