Why is this tube different from all other tubes?

Hi folks:

I was listening tonight to some LPs recorded in the mid-1950s: the Fritz Reiner/CSO "Lt. Kije" on RCA, recorded by Lewis Layton; "Provocative Percussion" on Command, recorded by Bob Fine, and "Music for Bang, Baroom and Harp", recorded by I don't know who, but it was in Chicago's Orchestra Hall.

The one thing that binds these recordings together is that they were all recorded with Neumann U 47 mics. Oh, and they all sound absolutely wonderful. But it started a train of thought that finished up with this question: what the heck was so wonderful about the VF14 tube used in the original U 47?

The word from everyone with U 47 experience is that the mic was never the same after they switched tubes. But -- why? What is it about the VF14 that makes it unique? Anybody got, like, curves on the thing?

Peace,
Paul

(Oh, and I couldn't resist the thread title...too many matzo balls last night, I guess)

This has been discussed a lot over at R/E/P.

I don't really know what to think since I don't have experience with using an original U47. I'm guessing the EF86 and Nuvistor follow-ups were thought to be crummy because the plate impedance was probably too high for the original BV8 transformer for a kick off.

Dale has mentioned here before why the original U47 circuit was quite a neat design which is probably worth researching.

As for a general answer though, my opinion on most recording gear which has a distinctive sound, often I find it has to do with the distortion characteristics, whether adding harmonics, or softening the overall timbre of a sound.

Well, one thing that makes that tube special is that I knew what this threat was about even before I saw the initial posting.

I suppose it must have something to do with the heater/cathode arrangement; at least that's something that makes the VF14 unique and separates it from its close relatives such as the EF14 or UF14. Also, from what I could gather, it is a very good tube performance wise. Very low noise, for instance. About 15 dB A self noise for a U47 (in excellent condition) is not bad for a tube mic.

The good news is that "Kind of Blue" sounds wonderful as well and that album was recorded using M49 mics. Same capsule, different headbasket, different tube circuit, different transformer - yet still a great mic.

In short, the VF14 and the AC701 are the only two tubes designed specifically for use as condenser microphone head amps - and they are both very good at it - much better than the next tube. Ans as the VF14 is shorter in supply than the AC701, it gains a subjective advantage

Jakob E.

in discussions with andreas grosser we came to the conclusion that the vf14 was not created/designed for microphone use.
it seems it was indeed intended for remote work (only one rail for heater&anode) for antenna amplifiers and similar HF applications.
and it seems most likely that it was choosen for it´s ability to work from one voltage only, rectifiers and capacitors were expensive in these days.

i think the vf14 tube - bv8 transformer marriage makes the circuit unique and interesting.

In deed.

From Neumann site:

"The VF14 was originally developed in the mid 1930s as a standard universal pentode for military field radios, where it was employed mainly in RF-front ends, mixers and fixed gain IF-stages, and rarely in AF stages."

The tube has low anode resistance in triode mode and works well with low voltages for the very same reason. It has also low noise because the transconductance is high. (noise resistance is 2,5/Gm)

I have not found the curves for it when operated as normal triode. You can find the curves for EF14 from Tube Data Sheet Locator, but triode curves are not there. I have some EF14 and have planned to trace the curves some day but have been too lazy. I would be interested in the linearity. I suspect that it is less linear than for example that great AC701 or 6072.

Here are the EF14 curves for pentode and several exotic "triode" configurations.
http://tdsl.duncanamps.com/show.php?des=VF14

The low internal resistance is certainly beneficial for transformer design. Lower turns ratio equals better treble response. Some people claim that the different filament voltage versions are not quite identical in materials and grid construction and therefore the EF14 conversion would not be the same. For example leaking grids. Anyone? Military field radio sounds like a thing to have the best and most reliable technology available in it, doesn't it?

The nuvistor conversion is really bad.  It has nothing to do with quality or engineering. The mismatch is serious. It was only done to keep the mics in service.

I don't really know what to think since I don't have experience with using an original U47. I'm guessing the EF86 and Nuvistor follow-ups were thought to be crummy because the plate impedance was probably too high for the original BV8 transformer for a kick off.

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This is what I've read - I think Oliver Archut has posted sentiment to this effect at other newsgroups. FWIW, not everyone I know who's used the real deal actually loves it. At the same time, not everyone who's used a U47i despises it. The U47i is said to be lacking in bass compared to the VF14m version, which would concur with the impedance statement. I seem to remember someone, possibly Archut, saying that the transformer was kept to try and keep as much of the U47's character, but the Nuvistor never had any hope of driving it properly.

Justin

gyraf said:

In short, the VF14 and the AC701 are the only two tubes designed specifically for use as condenser microphone head amps - and they are both very good at it - much better than the next tube. Ans as the VF14 is shorter in supply than the AC701, it gains a subjective advantage

Jakob E.

