DIY - wirewound resistor

Hey folks,

I'm building my own U47 and there is a 1780 Ohm wire wound resistor that looks like that:


Is it possible to do it on my own?

As base i have cut a the inner cardbox of a role of duct tape. Fits good!
As wire i would use this: http://www.spulen.com/widerstandsdraht-0-032mm-1650-ohm-meter-klasse-bis-1ppm-k.html
As I reckon in the worst case 1510 Ohm =1000mm
so                                                            1 Ohm = 0,66 mm
                                                                1780 Ohm = 1178 mm

the cadbox has an hight of  13 mm and a lenght of 65 mm. -8 mm for the connection = 57 mm.
as the wire is wound around this carbox i got ~ 26 mm for one winding.

1178mm/26mm  =45 windings
So there would be 8 windings per 10 mm.

I reckon that over this 1780 Ohm resistor will be a Voltage drop of 69 Volt.
From this i recon 0,039 Ampere flowing.
So there will be an load of 2,67 Watts.

2,67 Watts with a 0,034 qmm resistance wire?
I dont have a clue!

Cheers,
Zottel.

0,034qmm can easily handle 40mA of current. But I don't know how hot your self-wound resistor will get when under load. You simply have to try it out and see what happens.

Wouldn't it be easier to buy a couple of 1800 ohms wirewound resistors, and select the one closest to 1780 ohms?
(The difference between 1780 and 1800 is only 1%!)
I would prefer a commercially made resistor over a home-made one on a piece of cardboard...

Very interested in the results and heat generated as I need to refurb an original and would prefer not having to drill mounting holes into the base for a power resistor.

Anyone know of any other non-drill options?

Thx!
-jb

Get Zottel to build you one, he's now an expert! I would just buy a standard ceramic wirewound 1800 ohms, there are plenty of holes inside a 47 to attach a tagstrip  to.

Haha I'm not an expert at all, thats why I ask such questions  ;D

@RuudNL: My thoughts exactly! But someone in the Neumann Forum described the problem like this:
"Neumann build these wirewound resistors becaus they fit perfectly to the body, this results in a much better heat dissipation than a new wirewound resistor."

I guess i will give it a try with a factory build one. I still will be trying to attach it directly to the body with some PTFE Isolation between the housing and the resistor. If the mic get's so hot i cant touch it anymore i will try to build this thing on my own.

@0dbfs: why dont you use the metal part thats behind your resistor to clamp in the new resistor. I guess you could use the mica thats allready in your mic to isolate this "construction".

> 13 mm and a lenght of 65 mm

Sorry to work in units of a dead king's foot.

13mm = 0.52 inch
65mm = 2.56 inch
1.33 square inches one side
2.66 square inches both sides

A _ROUGH_ approximation is 100 degrees C rise per Watt per square inch

2.67 Watts on 2.66 square inches is 100 degrees C temperature rise.

In the open in a warm 30 deg C room it will reach 130 degrees C.

Well above boiling water.

Inside a small can, it will run hotter.

Pressed firmly against the can wall, it may run a little cooler.

130 deg C is 266 degrees Fahrenheit.

There is a famous (in English) book Fahrenheit 451, which assumes that the ignition point of book-paper is 451 deg F. (May really be 440 to 470 deg F.)

While a book may not start burning at 450 (or 439) deg F, it will "char". it will turn brown then black, and get very brittle.

I would use cheap cardboard at ~~130 deg C ~~266 deg F for a few minutes on the test bench, ready for smoke or fire. I would not put it INside a microphone.

I would expect hard wood to last longer. Thin veneer is easy to find. 3/4" (19mm) veneer for the edge of a shelf is a standard home fix-up product. Often comes with heat-glue on it. Trim to 13mm, wrap around a can, heat it, then cut to length.

I bet the original base was "electrical board". Strong clean paper mixed with Phenolic resin. The same stuff low-price terminal-strips and printed circuit board are made of. In thin sheets, also called "fish paper". CJ knows it as an insulator for transformers and motors. Our German friends may know what the word is around Leipzig. Or if you take the old one to a shop who re-builds transformers or alternators, they may have it (probably a more-modern stuff even tougher than the original). It will however be flat. Maybe you can get 2 or 3 thinner sheets, epoxy glue, and a curved form, glue-clamp it so it holds a curve.

How are you going to attach that fine Ni-Chrome wire to the terminals? Just wrapping it will not work for long (oxidation). As far as I know, it will not solder (lead/tin), and is very hard to weld (melt the Nickle). You may be able to weld it with a car battery, heavy wire to two copper bolts.

I have a fantastic set of wooden handled kitchen spatulas that say not to use above 451F.  Branded onto the wooden handles.  Best in-joke I've seen on a commercial product.

Carry on....

Thanks again for your replies!

I was talking about it with a friend and he also told me about this "fish paper" stuff. He is an machine engineering student and he want's to help me out on this. His father is running a museum about Konrad Zuse and they own a lot of stuff even old radio equipment, CNC Cutting machines etc. So he will be in the boat.
As we thought about building this resistor it came to my mind If some guys of you would like to get one too?

We will begin building in march and test it afterwards. When its working we would be open to make you guys an offer.

How are you going to attach that fine Ni-Chrome wire to the terminals?

Click to expand...

The terminals on a wire wound pot seems to be pressed or wrapped around the wire.
Could it be the same "cold welding" technique as in the wire wrapping method?

