A word on Carbon composite resistors.

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I’m quite a big fan of carbon comp resistors, but I know they’re a double edged sword.  This is a small guide to show where I use them, and why.
It goes without saying that there’s no magic inside this type of resistors, their construction and materials lend itself to their specific characteristics.

Carboncomp resistors consist in a few millimeters of resistive carbon composite material. While signal across a film resistor will pass through several cm of material and will surely produce some amount of inductance, signal across carbon comp resistors won’t.

However, they are very noisy, drift easily and are expensive and difficult to find nowadays if you’re looking for NOS brands. Moreover, they show a high voltage coefficient of resistance. That means that the value in ohms of the resistor itself will vary depending on how many volts you put across it. This variance is directly proportional to the volts applied, so high amplitude signals will get considerably distorted across a carbon comp resistor.

How does this distortion sound?? Some would say the difference compared to film resistors is almost unnoticeable, and the money you pay for them and the hiss they produce make them worthless.
I’d say they’re great if used in the correct place, cause that distortion produce some kind of sweet pleasant sound I really like.

That said, do not expect a big difference!!! At the end they’re only resistors. Changing the speaker, the first stage preamp tube, or the resistors’ value itself to name just a couple of things, will make a great difference compared to changing that resistor from metal-film to carbon comp.
As you see, it’s better to judge for yourself if they’re worth the money and use them sparingly in order to find a good compromise between the sound they produce, and noise.

I use them in places where there is a high DC voltage ( more than 100V), and a big signal swing. Moreover, I choose the maximum resistor dissipation to be high enough to withstand two times the average dissipation of the resistor, but don’t go further beyond that. All that will make the resistor distortion higher and more noticeable, keeping the resistor safe.
Plate resistors are a great place where this requirements are met, so I always use carbon comps as plate resistors.

Similarly, the amp input, cathode resistors, and preamp grid resistors are a bad place to use carbon comps, so I use film resistors. I try to put special attention in the first stages of an amp and in those places where the resistor value is higher, as resistor noise is proportional to its resistance. In those cases I tend to use good quality Dale Metal films like the ones in the image below.

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All that said, and once you’ve decided wether if you want to use carbon comps or not, there are several brands to choose. Allen Bradley, Radiospares, Stackpole …

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I like AllenBradley RCxx or mil-R-39008 RCRxx resistors and use them most of the time. The latter are established reliability types and usually have a fifth yellow ring as shown in the above image, failure rate according to mil specifications is less than 0.001% per 1000 hr. They’re good NOS carbon comp resistors, got long leads and take good solder. I wouldn’t go for new production carbon comps, they’re almost as expensive and I’d bet their characteristics are closer to film resistors than to old carbon comps.

That’s all!! Everything I wrote has already been said before here and there, but I wanted to share my personal opinion about this after much reading about the subject and my own experience. Hope you find this useful!!

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~ by fperezroig (22nanofarads.com) on March 5, 2013.

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