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[David Satz]

Soapfoot, I think the 10 dB/division scale was chosen because this is the frequency response of the entire microphone, including the capsule, to acoustic signals. If the electronics had been measured separately, a finer vertical scale could easily have been chosen and the rather obvious smoothing could also have been much less. But a graph of the complete microphone's response answers the question that some people would inevitably raise (and justifiably in my opinion), which is how the tube behaves in the context in which it would actually be used, i.e. not driven by a test signal from a relatively low-impedance oscillator, but via a capacitive capsule that has high impedance at low frequencies and low impedance at high frequencies.

--best regards

[klaus]

I agree with the theory as David explained it. The practical value of this particular graph is still not addressed: how exactly were both mics measured (if they were measured at all) to arrive at what can only be termed excellent results for the copy tube, and, more relevant: how well could even the most honest graph reveal sonic differences in the dynamic (i.e. real life) behavior of tubes?

[soapfoot]

Thanks David and Klaus, your points are well-taken

[broncoblair]

So should I bid on this VF14? I guess I could sell my plate... ! Or would I be better off with a nuvistor ? http://www.ebay.com/itm/Telefunken-VF14-VF-14-Tube-for-Neumann-U47-Microphone-Vintage-/262756917429?hash=item3d2d8860b5:g:0KQAAOSwojRYTCUs

[klaus]

The seller makes returns easy by not attaching any conditions. At this price for a non-M- stamped VF14 version, I would still have the tube authenticated and tested for health by a third party.

[broncoblair]

The seller makes returns easy by not attaching any conditions. At this price for a non-M- stamped VF14 version, I would still have the tube authenticated and tested for health by a third party.

Who's best to test the tube ?

[Timjag]

I thought you lot might like this - the BBC's research department assessment of the new Neumann U47 - seems that it wasn't quite up to the mark for Aunty Beeb!

http://downloads.bbc.co.uk/rd/pubs/reports/1954-23.pdf

Nevermind I'm sure they changed half the circuitry anyway!

Happy Christmas everyone!


Tim

[klaus]

One can sum the report up with these three conclusions:

"In the omnidirectional condition the microphone is of no special interest" I agree.

"These tests confirmed the impression gained when using the U 47 that it gives the best tonal quality of any condenser
microphone we have yet tested" No objections there, either.

"However, it is considerably larger than the Philips, and Telefunken omnidirectional microphones, and therefore not suitable for certain purposes, such as the Royal Festival Hall" They want good sight lines for the audience and orchestra. If that is the main criterium, I understand their downgrading the U47.

[Timjag]

the price of microphone including cables and mains unit is £100 that would be £1834($2252) - pretty reasonable I think, although the Beeb probably got a discount.

[gtoledo3]

Klaus, am I correct in remembering you having mentioned some U47s that have been running consistently by the original purchasers, without having been turned off for decades now?

(...Or was that someone else?)

[klaus]

That was me. I worked on two U47 (owners unrelated) which had been left on continuously in excess of 10 years (!) with no ill effect to the tubes or the mic, aside of dried up rubber components inside the amp cavity.

[mbrebes]

One of the hardest things on electronics, especially tubes, is heat up and cool down.  Ten years of leaving the power on is much less stressful on electronics than ten years of powering them off and on for the same period.

[klaus]

..but that may be a simpler suggestion for CERTAIN impedance converter tubes to make. The "always on" rule would be certified stupid and destructive for high-current / amplification tubes.

Differing opinions, especially those accompanied by a solid scientific explanation are welcome.

[Kai]

The most reasonable explanation might be that the VF14 in the U47 is largely underpowered, so there is almost no wear on the filament even when always on.
Opposed to that, every power cycle puts mechanical stress to all parts by thermal expansion.
This would not work on an AC701 e.g., which runs on full power.

One has to mention, that the VF14 is one of very few tubes that can be used underpowered without negative effects on the tube life.

[jazbrock]

An interesting graph and recorded comparison between the VF14M and VF14K is at this site:  http://www.foxaudioresearch.ca/VF14.htm. My Tele U47 #375 is original except for replaced power supply caps, but the original VF14M tube has developed low gain and distortion, so is pretty well shot. The price of the only "original NOS" VF14M I can currently find is $2750!! It resides in Mexico and who knows if it had the discreet Neumann "reject" marking or not. I'm guessing not because the vendor hasn't mentioned it. The VF14K looks pretty good on the graphs in the link for $2000 cheaper. Who knows definitively until using it in the studio? Of course I'd rather have an NOS VF14M that Neumann accepted but somehow got lost, and that a guy in Germany found in his GrossePapa's forgotton workshop and he doesn't have any idea how much it's worth. Dreeeeeam, dream, dream, dream..........