Microphonics in VF14 and U47

[rock soderstrom]

As I understand it, this tube was manufactured exclusively for Neumann and German Radio by AEG/Telefunken

That is not quite right. The VF14 was actually developed and also used for other purposes. Neumann then used this tube for his U47 and probably ordered most of the tubes of this type, but there were also other applications in reality. The VF14 was not only produced exclusively for Neumann and the German radio stations.

Maybe this applies to the M versions of the VF14, but I can't prove it for sure. Does anyone have a historical source for this?

Some info, in german. Scroll down to the comment section.

https://www.radiomuseum.org/tubes/tube_vf14.html
Edit: only the AC701 (Edit: and the MSC2) were exclusively designed for microphone use AFAIK. (e.g. for Schoeps and Neumann)

 

[Richmix]

It doesnt much matter what tube you have in a U47. a Nuvistor sounds just as good. The 1`3CW4 nuvistor was the official replacement for VF14.You could also use the 7586 if you adjust the filament voltage with the series resistor.7586 was used in the AKGmics.

 

[gyraf]

That is not quite right. The VF14 was actually developed and also used for other purposes. Neumann then used this tube for his U47 and probably ordered most of the tubes of this type, but there were also other applications in reality. The VF14 was not only produced exclusively for Neumann and the German radio stations.

Maybe this applies to the M versions of the VF14, but I can't prove it for sure. Does anyone have a historical source for this?

Some info, in german. Scroll down to the comment section.

https://www.radiomuseum.org/tubes/tube_vf14.html
Edit: only the AC701 was exclusively designed for microphone use AFAIK. (e.g. Schoeps and Neumann)

Yes, I agree that this is the official version - but three things make me suspect that it for some reason is not quite correct:

1) I have never been able to find any circuit (other than mics) that uses the VF14. I have an application note decribing a UHF>VHF converter - but have never ever seen or heard of this used "in the wild"

2) "Diese Röhre ist vor allem für die verwendung in Spezialgeräten und Spezialschaltungen bestimmt" = This is a special-purpose tube for special apparatus. I take this to indicate that the tube was designed for a specific purpose and customer (NDR) but was also allowed to be sold to others that could benefit from some of its special characteristics. Would be a good way to recuperate design costs? Only thing is that apparently no one ever did:

3) ..a total production of 5-6000 tubes of a certain type is absolutely nothing, barely enough to make Georg happy over those years, and certainly not enough to be used in general industry..

At least there's something here that dosen't quite add up..

/Jakob E.

 

[soliloqueen]

maybe it has a second designation and they're in loads of places and people just haven't noticed yet

 

[granger.frederic]

It doesnt much matter what tube you have in a U47. a Nuvistor sounds just as good. The 1`3CW4 nuvistor was the official replacement for VF14.You could also use the 7586 if you adjust the filament voltage with the series resistor.7586 was used in the AKGmics.

Yes.
I noticed that 13CW4 , and other higher internal resistance triodes (ef12 for example) in the U47 operation, (or penthodes in triode mode) needs hotter biasing (said previously in this forum)
7586 and other lower internal resistance triodes , often need high end adjustment with a film capacitor (50 pf to 330pF) between plate and ground, to match the response of the original VF14.
The operation point changes the internal resistance (or output impedance), amongst other parameters, and is set by the tube itself, the plate resistor and bias voltage...
when set up properly, a tube performs very closely to a vf14, and will do the job, even some small differences still exist, to be honest, but nothing critical for very good recordings...
You may sometimes even prefer the tube replacement...

 

[TLRT]

Yes, I agree that this is the official version - but three things make me suspect that it for some reason is not quite correct:

1) I have never been able to find any circuit (other than mics) that uses the VF14. I have an application note decribing a UHF>VHF converter - but have never ever seen or heard of this used "in the wild"

2) "Diese Röhre ist vor allem für die verwendung in Spezialgeräten und Spezialschaltungen bestimmt" = This is a special-purpose tube for special apparatus. I take this to indicate that the tube was designed for a specific purpose and customer (NDR) but was also allowed to be sold to others that could benefit from some of its special characteristics. Would be a good way to recuperate design costs? Only thing is that apparently no one ever did:

3) ..a total production of 5-6000 tubes of a certain type is absolutely nothing, barely enough to make Georg happy over those years, and certainly not enough to be used in general industry..

