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Old 17th June 2007, 03:24 AM   #1
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Default carbon composition - 2 types

Allen Bradley composition resistors seem to be the favorite. As you probably know, they have the solid slug element. In the 60s and 70s there were probably 20 different companies that made the slug type. Today there are about 7 companies making them.

There was, and still is, another type of composition resistor. It was made by TRW/IRC and is still available through IRC, but it is made for IRC by Kayama-Japan and there is no stocking distributor to be found.

This TRW/IRC (the TRW aspect of the name has since been dropped) resistor has a glass tube inside with a thick composition film on it. It is not spiraled and is purported by TRW to have better high frequency characteristic. As far as I can tell (TRW did not answer my email) it has the same surge capability as the slug type. It is not carbon film, it is composition film and drifts with age as the Allen Bradley type does.

I am looking for opinions on carbon composition.

(1) Is the Allen Bradley better than other slug types?

(2) Is the TRW type with the glass tube undesirable in any way?

It seems to be the consensus among technical folks that the reason the carbon comp resistor is unique is that the voltage coefficient of resistance is very high. That is, the resistance goes down when the voltage across it gets high resulting in a unique type of distortion. When used as plate resistors they distort the amplitude of the waveform.

The TRW type has the same voltage coefficient as the AB type.
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Old 17th June 2007, 02:38 PM   #2
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Investigate....Farnell also now do them....typical part/stock number 1265132 for 100K.

richj
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Old 17th June 2007, 07:15 PM   #3
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You have found yet another supplier. These appear to be the slug type.



I wasn't sure if this forum or musical instruments would be the right one.



Perhaps I didn't make the question clear.


I am trying to found out if carbon comp users do not like the TRW film on glass tube type. Also, is there a preferred composition resistor in general?
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Old 18th June 2007, 03:32 AM   #4
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Quote:
Originally posted by hailteflon

I wasn't sure if this forum or musical instruments would be the right one.

I am trying to found out if carbon comp users do not like the TRW film on glass tube type. Also, is there a preferred composition resistor in general?

I'm using metal oxide and metal film for both HiFi and MI.
Although in the past I've used carbon types in front end stages, the typical cracked carbon hi stab 2-5%% types aren't around anymore so I've had to resort to modern metal types to keep pentode 1st stage noise down. Despite the voltage v.s resistance not being linear with carbons I've not had any undesirable musical effects using on metal film types, metal on glass etc. I choose mainly the working voltage parameter.
I strongly dispute claims by those in the HiFi ratpack that can really tell differences. If there are subtle differences then as the audio frequency is so low compared to RF then something else in the circuit i.e amp circuit & output transformer quality is not up to par or using old tubes. Tube amp layouts are often so birds' nested that any extra inductance created by wiring far out-swamps any internal component inductance, and worse still loudspeaker manufacturers often use wirewound typs for crossover networks. So be it !
Sorry if I've taken my gloves off ?

richj
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Old 18th June 2007, 06:20 AM   #5
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Quote:
the typical cracked carbon hi stab 2-5%% types aren't around anymore
What do you mean by cracked carbon?
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Old 18th June 2007, 08:47 AM   #6
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Stuff from the junk box. Top LHS older vintage wax impregnated rod power carbons. Nom cold value as stated. Hot value +50% not unusual..
Top RHS rare wax impregnated <high stab cracked reworked carbon> with pink and other stability ind bands.These held values within stated % cold and hot. Hard to find.

Lower RHS bog standard carbon res. Some 20% values measure 50% Ohmage increase. Drift with time. Most vintage carbons can survive massive B+ surges whereas m/o types can change.

Lower LHS modern 1% 2% m/o film and glass TRW lowest. Stable cold and hot.

The most signifigant change is wattage v.s downsizing. The metal film power grey (lower left) can accept same power with hardly any coefficient change compared to wild temp drift of the top left rod types.
Considering most of the carbon types were fitted during the 1940-60s TV and audio sets, the sound quality of some vintage tube amps with these resistors fitted is still just as good as the modern stuff. Proof that tight tolerance is never required for tube work.

Carboot sales in the UK often come up with such goodies.

richj
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Old 18th June 2007, 09:26 AM   #7
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Quote:
high stab cracked reworked carbon
When you say "cracked" do mean that the resistor was cracked and they repaired it? possibly with wax?

The lowest on the LHS may be a TRW metal film, but it is not a carbon composition. TRW cc are molded dark brown. I don't think there is any glass in the TRW.

The fourth from the top on the RHS is a RCR20 (M39008) military carbon comp. That is about as good as they get. They are out of production worldwide. NASA published a bulletin about how they have no supplier. I found the NASA document on the web while googling carbon comp.

The three at the top on the RHS are what you are calling wax impregnated? They are so dirty they would have to have wax on them.

I recognize those, from sometime somewhere. Are those of British origin? The pink band is strange. Have you broken one open to see if they are carbon? Some of that type are wire as I recall.
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Old 18th June 2007, 11:00 AM   #8
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It's the carbon that's "cracked" not the resistor.
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Old 18th June 2007, 11:26 AM   #9
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Quote:
It's the carbon that's "cracked" not the resistor
Are you speaking of a chemical process, or are you talking about spiraled resistors? Is it "cracked" in the sense that it has a crack spiraled into the film?

I took a second look at the fourth resistor on the RHS of Richj's pic.

That doesn't look like an RCR. What are the colors on it?
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Old 18th June 2007, 03:14 PM   #10
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Radiotron hb/4th ed doesn't digress deeper than we are here.
The pink salmon end colour types are high stab, the cracked carbon is a solid composition but the carbon has been structually altered, (akin to annealing steel etc at the upper critical temperature structure is finer grained then processed.)
Farnell also stocks a carbon ceramic but that is spiralled with restricted values i.e stock no's 1219226.

I squeezed the larger 47K, this is cat cracked carbon BS.1852 1952 stock and despite faulty end caps the carbon is finer in one rod. I can bust some other open but need more time to set up EOS etc.

This may seem the perfect specimen which one is after, soon after carbons were no longer manufactured, the world went into helix/spiral construction.
I will look for a typical specimen out of the junk box.

richj
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