U47 hand wired with VF14 and BV8

[mad.ax]

AC701 and AC761 are interchangeable, the schemo would not be altered, BUT,
filament wires are reversed!

On such direct heated triodes, the heater is kinda polarized. It cannot work well with AC, nor with inverted DC.
If you mis-wire it, the noise will go up. So you need to find out which filament wire is the +, and which one is the -.

This is critical whether you use an AC701, an AC701K, or an AC761.

You can read this thread where Oliver Archut explain it well, and posted datasheets for both:
http://recforums.prosoundweb.com/index.php/m/1717/0/

Axel

 

[speakercoil]

Thanks mad... So now another question about A701... I know everyone puts A701k... Does the "K" have any significance like the M on VF14's? I know the "M" on the VF's stands for "suitable for use with microphones" and refers to the quality check that Telefunken did on the tubes... So are A701k's the same sort of deal... Or is A701 and A701k the exact same thing?

Also while I am on the topic... I plan on videotaping all projects after this preamp and doing a DIY Audio youtube and vimeo account... Anyone interested in collaborating on this? Essentially offering advice on how to build different popular pieces of gear, and maybe videos explaining details on tubes, transformers, and maybe even design theory...

It wil be a few months before I am ready to start this up, but I think it would be incredibly useful to DIYers, and answer a lot of questions with the aid of visual aids. Reading about all of this stuff is useful, but the ability to show people will help tremendously. Anyway let me know!

-Grant

 

[Skylar]

RE: AC701 vs. AC701k
http://recforums.prosoundweb.com/index.php/t/2927/0/

As a general rule-of-thumb, if you have any questions about any microphone, chances are it has been answered either here or at PSW.
These things are easy to find through a search of google or either of these two forums' search funtions.

 

[strangeandbouncy]

If I recall,

  AC701 was made for military handsets, and was slightly different in construction. I could be wrong. IIRC, AC701k was selected for microphone work. Maybe the failures went to Army?


  I have just a suggestion for you. I have a beautiful u48. 15 years ago, I had to replace the VF14. I had a tech install a EF14 with new power supply. It sounds absolutely fantastic, and I often put it up against "proper" VF14 tubed u47's. It always wins. Don't believe everything that you hear. I am certain that it might sound better with the VF14, but I am not about to tear my pride and joy to pieces to find out. I am equally not about to shell out more than £1000 for a 50 year old tube that might go Phutt! next week! Why not build with EF14? they are much easier to find, and cheep enough to get a spare at the same time. I am sure you wont be disappointed. Get Telefunken, and better not from late war production. I had 3 from 1944. All microphonic. You can tell date from codes. I have some new old stock, and some part used, still with over emissions. All are fantastic, and apart from the Wehrmacht ones from 1944, are quiet. Maybe they had to use inadequate materials. They all sound the same to me. You don't want to make a mistake with a real VF14, and you can afford to make a mistake. I would suggest that this approach is better than monkeying about with an AC701. It is very important NOT to wiggle the leads close to the glass body, and they are very easy to destroy one with over heater voltage I believe. I was warned NOT to use a voltmeter to test  for continuity in the heater. I could be wrong. They are very robust in a correct circuit, and should out live you if you are lucky. SO should an xF14. If you find a dead m7 or kk47, I can highly recommend Thiersch to reskin both mylar and PVC. I have not tried his new capsules. I have used several of his reskins, all perfect. My buddy is a well respected mic-tech who has repaired hundreds of Neumanns. He always uses Thiersch to reskin, and has NEVER had any complaints, just praise and repeat business. If you read all the forums, youmight think that thirsch was a rank amateur! I hate this attitude. It sucks.

Think I would prefer a Thiersch to something taken from a Gefell. Not quite the same. I prefer kk47 to m7 personally. More bass extension, and cleaner top end. Less "bakelite" sounding. ALso. new capsule will last for ages . . . .