Would it be possible to use thin mica wafers instead of paper?


Konrad Zuse, cool!

For those who don't know this "father" of the computer:
https://en.wikipedia.org/wiki/Konrad_Zuse

About the wire: http://www.spulen.com/widerstandsdraht-0-032mm-1650-ohm-meter-klasse-bis-1ppm-k.html?___store=english&___from_store=default

Its written down in the details sections:
Its possible to soft solder this wire. You have to remove the oxide coating and solder it with unleadet solder.


About the mica:

Lets quote some guys from this post:  http://groupdiy.com/index.php?topic=36948.40
mad.ax :
"More about the 1780 Ohms resistor.
This resistor will be dropping 105-36=69 Volts. You need at least a 3 Watts part.
Because of the precision required, and the need to heat-sink those 3 watts to the body, this was built in-house by Neumann. Definitely not an off the shelf part.
I don't know if it is an original one cause I don't see the usual mica layers, but attached is the only pic that I have found (on saturn's website)

You need good thermal conduction for proper heat-sinking, and of course electrical insulation. This was done with mica.
Heat-sinking is not as much necessary to protect the resistor itself (you could use a 10W part and forget about it), but to protect the transformer from excessive heat."

You can see the mica on this picture:

Just want to add some Stuff Klaus Heyne posted on the Neumann board:

"My experience has been that replacing the original flatboard wire wound resistor with a new commercially available resistor of correct value and wattage to work with the UF14 develops significantly more heat inside the U47's body cavity. That's why most conversions were done by feeding in separate heater and high voltages through new cabling and power supply. You are correct- the current draw stays the same, but the total lack of heatsink ability inside the mic cooks the body cavity beyond what I consider safe and stable. Neumann's ingenious design consisted of perfectly sandwiching a wire resistor between a curved metal plate and the body's housing bell, and distributing its heat throughout the mic's housing surface for cooling. Most heat felt when touching a stock U47 with VF14 tube after a few hours is that of the 1780 ohm sandwiched resistor, not the tube. As you might have observed, even with the optimal heat sinking the tight space allows, that heat is substantial. "

"For VF14 operation, the wire resistor is crucial: its design- sandwiched against the mic's bottom bell- allows for optimal heat dissipation of this rather large voltage divider: It needs to dissipate the bulk of the incoming voltage/current: down to 36VDC from 105VDC@40mA.
Alternatives, in the form of installing regular resistors, are flawed: resistors without proper ability to heat-sinking them, get so hot inside the mic's amp cavity that the tube's performance is affected: plate voltage drops and noise increases. Besides, every component will be baked and eventually will become unreliable with so much heat generated."


"Ambient temperature measurably and audibly affects VF14 plate voltage:
I have made tests where I used a fan to disperse what little ambient heat si generated by the wire wound and the the tube itself. Plate voltage rose by a volt or two, and tube noise retreated by a dB or two.
Then I made the opposite test: used the same, stock, tube supply voltage, but replaced the wire-wound with a standard 10W ceramic placed inside the mic. Then closed up the mic again, and ran it for a few hours: Plate voltage dropped to below acceptable values, with accompanying high noise levee, due to extremely high ambient temperature around the tube."

You should take Mr. Heyne's  advice with a grain of salt. As I know, he never suggested anything except the original in U47 because
........., you know.
This resistor can be nowadays done much better.  The original wounded as antenna acts also as  some sort of EM noise picker.

For example, you can use few caddock MP725 non-inductive resistors distributed (soldered ) arround  the thin round metal tube which is inserted and fixed using thermal compound into the chassis bell. This will be much better heater resistor than original.

http://www.caddock.com/Online_catalog/Mrktg_Lit/MP725.pdf

True story about Klaus!  8) I was thinking the same when iwas digging deeper into the Neumann board.

It's so hard to make a decision! 

I dont get how you would mount the MP725's.

I had a precision power resistor similar to the caddock in an EF12 (?) modded original U47 which became very hot and without adequate heat sinking transfer would also desolder itself.

That's a completely different case wrt: power dissipation from a traditional U47 setup but helps to illustrate the point.

Another approach is to manufacture an appropriate sized aluminum heatsink with a rounded side that fits nicely in the bell and a flat side for better mounting of the dropping resistor (s). It's a tight fit!

Thx!
-jonathan

Zottel said:

I dont get how you would mount the MP725's.

Click to expand...

:

I'm sure  you've seen this,  but this is what Oliver Archut did in the Lucas CS-4:

http://lucasmicrophone.lefora.com/reply/22855446/NEW-MICROPHONE-CONSTRUCTION#reply-22855446

Thanks for the drawing moamps! I have to try out some things.
Aluminium heatsinking  seems interresting! I got a CNC machine at hand ... so i guess this tight fit should be feasible.
How do you fix the Caddock's to stay in place?

As I'm reading about the method that Oliver Archut used, I'm getting more and more concerned about this small part. Do you guys think the temperature inside the microphone is THAT!!! important?

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-MICA-POUR-POELE-A-BOIS-59-X-81-DEMI-TARIF/202819076951?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!

Thanks for the report. 

I’d forgotten this thread.  I had my MK47 next to a real one last summer, and did note the much hotter body of the MK47 at the base where the resistor is mounted. 

What has Warm Audio done in their version?