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[opacheco]

If we can limit the discussion to the genuine Neumann U47 product whose only variable is the tube (and all necessary circuit/component alterations) then I can tell you from my 30-year personal experience:

1. None of the alternatives you mention will recreate the magic of a VF14 mated to the existing U47 circuit/capsule. And the reason is quite obvious: all the tube you cite are cheaper and more readily available than a VF14. That is the ONLY reason anyone has ever looked at any of these alternatives, not because they would yield superior audio, regardless how you define superior.

2. UF and EF 14, while somewhat related to the VF14 in construction and outline, have an overheating handicap which is only overcome by feeding a second voltage into the mic- a major mod. While these tubes can theoretically be biased in a similar way as the VF14 (fixed) the less noisy option-cathode biasing- will yield a substantial deviation in sound- thinned out, glassy, quite transparent, yet leaning towards body-less.

3. As soon as you exit the steel-tube world and look towards some of the high quality Telefunken designs like EF86/806/804 etc. you get into trouble with the existing transformer configuration: a bad match.

I gave up after these and similar kinds of experiments, and realized that the U47 and VF14-combination is a match made in heaven, and should be left alone. There ARE enough VF14 out there, but an increasing number of U47 seekers feel a five figure price is too steep for their pocket book (understandable), and are lured to substantially cheaper supposed alternatives by posters who claim authority- sometimes individuals with a vested interest (not understandable, in my book).

I do not wish to devalue the role U47 copies, re-interpretations, etc. can play for buyers who simply are not in a position to shell out five figures for a fifty year old microphone- some of these U47-inspired mics are quite nice in their own right- but I caution anybody who is after "that" sound not to fall into the trap discussed by hobbyists and eternal U47 wannabes: there is no substitute for the VF14. Period.

Dear Klaus,

What is the special inside of this tube or its Q bias point inside the U47 for be imposible to mimic with another tube ?......Could be its plates, Filament high voltage, the metal case......What kind of tube could be at least a little similar to its tone (at least!!) ??

Opacheco.

[klaus]

If anyone could figure out the special sauce that makes a VF14 so tasty, it would already have been copied and marketed.

The sound of a tube, even in the seemingly relatively uncritical application as impedance converter, is an amalgam of voltages, input and output impedances, current consumption, mechanical construction, thermal conditions, cathode doping, filament arrangements and hook-ups, and so on.

If you cannot afford a genuine Telefunken VF14, you will get there halfway (that word should be underlined!) with some of the derivatives: EF14, UF14, EF12 etc. but they all need careful integration into the circuit, often requiring rewiring, with special attention paid to supply voltages and biasing.

I also tested the "VF14ME" solid state version made in England by Grove Hill Audio a few months ago. It is available now on eBay and Amazon (try getting it even cheaper by buying direct). I thought it was not a bad sounding VF14 copy getting you three-quarters of the way towards the sound of a VF14; besides, it's direct plug & play- no voltage adjustments necessary. Though Grove Hill may have given up manufacturing and pursuing this tack further, there may still be a few of them out there to grab.

[J.J. Blair]

The VF14ME is available on reverb.com. 

[Koen]

How strange, the device is only available at Amazon US. Unable to locate  a dealer in Europe. Moreover, the company is based in Washington. Why the 'made in Europe' pitch I ask myself? Could this be a copy of Andreas Grosser's FET-replacement?

[klaus]

The British company was or is represented by an individual enthusiast located in Seattle. Contact the company directly, as I suggested.

[Koen]

Hello again,
that's what I was aiming at: I can find no mention whatsoever on the web about such company in the UK. Does any of you here have a URL or contact of the actual company in Britain?

[klaus]

Sending you email contact of the owner. Please check your PM.

[J.J. Blair]

Klaus, all things being equal, would you choose the VF14ME over a Nuvistor?  (Regardless of the circuit conversion needed for the Nuvistor.)

[klaus]

That's like asking: "Klaus, all else being equal, would you rather have your left ring finger or your right middle finger cut off?"

Both choices have significant shortcomings. The Nuvistor lacks heft, the ME lacks resolution. If you were to remove the associated RF suppression circuitry from the Nuvistor, I probably would go for that in a pinch.

There!

