U47-style bodies - who can make one?

[Skylar]

Finally, next round of samples coming next week.

 

[synchroman]

This is great news... Thank you for your hard work... I am working on a design for the 1780 Ohms resistor

 

[Benny]

Great news Skylar.

 

[Skylar]

I am working on a design for the 1780 Ohms resistor

Why not just use this:

 

[Skylar]

Or if you just HAVE to use wirewound...

Mouser Part # 72-RWM410-1K8-5

 

[synchroman]

I am trying to make a historically accurate U47... Also this resistor creates a lot of heat and should use the body of the mic as a heat sink

 

[Kingston]

You're over-thinking it. Whatever that bent wire-wound was doing is completely unnecessary these days. Won't affect sound, except modern resistors are lower noise.

 

[synchroman]

Oh but it will effect the heat inside the mic.... Bad for the tube, transformer, and capsule

 

[Skylar]

Oh but it will effect the heat inside the mic.... Bad for the tube, transformer, and capsule

If you're worried about heat, regardless of which resistor you use, just mount it against the body for heat sinking.


Heat vs. Capsule:
The capsule is in a separate chamber from the main circuit (and your hot resistor)...nothing to worry about.


Heat vs. Transformer:
I seriously doubt that the heat radiated by the 3W resistor inside a 60mm aluminum tube is going to damage or change the performance of a transformer.
Any experts have an opinion on this?


Heat vs. Tube:
Read how a tube works if you don't already know. The tubes that we typically use in audio have heaters that we run current through, which indirectly heats a cathode, which then emits electrons.
Tubes are designed to withstand high temperatures; they generate a lot of heat already during normal operation.
Depending upon the tube you use and its operating points in the circuit, the tube is likely to be generating more heat than the resistor.
Don't worry about the hot resistor affecting the tube.


I agree with Kingston that you're over-thinking it.
I certainly don't mean to offend, but using an existing, readily-available resistor is the simplest & most logical solution.

So, why then did Neumann heat-sink their resistor?
My guess is to protect the resistor itself, not the other components.
If memory serves correctly, there's over 2.5W of power to dissipate by that resistor.
Personally, I would use a 5W resistor there.


Of course, this is all just my opinion—I could be wrong.

 

[Hank Dussen]

And of course if you use a tube mic upside down, the heat rises away from the capsule.

 

[ppa]

there are no problems for the tube for the heat, it was designed to work with other tubes near. Inside old gears, there were often several tubes that worked together.

 

[synchroman]

I am overthinking this and I do not mean to hijack this thread...

The original U47 runs at about 60 degrees centigrade internally - Modified U47 run at about 72-90 degrees centigrade

Heat vs. Capsule:

M7 capsule are made of PVC and PVC does not like heat...

Heat vs. Transformer:

"Regarding the heat sink resistor, the power dissipated inside the mic is the same regardless if you have the resistor on the body tube wall or inside, but the one that has the resistor inside has a higher temperature inside the mic that means all components are subject to a higher heat. Parts like the x-former do not like heat and in average a higher temperature with a nickel core x-former will reduce the inductance what means less low end."

quote from Oliver Archut of TAB Funkenwerk

http://recforums.prosoundweb.com/index.php/mv/msg/22971/0/0/0/


Heat vs. Tube:

"Degenerative failures cause the performance of the tube to slowly deteriorate with time.

Overheating of internal parts, such as control grids or mica spacer insulators, can result in trapped gas escaping into the tube; this can reduce performance. A getter is used to absorb gases evolved during tube operation, but has only a limited ability to combine with gas. Control of the envelope temperature prevents some types of gassing. A tube with very bad internal gas may have a visible blue glow when plate voltage is applied.

Gas and ions within the tube contribute to grid current which can disturb operation of a vacuum tube circuit. Another effect of overheating is the slow deposit of metallic vapors on internal spacers, resulting in inter-element leakage.

Tubes on standby for long periods, with heater voltage applied, may develop high cathode interface resistance and display poor emission characteristics. This effect occurred especially in pulse and digital circuits, where tubes had no plate current flowing for extended times."

http://en.wikipedia.org/wiki/Vacuum_tube

"Tube life is sharply dependent on temperature, which means that it is dependent on filament or heater operating voltage."

http://www.vacuumtubes.net/How_Vacuum_Tubes_Work.htm

 

[MagnetoSound]

Don't forget that the VF14m in a U47 is marginally underheated to begin with, in part to prolong it's useful lifetime.