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Hey Jakob, nice meeting you (and Tim) at the Frankfurt Messe!

I'm quite sure I remember Oliver Archut stating at Klaus Heyne's forum that the VF14 was developed for the V2 missile. Not sure that's true but it's a good story. And it does make sense in a way because a single power supply would make things easier in a rocket. I guess. Never actually built a rocket (and I'm not sure I'd use tubes if I did). Anyway, I doubt the VF14 was specifically designed for microphones. Neumann was pretty small back then, and mics are a small market to this day. It's not likely Telefunken made a tube just for them. And remember they used a specially selected VF14m version. The regular non-m version was probably used in other pieces of gear. I rememember reading about the discontinuation of the U47 that at some point Telefunken told Neumann that they were the only users of VF14 tubes now and that Telefunken would only do one last run. So there must have been other users of VF14 tubes - the question is who and for what? I know that Gefell used VF14 tubes in some (rare) versions of their CMV series. Maybe they were used in some laboratory equipment or some military stuff.

Yea - you might be right - though the tubes used in V1 and V2 were EF14, not VF14.

And I'm aware that the EF14 was a widely used quality tube at mid-thirties.

The reason why I think VF14 was ordered/made specifically for microphone use is that it really wasn't used anywhere else (execpt for the more than 60% Neumann rejects that were used in manufacturing of some locally-built budget radio receiver, imagine VF14 for both RF and audio, including power output!!) - and so relatively few were ever actually made (I forget the numbers, but some 10-20.000 in total iirc)

Jakob E.

The reason why I think VF14 was ordered/made specifically for microphone use is that it really wasn't used anywhere else

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So this quote from Neumann site is BS:

"The VF14 was originally developed in the mid 1930s as a standard universal pentode for military field radios, where it was employed mainly in RF-front ends, mixers and fixed gain IF-stages, and rarely in AF stages.

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I've heard this military field radio story from several sources and it makes sense. (which doesn't make it more true, off course). It seems to be very difficult to source the data sheet. Would be so for military only tubes? Just speculating. But I'm getting skeptical after all.
What is the total number of U47 and U48 mics made? That info should be easy to find.
More quotes:

To add another historical detail: in 1993, I contacted the former Telefunken tube factory in Ulm, about the possibility of a VF 14 re-issue. While my request was kindly declined, I at least got to know that between 1946 and 1957, exactly 27,548 VF 14 tubes were produced.
Mathias Myka 

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Jacob seems to be on the right track. 27000 is still very low quantity for a tube.
another one:

There is also a common story that the VF14 is a WWII surplus tube but the VF14 was not produced before 1944. The first time it shows up in Telefunken databooks and price sheets is in 1946.About 27.000 VF-14 was made in the Telefunken plant in Berlin.

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So, the total number _after_ 1944 is 27000. What about before? None? It seems so. Of course it could be that military tube is not listed in ordinary catalogs.
What does the big Neumann book say about this subject? I don't have it but some of you surely got it. I can not remember if there was any info.

-Jonte

Sorry for misspelling your name, Jakob. I was kind of tired.

About VF14. I begun to wonder why the data  sheet mentions 60V (or 55V in another!) heater if the tube was specially modded version for the mic only and it was probably no ones intention to run it at full heat? Was it to keep the data sheet comparable to EF14? Also why does the data sheet show pentode connection only and with transformer input? Was this done to help to dump the "bad" tubes to radio manufacturers or just the normal procedure?
I guess a special "from the scratch triode for mic" was not possible right after war and EF14 was redesigned for mic use. Makes sense.
I found tube specks for one (yeah, just one, doesn't prove anything) WWII German field radio and VF14 was not there.

-Jonte

I found this long discoussion on www.filmsoundsweden.se a few weeks ago
http://www.filmsoundsweden.se/backspegel/ror-pdf/vf14-forum.pdf
it's a bit long but seems to hold some knowledge on both origins and why the EF14 doesnt sound the same...
johan

I think that answered or confirmed (almost) every issue raised here so far. Grid leak, cathode material, history, other uses etc. Thanks! I think I'll try to get one specimen BTW, I found that it was also used in some measurement mics. So, look around.

I'm sure there are tens of urban myths regarding the VF14's origins. I heard from one source that it was developed as part of a project to design the ultimate microphone for a certain fascist to address large rallies with...

Maybe we should have a dedicated thread to myths surrounding the VF14 and add it to the microphone META?


Justin

Johan said:

(..) http://www.filmsoundsweden.se/backspegel/ror-pdf/vf14-forum.pdf

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Thanks! Good info in there...

Jakob E.

So what are we gonna do when there are no more VF14...will we abandon audio? I doubt it. The old will speak of it and the young will ignore it...I stand with the young.

I agree with Analag.  Let's move on.  I'm more interested in what we can make now. 