[usattler]

"If you were to remove the associated RF suppression circuitry from the Nuvistor, I probably would go for that in a pinch."
None of the schematics I have for the U47N shows any 'RF suppression circuitry'. The 6.8 pF capacitor between anode and grid provides negative feedback and prevents oscillation. In conjunction with the 60 MΩ grid resistor the corner frequency of around 390 Hz makes this feedback effective rather broadband and results in a gentle equalization for low frequencies. Since the considerably different internal construction of the VF14M and circuit layout results in considerably larger inter-electrode capacitances this external feedback was not necessary, and some U47 and U48 versions with the VF14 feature a functionally equivalent 'gimmick' capacitor of a short pair of twisted wires or twin lead between grid and anode terminals. The length of which (= capacitance) was then custom trimmed to achieve the desired frequency response and gain for the specific tube/circuit/transformer combination. Many other similar tube microphone circuits, for example the M49,  specify the capacitance as 'SELECTED'.

[klaus]

Never seen the "twin wires" in any of the U47 I have worked on, but interesting concept, nevertheless.
I regard the global feedback capacitor like any other such device in this position of a mic circuit as detrimental to good sound, whatever other positive functions it theoretically may have.

By the way, I was not referring to the feedback capacitor, but the choke coil on the Neumann-made plug & play "Nuvistor Kit" in my comment about RF suppression.

Best,
KH

[usattler]

With a value of 100 µH for the choke in in series to the grid terminal of the (long obsolete) Neumann AR47 conversion kit and 60 MΩ for the grid resistor the resulting corner frequency of nearly 38 kHz does place any effects well above human audible perception. In addition to the feedback capacitance between anode and grid this choke adds in the prevention of the circuit to oscillate at high (radio-) frequencies. Suppression of (external) RF-interference at best is an unintended benefit. With the effective Faraday cage construction of U47/48 and its transformer output configuration it is highly unlikely for any RF-interference to be picked up anyway.

[broncoblair]

Yes,
I can comment that, despite repeated promises by representatives of Telefunken/USA, I have not received a tester of their glass tube-in-a-metal jacket they call "VF14k". I can therefore not make any statements what such a tube would sound like.

Klaus, did Telefunken USA ever send you a tube? What do you make of this graph ? I find it interesting how close it seems to be but the loss of a few DB's in the lower part of the scale no doubt make a difference. And we know it can't be as quite as a VF14, the good ones I've heard are non existent.

[klaus]

Telefunken North America has not sent me a tube for testing, but I have had access to several of them through clients through the years. So far, every one of them was too noisy to allow for more critical listening tests.

But if you have followed the lengthy discussions on many forums about the construction details of this VF14 copy, even if you don't have access to the conversations I had with the inventor of the"VF14k", little doubt remains that even under the best of circumstances it cannot approach the unique timbre of a VF14.

Regarding the graph you provided.
If you venture into the scientific world by producing a graph to try to prove something, you should play by the rules scientists have established for the advancement of good scientific knowledge, where careful sourcing and critical evaluation of the information is required: Who made the graph? What were the exact circumstances of its making? Was its creation peer-reviewed?
As long as I don't have that information, the graph is not useful as a basis for any discussion, in my opinion.

 For the rest of us, I am happy to form an opinion in the realm where it really matters-listening

[soapfoot]

Something else I notice about the graph--

It seems like the overlay was done in a manner that minimizes as much as possible the low end discrepancy in two ways--

Firstly, the calibration point between the two traces seems almost arbitrary (about 180 cycles?). Notice that the trace for the new tube is about a dB louder than the other at most frequencies. This has the net effect of making the low-end discrepancy psychologically appear to be about a dB not-as-bad. If the two traces were calibrated to be the same at, say, 1k, the deviation in the low end would look even more pronounced.

But more importantly, the scale shown on the graph is a massive 10dB per division-- there's absolutely NO reason to have a resolution that coarse on this graph, except that it makes the two traces "look" more similar and close together. If the resolution were 2dB per division, both traces would still fit on the graph entirely, but the low end discrepancy would be about a division-and-a-half at 50 cycles... which would LOOK much more serious.

Consequently (and also considering the source) that graph appears to me to be marketing material, rather than a study of the relative behaviors of the two tubes. Its objective is NOT to give you an honest idea of the relative behaviors of the two tubes.

Remember also that the principal measurable difference in performance between a VF14 and a 13CW4 Nuvistor is also in the bottom couple of octaves, just like we see here... and that the low end is where much of the U47's signature lies.