R/E/P Community

Please login or register.

Login with username, password and session length
Advanced search  

Pages: [1]   Go Down

Author Topic: Reversing 13CW4 nuvistor modification to U47  (Read 4065 times)


  • Sr. Member
  • ****
  • Offline Offline
  • Posts: 275
  • brad allen williams
Reversing 13CW4 nuvistor modification to U47
« on: September 08, 2013, 01:43:26 PM »

One of the owners of our studio just acquired a long-body Telefunken-badged U47 that had been previously modified to use the 13CW4 nuvistor in place of the VF14. We will probably also need to source a good power supply-- the supply that came with the mic appears homemade (and a Nuvistor-suitable PSU is slightly different from a VF14 supply anyway, correct?)

Obviously, our objective (whether now or in the near future) is to reverse the modification and install a good working VF14.

Finding the VF14 will obviously be the most challenging part. I have located the original 1968 service bulletin describing the conversion (here). Oddly, it does not contain a schematic for any of the changes-- it merely gives a step-by-step on how to complete the original modification (and no information on reversing it).

The subject of buying good tubes has been discussed here before (i.e. return policy, etc). This will all be taken under advisement-- however, if anyone knows anything we would appreciate any leads on where we might begin our search.

The nuvistor in our mic appears to be the plug-in type, which bodes well for the modification reversal. It also appears that the resistors in the mic have been replaced with inferior inexpensive carbon film types. However, the capsule is a K47 which looks original and good. I'll be able to check into the mic more closely today and form a full status report.

I'd appreciate greatly any info that could help us on our way.


  • Moderator
  • Hero Member
  • *****
  • Offline Offline
  • Posts: 2124
Re: Reversing 13CW4 nuvistor modification to U47
« Reply #1 on: September 08, 2013, 04:06:38 PM »

Neumann's instructions are correct: aside of lowering the (original) power supply's output a tad, from 110VDC to 105VDC, by removing the doubled-up resistor and putting it back in series with the others, no additional work is necessary, when you have the AR47 Nuvistor plug-in.

That was the beauty of the AR47 kit.
Klaus Heyne
German Masterworks®


  • Sr. Member
  • ****
  • Offline Offline
  • Posts: 275
  • brad allen williams
Re: Reversing 13CW4 nuvistor modification to U47
« Reply #2 on: September 08, 2013, 11:31:21 PM »

Thank you Klaus for that info.

I had a chance to check out the microphone today. Considering the nuvistor, it sounds quite nice. The back side of the capsule seems to sound a little nicer than the front side. There's also a bit of noise, and the home-made power supply is amateurish and has to go... I'm sure it's not doing the sound of the mic any favors.

I took some detailed photos. I'd love any insight into what we have here.

Serial No. 3082

BV8 transformer-- "BV No. GN 8"

K47-- would this have been made in 1963 or 1973? An older replacement perhaps, or perhaps replaced when the Nuvistor retrofit took place?

K47 rear side... a bit dirty, but looks nice enough to me.

This 1µf capacitor is branded Bosch and has the designation "MP." I'm not sure about the meaning of the star of David ink-stamped on it, but it's certainly peculiar. I'd rather not speculate on the origins of that mark. Is that a date code of 1956?

I'm not sure if this green capacitor would've been original or not, but it doesn't look familiar to me-- I suspect it's some sort of replacement. Its make is "ERD" in a shield logo, and it says "MKT 1813" along with "0,47 µf" "±10%" and "400V RD"

Tiny 1/4W carbon composition resistors up top, in the 100M and 60M values. A little surprising to me that they'd have used carbon comp resistors in that location-- they don't look like other 47s I've seen. Unoriginal? The orange .01 capacitor... is that an axial ceramic? Odd choice for that location-- is it normal?

One side

...and the other. A clearly-unoriginal mix of low-cost carbon film and metal film resistors here. I'd suspect these should be changed out for something nicer. No clue why they were replaced in the first place. The 100k carbon film hanging off the end looks odd, but I'm not looking inside 47s every day.

One end of the cable has this connector that looks like it may have once been a part of a stand mount, but it's missing a piece. Not sure if it's even possible to locate the part to complete it.

Would love any info, input, or thoughts.
Pages: [1]   Go Up

Site Hosted By Ashdown Technologies, Inc.

Page created in 0.058 seconds with 21 queries.