At least there's something here that dosen't quite add up..

/Jakob E.

It was used in HF radio devices(those went quickly obsolete in the fifties I think), here you can read more infos in the comment sections:
https://www.radiomuseum.org/tubes/tube_vf14.html

 

[gyraf]

Do we know if it was actually, really, used for HF-purposes - or was that only a "recommended use"? As mentioned, I have never ever come across it anywhere outside a mic, and I've been actively interested in the topic for now some 45 years.. (time flies!)

 

[rock soderstrom]

1) I have never been able to find any circuit (other than mics) that uses the VF14. I have an application note decribing a UHF>VHF converter - but have never ever seen or heard of this used "in the wild"

I actually saw an VF14 "Allstrom" domestic radio receiver with schematic on ebay years ago.
These things were probably not that rare, but became obsolete relatively quickly due to their outdated technik and ended up in the rubbish.

"Diese Röhre ist vor allem für die verwendung in Spezialgeräten und Spezialschaltungen bestimmt" = This is a special-purpose tube for special apparatus. I take this to indicate that the tube was designed for a specific purpose and customer (NDR) but was also allowed to be sold to others that could benefit from some of its special characteristics. Would be a good way to recuperate design costs? Only thing is that apparently no one ever did:

Sorry, no. The tube was clearly designed for other "Allstrom" applications such as UHF reception and HF jobs as needed in a radio/radar environment. One should also consider the time context when this tube was designed. There was an increased demand for such tubes in the military sector. Rumour has it that the VF14 had a experimental predecessor with similar specs in WWII. However, the VF14 was not an active "Wehrmacht" tube as its production only began after the end of the war.

If this tube was designed for microphones, it would be a triode (like the AC701). Why build a pentode and then use it exclusively as a triode? It was taken because there was no suitable "Allstrom" triode!

"Allstrom" (or better the high filament voltage) is actually the key to explaining why this tube was used. One could do the job with a well-filtered single voltage PSU.

3) ..a total production of 5-6000 tubes of a certain type is absolutely nothing, barely enough to make Georg happy over those years, and certainly not enough to be used in general industry..

Other figures say that over 27000 VF14s were produced in Berlin (West). RFT in the GDR produced aswell some VF14.

Edit: "Allstrom" means that the unit can be operated with AC or DC voltage. At that time, there was not yet a completely homogeneous electricity grid in Germany. Some radios were also operated with batteries. There was no electricity grid on the Eastern Front.

 

[Tubetec]

Regarding AC701 being the only tube designed for tube mics ,
What about Hiller's MSC2 tube ?
It also had the top cap style grid connection ,

 

[rock soderstrom]

Regarding AC701 being the only tube designed for tube mics ,
What about Hiller's MSC2 tube ?
It also had the top cap style grid connection ,

Good point, I think you are right. It seems that the MSC2 was exclusively designed for mic use, too. This is also stated in the data sheet.

I've edited the corresponding post.

 

[gyraf]

Thanks - this makes more sense now..!

 

[Johnny1234]

A buisness would still lose money creating and manufacturing this tube even if they charged 5000 per tube. Enjoy them while they are still alive.

 

[Tubetec]

Hiller also produced an AC701 equivalent , MSC4 ,

 

[Whoops]

I have never ever come across it anywhere outside a mic, and I've been actively interested in the topic for now some 45 years.. (time flies!)

I also never seen it outside Neumann microphones, and I also beleive if the VF14 was used in other devices that because of it's scarcity nowadays and the high value people pay for it that if there were other devices that used that tube that would be pretty well known and those devices would be scavenged just for the tube.
saying this Radium Museum website states 2 devices (other than Neumann Mics) that used the VF14:

https://www.radiomuseum.org/r/bogenwo_novaphon_tonbandgeraet.html
https://www.radiomuseum.org/r/hewel_ukw_vorsatz.html

 

[rock soderstrom]

https://www.radiomuseum.org/r/bogenwo_novaphon_tonbandgeraet.html
https://www.radiomuseum.org/r/hewel_ukw_vorsatz.html

Thanks for the links. The second one shows the unit I very likely saw on ebay years ago. I remember that it was equipped only with two tubes, it also looked like this or very similar.