I do like the BV8 concept -- that it's full of highly complex harmonics which makes the circuit sound rich.  It's not "clean."  It's dirty and complex.    The U47 I've heard sounded just like that, big and fabulously rich and complex.  Let's see what we can do in that direction with what's at hand.  I'm sure we can. 

The VF14 belongs in a museum at this point.  Let's see what we can do with a 12BH7 or something.  ;D

rodabod said:

This has been discussed a lot over at R/E/P.

I don't really know what to think since I don't have experience with using an original U47. I'm guessing the EF86 and Nuvistor follow-ups were thought to be crummy because the plate impedance was probably too high for the original BV8 transformer for a kick off.

Dale has mentioned here before why the original U47 circuit was quite a neat design which is probably worth researching.

As for a general answer though, my opinion on most recording gear which has a distinctive sound, often I find it has to do with the distortion characteristics, whether adding harmonics, or softening the overall timbre of a sound.

Click to expand...

There are endless unverified (and unverifiable) myths and legends about this tube. As all the people who designed and used it are no longer with us all this must be considered as pure speculation, at best... We should leave the pure subjective domain and try a more scientific/technical approach. No "Black Art" here (only a black tube!) , after all this is only an old triode-connected RF pentode in a metal bulb. His rarity and very high price doesn't necessarily means it can't be substituted. I doubt NEUMANN enginners then spent thousands of hours in comparative subjective listening tests,nor did they had thousands of different tube types to choose from. Selection of the VF14 was done strictly on technical engineering considerations and tube availability. These are my own (unverifiable) speculations to be added to the story: choosing the VF version was only a consequence of power supply implementation:low noise/ripple heater DC supplies were not easy to make then and it was much simpler to use the 55V/50mA heater version: all you needed was an extra dropping resistor from the already available well filtered B+ plate voltage.(and one less connecting wire !) A good and logical engineering trick then... when the VF14 was widely available !  Again, no Black Art here. Apart the heater ratings,an EF14 has exactly the same electrical spec's and there is no reasons why he should "sound different" ,if correctly implemented. None of these tubes were "specially designed for microphone use", the VF14 was simply "selected" for low noise and grid current and then re-labelled VF14M (like the earliest RE084K MIK used in the CMV Bottle). ANY tube can be selected for low noise/microphony/grid current. I once opened (actually,sawed) a defective VF14 and found nothing "magical or esoteric" inside,only a rather crude old radio pentode typical of that era. It's not the tube but the way it is used in the U47 which is special: as you probably know it is quite under-heated and this has  considerable effects on the electrical characteristics of the tube. (see other thread about lowering tube filament voltages), this was a deliberate technical choice by  the NEUMANN engineers, another well know trick then,also called "heater starved mode". Does ANY "VF14 Experts" ever took the time to actually trace the curves of a triode-connected VF14 at reduced filament voltage,exactly as used in the U47 ? It would be the first thing to do for any serious analysis purposes. So far, I've never seen this information. Of course, a EF14 used @ 6.3V will have different electrical spec's and will "sound different" ! At reduced filament voltage (and same cathode t°) it will be indistinguishable from his (50 times more expensive) VF brother. Due to their peculiar heater scheme there are unfortunately no know "plug and play" substitutes for the VF14 in the U47. With some changes in the power supply/wiring/cathode bias resistor there are LOTS of possible (cheap) substitutes : all you need is a triode (or triode-connected pentode) with the same electrical characteristics/curves as the under-heated ("starved") VF14,low grid current/leakage and low noise/microphony. Such a tube is not impossible to find, but stay away from 13CW4 nuvistors ! (probably the worst VF14 substitution ever !)  The U47 is still a wonderful-sounding mic with a good VF14 (and capsule!) I wouldn't touch it. Unfortunately,vintage genuine VF14's in good condition are extinct and will never be reproduced. (Sorry, TFKUSA !) At some point a substitution will need to be considered to preserve these mic's and it's time to break the myth that this tube has some "undiscovered/inexplicable magical properties" and can not be substituted without loosing all the U47 outstanding qualities. Nothing is farther from the truth.        

analag said:

So what are we gonna do when there are no more VF14...will we abandon audio? I doubt it. The old will speak of it and the young will ignore it...I stand with the young.

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I agree. There is nothing really special about the sonics of a vf14. I would guess the mojo comes form the fact they are rare.

tubologic said:

At some point a substitution will need to be considered to preserve these mic's and it's time to break the myth that this tube has some "undiscovered/inexplicable magical properties" and can not be substituted without loosing all the U47 outstanding qualities. Nothing is farther from the truth.

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Well, that was kind of what I was thinking about when I asked the question originally: what the VF14 was actually doing in the U 47 in terms of gm, mu, and resistance, and the ways those changed as you moved along the load line. Those are the first things we'd need to know when looking at possible substitutes, or emulations.

Peace,
Paul