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What type of resistors are best?

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7/4/2003 6:14 PM
Thijs de Vries	What type of resistors are best? What are the advantages or disadvantages between carbon comp, carbon film, metal film, metal oxide or any other resistors that may exist?

7/4/2003 7:58 PM

Left out wirewound.

The one that's got the main disadvantages is carbon comp. The mixture's inhomogeneous, so they're quite variable as manufactured. They're temperature sensitive, their value rising quite a bit (and rather unpredictably) from heat of soldering. They're noisy, hissing and crackling as the current jumps from one carbon particle to another in the muddy mix. And they deteriorate over time, straying even farther from their nominal values and becoming noisier still. They may have "mojo," but it's not good mojo.

7/4/2003 8:26 PM
Thijs de Vries	so that leaves carbon film, metal film, metal oxide and wirewound. I read metal film is the most quite and most accurate but can cause alot of damage if they are exposed to too much voltage. Is this true?

7/5/2003 3:45 AM

Any resistor can arc over more or less disastrously if exposed to too much voltage, and the voltage rating is sort of proportional to the wattage rating. I don't have the ratings handy in front of me, but if you're running B+ in the 400-500V range, I think you may want to use at least 1W resistors everywhere they'll have full voltage across them. Most places in an amp, of course, they won't see that much voltage. Carbon resistors, of course, have flammable material inside them and many resistors have the body molded of material that can burn. "Flameproof" resistors add a little bit of safety. I like metal films, myself. I can't tell that metal oxide ones sound any different. I'll use what's handy, including carbon films and occasionally carbon comps. But I just don't like carbon comps. I've had to replace too many of them that have gone bad in old radios and stuff. Maybe it's just my bad luck, but I've had more nonfunctional equipment from failed old carbon comp resistors than any other component.

As for wirewound resistors, they can be made to very precise tolerances, and they're very heat tolerant. But most of them have the wire wound in a coil and have significant inductance that can be a problem at higher audio frequencies and above. There are noninductive wire resistors, but I don't commonly see them for sale.

7/5/2003 11:42 PM

Sleebo_The_Beebo

Usually I find that carbon resistors go bad when some other component fails such as a cap shorting out that causes the resistor to pull lots of current. Whenever I find a bad resistor, I usually replace any caps around it also, for safety and reliability reasons.

I don't have tons of experience at this kind of thing but the only resistor I've ever had go bad by itself was a wirewound in a PS that opened. Seems the more heat you have on a resistor the more it is prone to open up. And it seems they always open up rather than short when they go bad. Moral of the story is to use high wattage values, way overrated.

But I agree with you that carbons drift overtime and are probably less reliable than films.

7/5/2003 2:52 PM

Low noise + hard work

John,

I totally agree about using a good carbon film/metal film/wirewound for lower noise applications. I will say that I disagree with you regarding the carbon comps in this one instance: I used a bunch of NOS Allen Bradley carbon comps from Angela Instruments in my 5E3 and 18 Watt. Well, both amps measure less than 8mVAC residual hiss/hum/noise at idle. Of course, that is after I spent some serious time tracking down and fixing a few mistakes I made in layout. To my delight, they are both extremely quiet and well behaved amps.

So here is where it lies for me: If you are careful in your application/layout of components, and your components are within spec, usually a tube amplifier can be built to be extremely quiet--even using the carbon comps.

I may have been lucky here though--I won't rule that one out. I may have been blessed with a decent batch of components. However, I spent enough time prodding and experimenting to get less noise--I attribute the low noise to hard work.

Jason

7/5/2003 4:47 PM
Thijs de Vries	regarding low noise. I hear by making sure all the grounds connect to a single point and are grounded from there is less noisy than just connecting everything to the chassis. Is this true? Would I notice a huge differance if I rewired my JCM800 2204?

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