If you want to build something m49-like, check out the Sela t24 or t25. Uses AC701k, and is absolutely my best sounding mike. So much simpler than an m49 - far fewer components!


   just don't believe everything that you read. I think that many folks have something of a closed shop, and vested interests in perpetuating the "original-or-die" mentality. Maybe many of them don't actually RECORD with such mic's. I do.


   Happy new year,


   ANdyP

 

[speakercoil]

Hey Andy,

Thanks for clearing up the AC701... That makes sense.

As for making the U47 an exact replica... It is partially, because I don't believe anyone has really done it all the way... I kind of want to do it from a pioneering stand point. Everyone seems to cut a corner somewhere. Does that make the mic sound worse? Of course not... It many cases it probably tops the original in many ways... Less noise, clearer bass, etc... However I kind of want to do it for the sake of doing it. The M50 I am not as worried about. That I don't mind making slight mods too.. The parts of the M50 I am mainly worried about are the capsule and the body, because that is really what makes that mic special. Sure the rest plays a part, but I think a different tube would be fine in it.

As for monkeying around with a VF14... TRUST ME... I will not be testing the mic with it. Regardless of if I get a VF14 or not, I will put a RV14 in there first to test the sucker. I refuse to stick in a 2000 dollar tube to witness it burn out in seconds... I mean it would be more fun and less of a waste to light fire to 2,000 dollar bills piled up in my back yard. I will use the RV14 for all demo work... That way I can make sure I didn't screw anything up... If all goes well for about a week or two of testing, then I will throw in the VF14.

I have heard a lot of good things about the EF14, however I think I would go with an RV14 first (if I don't get a VF14)... This way I don't have to mess around as much with the schematic to make it work. Especially since I have seen multiple ways of achieving it online so far. I have also heard arguments for the Nuvister as well... But there is a lot more "bad press" on that tube than on the EF14 or the RV14... Actually the RV14 is supposed to have a nicer top end than the VF14!

Between a better bass or treble response... I think I would go with the treble... I don't plan on putting my newly build baby next to a kick drum or bass cab anytime soon... Though it would be nice for that "chesty" vocal quality on soft vocals. However I think the high end would be nice for things that it will be used on frequently like acoustic guitar, violin, and for the airy vocals.

The VF14 will probably be my last decision... and I will probably end up purchasing it last... possibly after the whole thing is built. I will probably spring for it though since I want to do everything else as close as possible.

I did think about purchasing a busted cap and then getting it fixed... However wouldn't that end up being just as costly as getting one in good shape or getting a new one? I wish I could find this link I had months ago of a guy in the UK who makes M7 and K47 capsules... He has been repairing neumann's for like 40 years or something like that... I remember stumbling upon it once and never being able to find it again!!!! I would recognize the website if I found it though.

I mean keep in mind... if I do this up right... I could end up with a more authentic clone than Wunder, Wagner, or any of those guys... At about the same price as purchasing one of their clones. That would be a hell of a feat....

-Grant

 

[speakercoil]

PS

HAPPY NEW DECADE!!!!!!!

 

[Gus]

I have not heard or seen inside a Wagner but it looks like it is very close
http://www.wagner-microphones.com/

 

[strangeandbouncy]

I believe he uses yet a different tube . . .EF12k I think. Perhaps he has a different output transformer to match(?). I think I have seen a Flea advertised with VF14. I know that they do both EF14 and UF14 versions. I have used UF14 version. It was much darker than my mike.

 

[speakercoil]

That's saying a lot since the U47 is kind of dark to begin with... Well I guess smooth is a better word than dark. As for the Wagner... He does use a different tube, but I don't think he specifies which one... I know it's not an EF14, because he mentions those tubes and then says his circuit doesn't need a dummy load. It says that he uses hand selected Telefunken steel tubes, but never specifies which model. However if the circuit is the original as it claims to be, this means it has to be a plug and play replacement for the VF14.

