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[River]

Hi Everyone -

I've got a U47 that has had some pretty shoddy work done to it in the past that I'm trying to correct.

This mic is missing it's 1780 ohm resistor, although the mounting plate and mica insulators are still installed.  In it's place are a series of resistors in a TO-220 type package, which, due to the way they're mounted, make it difficult to replace the correct tube in the socket.  This particular mic has had a 407A tube installed.  I'd like to correct the electronics and start the search for a decent correct replacement.

Is there anyone who is making a replacement for the wirewound resistor?  It seems like kind of a simple part, but I can't find any kit manufacturers or replacement parts manufacturers who are making these.

I'd like to mount the resistor back where it belongs if at all possible.

Alternately, what are other people doing if this resistor goes bad?

Thanks!

Tim

[klaus]

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.

I would contact Wagner in Australia, Telefunken North America, TAB Funkenwerk, and FLEA: All of these companies at one point or other used to market U47 copies with VF14 as an option. Maybe they still have one of these resistors lying around. Ina pinch, you may want to investigate winding one yourself on a piece of heat-resistant cardboard. I alos heard of one person who made one out of the heater wire of an old toaster.

Please let us know when you find one, so others can be helped as well.

[River]

I suppose that one could be made using standard nichrome resistance wire around a high temperature cardboard, as you recommended.  I don't think it would be too hard to make.

This particular mic has 3 Ohmite TCH series resistors mounted in series (1K, 750, and 47 ohm) with each one mounted to one of the ribs of the microphone.  There was a crude piece of metal joining two of the ribs at the mounting point to add additional heatsinking.

It worked, but not as simple and elegant.

Anyone have any further thoughts on the correct material to try to recreate one of the original style resistors?

Tim

[klaus]

Try the easy route first: contact the companies I mentioned.

[J.J. Blair]

I've tried making one before, and it was a failure.  Insulating the wire from itself proved problematic.  Oliver has made them in the past, but I don't know if it's anything he has in hand. 

I know this is going to sound sacrilegious, but one idea I've thought of in the past is too take advantage of one of the unused wires in the cable, like #3, using an external an separate resistor in the PSU, and feeding the 36V directly to the heater through contact #3.  What is the downside to this approach, that I'm maybe not thinking of?

[klaus]

For starters, you would need to completely redesign the voltage divider network in the mic AND add such network in the power supply.

You would also need to consider unforeseeable side effects to the sound of the mic: what is the sonic effect on the tube's performance if the ambient heat it is operating in is drastically changed? (The mic's body cavity heat stems to a large part from the wire wound resistor heating up the cavity-the tube itself barely gets hand warm).

[J.J. Blair]

The warm body I understand.  I've actually measured the inside heat of U47s with historical resistors, and with non heat synced cement encased power resistors.  The lower air temperature change inside the mic should not really affect the performance of anything.  My understanding is that the real concern is the transformer, because the nickel core causes bass response loss, when heated over a certain temperature.  I provided these findings to a U47 clone maker who was using the latter type of resistor, which was causing extremely high temperatures inside the mic.

[klaus]

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.

[Kai]

...wire-wound with a standard 10W ceramic...

there are metal-cased resistors quite similar to ceramic types.
The resistor element (wirewound) is isolated against the case, so it could be screwed directly to the bottom bell. Get yourself a 1.8 KOhms one - that's close enough to 1780 Ohms. Or select one out of some - 1780 Ohms lies within the tolerance of a 2%.
They look like this (10W resistor):
http://docs-europe.electrocomponents.com/webdocs/0099/0900766b80099672.pdf

Regards
Kai

[klaus]

Yes, familiar with the surface screw-in type, and thanks for the suggestion. 

But I see several problems:

* No space to mount a straight resistor in a circular bell of limited diameter, AND use the existing frame screws and bolts as attachments. Mic components are in the way, mainly two large capacitors

* I also doubt that enough heat-sinking can be accomplished, given that little or none of the resistor's main body can have direct contact with the bottom bell with this resistor shape

[J.J. Blair]

Once upon a time, I took somebody's T-USA 47 and replaced concrete encased resistor with two flat power resistors in series, totaling 1800Ω.  I made some device to keep them as flush as possible to the bell, and then had some heat sync grease, like you would use on a heat synced IC, in-between the wall and the resistor.  Worked pretty well, and the temperature inside was fine.  It was so long ago, though.  I don't remember the brand.  I posted photos somewhere in this forum, and you might be able to find them.