U47 TUBE Issue

[sr1200]

OK the  bud of mine that has the EMT 140 plate also brought this thing out with an issue.  First off, the power supply has some NASTY leaking caps that need to be changed (pic in next post) but beyond that, the mic is making a low pitched "fire" noise that I've heard before when a tube starts to go (or when a cap is mis-behaving).  Been trying to find a replacement tube for it (RCA 13CW4 Nuvistor (39z)) since a tech came and told him the tube he had in it (telefunken original was no longer good, and swapped it out for the RCA... the mic has worked beautifully up until a couple days ago).  Anyone know where I can procure NOS caps for the PS and a replacement tube for this?  Would telefunken or neumann have these things?

 

[sr1200]

Thats not tape residue.... and theres this grey gunk oozing (now hardened) out of the top of the caps.  Again, if anyone knows a source for NOS parts, id rather try and keep things as "original" as possible.

 

[gemini86]

Personally, I'd stuff modern parts into the old cap cans to take advantage of modern cap technology and retain the original aesthetic. But that doesn't answer your question... sorry.

 

[hop.sing]

http://www.die-wuestens.de/dindex.htm
Search that site, he has got replicas of those beautiful old caps. Modern technology in there, too, I guess. I would not use NOS electrolytic caps, makes no sense at all...

 

[mjrippe]

Agreed, putting vintage caps in a power supply is just asking for trouble.  For the Nuvistor, try Leeds Radio on North 7th in Williamsburg. (www.leedsradio.com)

 

[sr1200]

Having a heckuva time finding caps for this thing.  I checked the linked site, best i can tell, the closest thing is a 40uf 450v cap (which are going for about $50 ea USD without shipping)!!! YiKeS!

 

[sr1200]

http://www.mouser.com/ProductDetail/Cornell-Dubilier/3186BA400T450APA5/?qs=sGAEpiMZZMvwFf0viD3Y3U0mFRNR81Zy3CExS3PyHwY%3d

out of curiosity, anyone care to chime in on this choice?  These might be a bit too wide though.

 

[mjrippe]

Do you need screw terminals or can the caps be soldered in?  47mfd @ 450v caps with wire leads are available from Mouser starting at $1.77 each!

 

[Mike Cleaver]

Lots of Nuvistors on Ebay.
Search by tube number for the best deals and try to get an RCA one.
As for caps, if you're not worried about the power supply looking the same as with the old ones, any good quality electrolytic of a similar or larger value will work just fine.

 

[hop.sing]

You cannot link to the things on the wuesten site, which is a pity, but look for that Number: KON5650
That should be the perfect replacement for your defective capacitors. It looks, feels and screws like the old one, but might be a little smaller.And costs something like 8 Euros a piece.
I have used them a lot in old radiorepairs and tube stuff from the 50s.

 

[sr1200]

So increasing the value by 10uf is ok?  (I know going to a higher voltage rating is ok) Is that because these are filter caps that you can do this?

 

[mjrippe]

Increasing the value (within reason) is nearly always OK.  Old caps were generally rated +20/-10% so a 40mfd could be ~36-48mfd when new. 

 

[sr1200]

Very cool to know thanks.

On to one other question.  Originally , the mic had a VF14 tube in it, it was fitted with a different tube socket to fit the 13CW4 nuvistor.  Anyone have any input on performance of the two (differences, similarities) and would it pay to go back to the VF14 socket/tube (EXPENSIVE TO FIND) which is probably why that mod was done.  Also, would anything have to be changed on the mic to go from one tube style to another.  It doesn't look like any other components were added (except for maybe a WIMA looking cap thats suspended from the top of the area where the headbasket mounts.

 

[Moby]

Very cool to know thanks.

On to one other question.  Originally , the mic had a VF14 tube in it, it was fitted with a different tube socket to fit the 13CW4 nuvistor.  Anyone have any input on performance of the two (differences, similarities) and would it pay to go back to the VF14 socket/tube (EXPENSIVE TO FIND) which is probably why that mod was done.  Also, would anything have to be changed on the mic to go from one tube style to another.  It doesn't look like any other components were added (except for maybe a WIMA looking cap thats suspended from the top of the area where the headbasket mounts.