It is a good idea to heatsink the resistor due to it's proximity to the transformer, but part of the reason it is down there in the bell to begin with is to keep it away from the tube.

 

[ppa]

Heat vs. Tube:

I don't know the tube schematic but I suppose that's important the following thought on mic's tube temperature.

We should do to discriminate the following two tube's sections:


1) the tube's section that uses the electrons to make the tube an active device, the section formed by plate, grid and catode for to be explicit:
this section's life depend from: plate current, plate voltage, temperature.
Since, in a mic the tube is used with a low plate current and plate-catode voltage to reduce the noise as well as possible (consistent with the need to have a good gain), an over ambient's temperature isn't a problem so high in my opinion.

However, if we speak of tube temperature, it's important to write the tube's temperature depend by the power made by itself, the heater's power and the ambiet temperature.

Since, the 1st power depend by plate current and plate voltage (being this power = plate current X plate voltage ). So a low plate current and a low plate voltage give lower temperature than in a normal case, so the high ambient temperaure tends to be compensated by the lower plate current a plate voltage.
But the most important thing is that in a tube lower is the power dissipated by its plate lower is the effect of the temperature in its safe.
 
2) the heater section that makes the free electrons for the tube current:
this section runs at high temperature so it's less sensible to high ambient temperature than the 1st section, but , since its temperature depend also by tube temperature and, as I've written, this temperature isn't so higher than normal, there is not a real problem.

I'm going to view the schematic.

Pier Paolo

 

[Skylar]

Heat vs. Capsule:
M7 capsule are made of PVC and PVC does not like heat...

Again, the capsule is in a separate chamber. The heat from the resistor will not affect the capsule.

Heat vs. Transformer:

"Regarding the heat sink resistor, the power dissipated inside the mic is the same regardless if you have the resistor on the body tube wall or inside, but the one that has the resistor inside has a higher temperature inside the mic that means all components are subject to a higher heat. Parts like the x-former do not like heat and in average a higher temperature with a nickel core x-former will reduce the inductance what means less low end."

quote from Oliver Archut of TAB Funkenwerk

http://recforums.prosoundweb.com/index.php/mv/msg/22971/0/0/0/

It makes sense that the inside temperature of the mic will increase if the resistor is not heat sunk to the aluminum body.

"Parts like the x-former do not like heat"

This statement is worthless without an actual number.
How much "heat" is he talking about? 50°C? 150°C?

For what temperature (or temperature rise) is your transformer rated?
As I understand it, transformer temperature rating has more to do with how much heat the insulation can handle.
So I think you'll be melting insulation before you will affect the performance of the transformer.
I believe most audio transformers would use Class A insulation, which should be rated for 105°C.

Also is this even true..."a higher temperature with a nickel core x-former will reduce the inductance"?
And how much temperature rise (above 40°C ambient) would we need to see in order to measure a meaningful reduction in inductance?

I'm reading Radiotron 4, and I cannot find any relation between inductance and temperature regarding transformers or coils.

Heat vs. Tube:

"Degenerative failures cause the performance of the tube to slowly deteriorate with time.

Overheating of internal parts, such as control grids or mica spacer insulators, can result in trapped gas escaping into the tube; this can reduce performance. A getter is used to absorb gases evolved during tube operation, but has only a limited ability to combine with gas. Control of the envelope temperature prevents some types of gassing. A tube with very bad internal gas may have a visible blue glow when plate voltage is applied.

Gas and ions within the tube contribute to grid current which can disturb operation of a vacuum tube circuit. Another effect of overheating is the slow deposit of metallic vapors on internal spacers, resulting in inter-element leakage.

Tubes on standby for long periods, with heater voltage applied, may develop high cathode interface resistance and display poor emission characteristics. This effect occurred especially in pulse and digital circuits, where tubes had no plate current flowing for extended times."

http://en.wikipedia.org/wiki/Vacuum_tube

"Tube life is sharply dependent on temperature, which means that it is dependent on filament or heater operating voltage."

http://www.vacuumtubes.net/How_Vacuum_Tubes_Work.htm

I think these two articles are warning about the dangers of applying a higher-than-specified voltage to the heater instead of increased ambient temperature.

