U47 custom wirewound resistor

[tomas.borgstrom]

Hello, my name is Tomas and I'm from Malmö, Sweden. Although I've been repairing, modifying and building studio gear for a couple of years this is my first post in a forum. I primarily service microphones and I'm currently working on restoring a vintage U47. When I got it most of the interior hardware were missing and a hole was drilled through the back of the body. After some research I found the answer in a picture on the fantastic page filmsoundsweden (only in swedish) that showed a U47 modified in 1981 to a custom FET circuit by a Swedish company (AVOX).

Fortunately the custom flat 1780 ohm wirewound resistor (R4) that drops the voltage to the filament was still there but the connection was bad on one side. I actually managed to fix this by wiring it back one turn, scrape of the outer coating and resolder it. Now to my specific questions:

1. Does anybody know what material you can use if you want to wire one of these resistors yourself? My own research ended up with enamel coated nichrome wire. What material can you use for the flat core? Remember this resistor gets very hot during operation.

2. These resistors are surrounded by some kind of oil, which I assume is to increase the contact with the microphone body to cool the resistor. Does anyone have any knowledge regarding this? Is it possible to use the standard white thermal grease?

 

[mhelin]

Welcome! There was thread about the subject here: DIY - wirewound resistor

 

[tomas.borgstrom]

Ok, thank you for the reply. A bit embarrassing I missed that thread since I've digged quite deep in the forums regarding info on original parts in vintage tube microphones. I'd like to quote remsouille since that post is really informative on this matter.

Hi guys, i just wanted to let everyone know that I successfully wired a resistor for my U47 using 173ohm/meter 42AWG Isotan insulated resistance wire.
You can easily find insulated resistance wire on ebay. The original U47 resistor used Silk+enamel insulated manganin wire, which you can find as well.
It is important to use a material that is around 170/200ohm/meter so the count turn is ok and the resistor fits in the cavity.
Under 170ohm, the wire would be too thick and the resistor would be too big, over 200ohms, the wire would be too thin and wouldn't handle the 40miliamps.
This was validated by Andreas Grosser a couple of weeks ago when I showed him my U47, though he seemed to have never thought of building some!
I wounded mine around some 1/1.5mm vulcanized fiber (also sometimes called fish paper, it's the same stuff that's used for guitar pickup bobins), but I'm almost sure neumann just used regular stiff cardboard!
The mica sheets used to insulate the resistor from the body can also be found on ebay (like these https://www.ebay.fr/itm/FEUILLE-DE-...076951?hash=item2f38f58357:g:WogAAOSwjDZYchjb
but in a pinch, you could use a thin teflon sheet.
It really is an easy build, just needed a bit of research!

The only question that remains is the use of some oil or thermal grease to increase the contact with the bell. Anyone who has any experience?