Now for a few new questions...

Bosch Capacitors... I am having a hell of a time trying to find these online. Anyone know a good source for them... Hell they don't even have to be NOS, but just in good condition. I thought about buying up a bunch of old broken telefunken electronics and salvaging parts... But that is just taking it a bit too far isn't it?

Resistors... Still no one has any answers on what kind of resistors were used? Brand, Type... anything? I mean I originally assumed they were carbon comb, but looking inside old U47's it doesn't seem to be the case. If I can't find the answer, I will just use a mixture of NOS "period" resistors.

-Grant

 

[MagnetoSound]

That's saying a lot since the U47 is kind of dark to begin with... Well I guess smooth is a better word than dark.

'Dark' is not a word I would ever use to describe a well-functioning U47.

Bosch Capacitors... I am having a hell of a time trying to find these online.

Not worth the trouble, really. There are plenty of PIO (and other paper film) caps to be found, including those in similar square cans and cylindrical packages. Look for the Russian caps.

Resistors... Still no one has any answers on what kind of resistors were used? Brand, Type... anything? I mean I originally assumed they were carbon comb, but looking inside old U47's it doesn't seem to be the case.

Certainly carbon comp, apart from the wire-wound filament dropper. If you don't see them they have been replaced. You can find NOS or current production versions, though the high values can be harder to find, but don't get hung up on it - modern film types are more stable and tighter in tolerance.

 

[strangeandbouncy]

I don't find the u47/8 particulaRLY dark . . . just the Flea I tried. It might be just that one. There is NO Telefunken metal tube replacement for VF14. I am pretty sure he uses EF12k. He probably has separate supply for heater, so no dummy load needed. General consensus is that make of resistor not important, just maybe TYPE. They were produced for many years, and I am aware of many variations. Modern film is much more stable, and easier to find. Doubt if much difference in sound. As for Bosch caps, the early ones have big square caps. I remember someone saying they were designed for V2 rockets. Who can say what is "authentic?", if there are more than one type? I personally think you will find more variation between capsules than caps, but hey . . . I would prefer a nice new cap over anything that might fail . . . . Someone mentioned Russian paper in oil caps, that are built from machinery appropriated from Eastern German factories after the war.



EDIT. Magneto Sound beat me to it . . . diddn't mean to duplicate

 

[speakercoil]

So I found out they were wire wound resistors made in house... So no good getting my hands on those. I will have to get a custom made wire wound solution. As for the capacitors... If I cannot find any Bosch, I guess I will just go with something hi-end like auricap... However it would be REALLY sweet to find some NOS Bosch... However they seem to be impossible to find.

 

[speakercoil]

Yeah I read that about them being made for rockets as well. I guess the company Bosch is still around too, but they no longer make capacitors... In fact it doesn't even seem like they do anything with electronics anymore. I guess if I can't find NOS Bosch then I will go with something hi-end like I mentioned... Auricaps would be nice, because I know they won't be adding anything to the sound.

As for the resistors... Yeah I agree it's more "type" than "brand" that I am looking for. I guess for the carbon comps I will go with Allen Bradley since those are sort of the correct time period and used in many old school pieces of gear... Also the price is right on them and I have found them in some of the extreme values. A Neumann tech on Neumann's site said the resistors were all wire wound and made in house... However maybe this is just referring to the filament dropper as you stated...

I know a couple of hi-fi companies which produce wire wounds and will do custom ones for specific values... Which resistor si the filament dropper anyway? I am good with schematics as far as assembling things, but not very good at the whole "design theory"... So it is hard for me to point at a part and say "this is for this" or "this is for that".

I also played with the idea of using extremely high end components for these areas... Such as bulk foil resistors, etc... However I think I would like it to have that vintage charm that NOS components make... Both in aesthetics internally and sound coloration. I know metal film are tighter and more accurate, but part of the slippage is what gives vintage electronics their individual characteristics.