As I can see your 13CW4 adapter is not original Neumann's . See the pic of the Neumann's mod from 1970.

I hope that person who did it followed the original documentation. If you want to go back to VF14 few reverse mods must be done before that, check the document. BTW, Nuvistor can sound great  with slight different tube bias point. In that case you end with same K² distortion point but less noise than any VF14. Only bad thing with 13CW4 is that tube is more microphonic liable so it must be selected. Also, you can call Mr. Andreas Grosser and ask for his VF14ER or VF14EF tube. It sounds great Hope this helps

 

[sr1200]

Moby thanks for the info, great document!  I think I should have him keep that nuvistor in there since the tube is so hard to get.  You dont happen to have a document on setting that bias point do you?  I ordered a new (well, NOS) tube for the mic. which I hope is good enough to solve the issue.  If not, could the poly cap thats sitting on your pic (the one im working on has a slightly different size, but hopefully same value, be an issue?

 

[emrr]

Increasing the value (within reason) is nearly always OK.  Old caps were generally rated +20/-10% so a 40mfd could be ~36-48mfd when new.

1960 and before electrolytics in many cases are rated +100/-10%, and I find this to be true in practice, with many old/good caps reading much higher than the target value.  I find this true of non-electrolytic caps too.  Look at it as a time when they were concerned not so much with an exact value, but instead a value that met a minimum standard.

 

[Pip]

Hi:

Sadly it won't do to go back to the VF14 tube. Personally I have never seen one outside a mic that was good enough to be inside a mic. Some people have gotten lucky and found a pull that has some life left but that is rarer and rarer these days. There is also some cryo freezing techniques that some very reputable people have mentioned in there forums/blogs. All of this is big buck territory, think thousands. You could swap the Nuvistor for a UF14 but then you need to add a filament circuit to the PSU and do some rebuilding of the mic guts.

There is another option but get  your wallet out. This guy is the man and does some of the best work out there. His name is Andreas Grosser and he is a legend in the Neumann community. He has a FET solution but he must install it in the mic himself as the PSU must be to spec. http://www.voxorama.com/service.html

Lastly Telefunken-Elektroakustik is offering repair and restoration service and does have a "NEW" VF14 option but again pricey. http://www.telefunken-elektroakustik.com/services/mic-repair.php

As to the Nuvistor there are a lot on ebay but if you want a source that will deal with problems try:

http://www.pelusomicrophonelab.com/parts/Tubes.html

Seems that you have sorted the filter cap situation. If you can handle another opinion; When it comes to old PSU's I say replace all the caps and check for spec on the resistors as well. A mic is only as good as the quality of the power its getting. A PSU in spec can only help a mic not hurt it. Unless there are other problems. It is possible I suppose if your capsule is old, and of the PVC variety, that bringing the Voltages back to spec might send and old tired capsule south but it would happen sooner than later anyway.

http://www.mouser.com/Nichicon/Passive-Components/Capacitors/Aluminum-Electrolytic-Capacitors/Aluminum-Electrolytic-Capacitors-Leaded/_/N-75hqw?P=1z0wrk9Z1z0z7l5Z1z0x818Z1z0zlbm

 

[Pip]

Agreed, putting vintage caps in a power supply is just asking for trouble.  For the Nuvistor, try Leeds Radio on North 7th in Williamsburg. (www.leedsradio.com)

Yes I didn't think of Richard my bad he is the man and being in the NYC area you should probably get to know him. http://leedsradio.com/

 

[Moby]

Moby thanks for the info, great document!  I think I should have him keep that nuvistor in there since the tube is so hard to get.  You dont happen to have a document on setting that bias point do you?  I ordered a new (well, NOS) tube for the mic. which I hope is good enough to solve the issue.  If not, could the poly cap thats sitting on your pic (the one im working on has a slightly different size, but hopefully same value, be an issue?

I'glad you find the document  useful. If you need more info you can drop me email, also, If it's not too late try to order Sylvania nuvistor, they are usually less microphonic. Regarding cap, yes, everything is possible after 50-60 years but drop me some more detailed pictures so I can give you piece of advice.
Cheers

 

[sr1200]

Ill post what i have here