They're saying that overheating the internal parts above what they're designed to handle will dramatically shorten the life of the tube.
Being that the tube and its internal components are already very hot, it would take some seriously hot ambient temps to degrade these internal components.

How hot is typical for these internal parts during normal operation?
Well, let's find out how hot the Cathode must be in order for thermionic emission to occur.
In Principles of Electron Tubes on pg. 22, Reich says this point is above 1000°K (or >700°C).
If the Cathode is heated indirectly, the heater filament must be even hotter (~1200–1300°C according to Electron Tube Design pg. 216)
Pages 261–264 of Electron Tube Design show how to calculate temperatures for the remaining electrodes (Plate & Grids).
These work out to between 300–500°C.

Do you think a resistor dissipating less than 3 Watts of power in a mic body is going to raise the ambient temperature enough to push the internal components of the tube to a point where tube longevity will be affected?
I certainly do not.

Check out some tube data sheets for maximum bulb temp.
This will be from 170–300°C from what I've seen.
How high do you think the ambient temp would have to be in order to get the tube above this rating?

 

[Skylar]

I'm running OCR (optical character recognition) on

Principles of Electron Tubes
Electron Tube Design
Inside the Vacuum Tube

so that the PDFs are searchable with Acrobat.

I will upload these to the GDIY gmail once finished so that you can read through my referenced texts.

 

[Skylar]

Just uploaded a new version of the Radiotron 4 PDF to the Google Docs section of the GroupDIY Gmail account.

This version is fully unlocked and has no printing/copying/extraction/commenting/etc. restrictions.

But more importantly, I ran OCR on the full document so that you can search, select, and copy text.

I also uploaded a few other old tube books with full OCR.
Inside the Vacuum Tube - John F. Rider 1945
Principles of Electron Tubes - Herbet J. Reich 1941
Electron Tube Design - RCA 1962

 

[synchroman]

Great...

 

[maxwall]

I realize the original U47 power resistor by todays standard is completely obsolete by design However....

I suppose the real question about the big resistor is why did they use it instead of just using a typical wire wound power
resistor , it seems to me that at that time ceramic wire wound resistors were available just look in the original
U47 power supply. If one resistor was not enough two could be run in parallel to dissipate power in the form of heat.
Nevertheless, they used wire wrapped around a rectangular piece of cardboard insulated by two pieces of mica plastic
a pressed up against the bottom bell area of the mic. Very odd. Could it be that this resistor has some kind of inductive
quality about it as well as its resistance value ? As we know , most inductive chokes in B+ power supplies occur at the
end of the chain as some kind of filter to smooth out the dc voltage. In this case, the cardboard resistor is placed similiarly in the chain after the last resistor and filter capacitor from the power supply, but seperated by a short or long cable run. My second question is why do they put the cardboard resistor in the mic rather than the power supply box ? I'm thinking the distance in the mic cable between the mic and power supply could have something to do with this and might contribute a capacitative effect.

These are just some general questions I have always wondered about in the original design of the U47 , but I realize that
today's designs have acceped that a modern equivilent 1780 ohm resistor be used in place of the original cardboard wrapped one. However, in a small percentage of the original design,  I still feel the U47 designers knew physics and electronics intricacies on a level we are still trying to understand today.

 

[riggler]

Brainstorming:

#1. Making that resistor in that fashion required substantial effort on their part at that time. There must have been a motivation.

#2. Making a resistor like this has a much larger surface area, thermally coupling to the mic body.

When against the mic body like this, the body becomes a pretty efficient heatsink, and heat can be dissipated off the body to outside air.

When the resistor is conventional, heat dissipates to the air (not too efficiently) around the resistor IN THE MIC BODY and through the leads to other components (much more efficiently).

Now, what components will drift in value with temperature? Carbon resistors! Take a look at this article for an eye-opening experience:

http://www.cliftonlaboratories.com/carbon_composition_resistors.htm

How warm does it *really* get inside the mic body? Maybe these guys knew something about component aging and heat. Maybe not failure, maybe more like consistency and tolerances for intended operation.

So maybe this is all mitigated by using metal films, etc...

Now as far as inductance... Maybe the larger coils of this "Cardboard U" resistor reduce some inductance effects, maybe they found ringing with a "standard" resistor?

Good read:

http://www.physicsforums.com/showthread.php?t=67998