And yes... I agree not "dark"... Hence why I said more "smooth" than "dark". When I said "dark" I meant that the high end is not overbearing like a lot of modern mics tend to be.

I think I found my capsule btw... I found someone selling a vintage M7 bottle head and might just dissect it and remove the capsule. It's kind of a shame to do that to the head, but it is worth it for the real capsule.

For those in the same boat as me... Here are some useful links I found today.

http://www.neumann.com/forums/view.php?bn=neumann_mictec&key=1192301288&v=f
http://www.tangible-technology.com/microphones/u-47.html

Oh and the inside of the U47 I have been looking at has been modified... It's a nuvister... So yeah it definitely has been tampered with, but it does have the old school square Bosch in it.

As for the wiring I am going to go with a silver wire I think... Just for clarity reasons, and going vintage on the wire would be WAY overkill... Not that trying to find the same caps and resistors isn't... 

Well back to research...

-Grant

OH... Also I read somewhere of someone modding out a U47 to use a AC701k... I thought that was kind of funny considering I brought up the AC701k for an M50 build and not the U47, but then I found someone using it for a U47!

 

[speakercoil]

VF14 question... Where on the tube is the "m" stamped... I am finding lots of VF14's for sale, but no one is specifying "m", but are still charging a fortune for them... Just curious so I can ask for pictures proving it is an "m" and not just going on their word.

-Grant

 

[Johan]

VF14 question... Where on the tube is the "m" stamped... I am finding lots of VF14's for sale, but no one is specifying "m", but are still charging a fortune for them... Just curious so I can ask for pictures proving it is an "m" and not just going on their word.

-Grant

http://www.filmsoundsweden.se/backspegel/ror-pdf/vf14-forum.pdf
lots of info on tube mic's on filmsoundsweden ..most in swedish thou..perhaps there is a good online translator somewhere..

j

( I've been reading up on SELA mic's the last couple of days, being extra intrigued by learning they where produced next door from where I live...about 20meters from where I write this.. )

 

[strangeandbouncy]

Johan,


  Do try to bring a Sela back to it's birth place! I think that the T24 or T25(only difference is m7/kk47) are my favourite mics. I have a T25. Like an m49, only better.(but no pattern switching.) I am very keen to try the older 6AK5 tubed versions one day.


  Kindest regards,



    ANdyP

 

[Sorr]

"Auricaps would be nice, because I know they won't be adding anything to the sound."

Are you sure about that?

 

[rodabod]

Which resistor si the filament dropper anyway? I am good with schematics as far as assembling things, but not very good at the whole "design theory"... So it is hard for me to point at a part and say "this is for this" or "this is for that".

It's the 17K8 resistor.

 

[speakercoil]

You know... Out of all the boards across the internet, this board always surprises me with how quick and accurate it's responses are... So thank you to everyone.

Well as far as the auricaps go... They are very clear with little color addition... Unlike others which tend to color the sound (mostly in desirable ways)... There are caps that are more transparent, but cost much more than auricaps. I have used Auricaps before, and have been very happy with the results.

Thanks for such a detailed picture... So those M's are REALLY visible... I don't think any of these were M's then... a few had an N underneath it, but I have not seen a giant M stamped on any of the pictures they have shown.

Sela mics... Just did some research as I had never heard of them before... They look like they would sound amazing. Crazy, I have seen soooo many beautiful rare mics before, but have never stumbled upon these beauties.

I live close to where Charter Oak Mics and where Neumann's USA offices are which is pretty cool.

Cool now I know to get a 17k8 wire wound... and the rest I guess I will go carbon comp... probably allen bradley.

-Grant

 

[mad.ax]

About the 17K8 resistor...
This is the problem with that supersonic schematic... R4 is written: 1780Ohm
Easy to confuse for a 17K8... I have done that very mistake myself!
The correct value, as shown in the original Neumann schemos, and as confirmed by calculation, is 1780 Ohms.

Axel