It is a "UKW Vorsatz", so a ultra shortwave VHF reciever for retrofitting an existing radio system.

The reason why there are not many more units with VF14 tubes is simply that the "Allstrom" technology became obsolete relatively quickly after the VF14 appeared, there was no longer any need for it.

Strictly speaking, the VF14 was already an almost discontinued model with its appearance, since this transitional period was quickly ended after the war. There was quickly a unified power grid, no one needed such devices anymore. "Allstrom" devices do not have a mains transformer, so they are not very safe and you have to elaborately ensure that the user does not come into contact with the mains voltage. (no galvanic isolation due to a mains transformer!)

Just to avoid any misunderstandings. The U47 was not an "Allstrom" device but used a tube that was built for it. The reasons from my point of view, as already mentioned were the simple "one voltage" power supply and probably also availability immediately after the war.

Both of the devices linked by Whoops are "Allstrom" devices.

 

[rock soderstrom]

View attachment 109615
Hiller also produced an AC701 equivalent , MSC4 ,

The Hiller company has a very exciting history, which is great described in the following link! Turn on the translator

https://www.jogis-roehrenbude.de/Roehren-Geschichtliches/Hiller/Hiller.htm
Edit: The Hiller MSC2 was many times more expensive than the VF14, which clearly shows who was the specially designed tube at the time. The VF14 did not cost much more than its EF14 and EF12 steel tube siblings at launch.

MSC2= Microphone Specialtube with one C ( for triode system!)

 

[Marc HENRY]

As soon as I stumble upon one I will test it. I've designed my own VF14 replacement tube with one 408a (I know it's uncertain if it's actually a 408a inside VF14k) and a small circuit around it. I will present it with some sound clips some time in the future.

I selected the 408a with the least amount of noise and microphonics. I also fixed it with silicone to the socket. This measurement is made in a 1:1 clone of my short body made with Flea hardware.

View attachment 102740
The microphonics are low but something unwanted happens around 100Hz. Perhaps some component in my clone is responsible for that.

Vf14k is still a 408a. Sound is not good enough to give the will to do any reverse engineering to my opinion... When your vf14 IS dead, add one heating wire and give it an ef12 or anything else.

 

[tomas.borgstrom]

Vf14k is still a 408a. Sound is not good enough to give the will to do any reverse engineering to my opinion... When your vf14 IS dead, add one heating wire and give it an ef12 or anything else.

The sound of 408a varies quite a bit between the various brands. They respond differently to the bias as well. I find that lowering the plate resistor makes them sound closer to VF14. It might not be a perfect replacement but you don't need to modify the U47 circuit (parallelling the 100K resistor can be done and reversed in 30 seconds).

EF12 is probably the best alternative to VF14 but you need to do a lot more to the microphone and PSU than just adding a wire. A U47 without a VF14 will of course drop substantially in value but I would expect an even larger drop if the microphone is modified for another tube. If you build a U47 copy from ground up it's better to use something else than 408a (if your not obsessed with building an exact clone of the electronics).

 

[Drea]

Noise is almost not an issue because after the mic there is almost no noise. No tape no mixer, no medium
I almost say the noise make this mic THE MIC if it’s general.

 

[rock soderstrom]

The sound of 408a varies quite a bit between the various brands. They respond differently to the bias as well. I find that lowering the plate resistor makes them sound closer to VF14. It might not be a perfect replacement but you don't need to modify the U47 circuit (parallelling the 100K resistor can be done and reversed in 30 seconds).

What total anode resistance value do you suggest for the 408a tube? Just another 100k in parallel with the existing resistor?

I am currently experimenting with the 6.3V siblings (5654/6ak5w) of the 408a.