Resistors: Carbon Film Versus Carbon Composition

[admin]

My amp tech has found noise in the preamp stage of my Bassman 50, apparently the plate load resistors may have deteriorated.

He has asked if I would be willing to replace them with carbon film versus carbon composition types?

I understand that the Carbon Film resistors are warmer in comparison but this may be at the expense of increased noise, the very reason that the amp is in for servicing to begin with.

I would be interested in any recommendations that amp techs may have.

[soundmasterg]

Well carbon comp has that traditional sound and look associated with it because it was used in all of the older amps. Older carbon comps were 20% tolerance, whereas newer stuff is usually 5% tolerance, so any new carbon comp will be greatly superior to old ones. Moreoever, carbon comp eventually wears out and gets noisy, which are the noises you are probably hearing. I say probably because it could still be something else but you won't know until the resistors are replaced. Here is a wuote from a site I found on a google search that pretty much outlines what noise the plate resistors usually do when they are bad.

"In guitar amps, because of the higher amounts of gain used, the plate resistor can be a source of noise in the amp. The type of noise caused by bad plate resistors tends to be the crackly, weak popping sounds."


Carbon film will be quieter than carbon comp, and metal oxide or metal film even quieter still. They will also have better tolerances than the carbon comp. Some people say they sound different and carbon comp sound better. I've found that they do sound different, but is a VERY subtle thing. Changing a tube or a cap will have a much larger sound difference than changing a resistor as long as the value of the resistors swapped are the same.

Carbon comp resistors introduce their own distortions due to their type of construction, and they also drift in value under high temperature and/or a high load on them. So they are really only useful in certain locations in the amp where there is a large voltage swing, such as in a plate resistor. In a location such as the 1M grid resistor on the input tube, a metal film would be the best to use because it will be MUCH quieter as far as overall self-induced noise and hum level.

I've used all types of resistors, including wirewound, even though they are often inductive, and in a plate resistor I would use whatever works best for you. As I said the sound difference is very subtle, and if it was an amp that was worth a ton like a '59 bassman, then I'd probably use a carbon comp to keep it closer to original. Since its not that vintage, I'd say put in whatever works as long as it has a high enough wattage rating and is the correct value.

If you haven't yet, read Randall Aiken's take on the resistor types, and any of his other articles too. The resistor article is here:

http://www.aikenamps.com/ResistorNoise.htm

and the others are here:

http://www.aikenamps.com/TechInfo.htm

Here are another couple links on the subject. Keep in mind that some people approach this subject from a hifi perspective, where transparent sound is the goal, and that is another completely different world from guitar amps and tone for those.

http://www.angela.com/catalog/resistors/Resistors.html
http://www.tone-lizard.com/Electronic_Parts.htm
http://www.geofex.com/tubeampfaq/TUBEFAQ.htm#CarbonComps

[admin]

Much appreciated Greg.

I hope we get to the bottom of the snap, crackle and pop. It looks as if the problem is in the preamp.

[soundmasterg]

Sometimes these noises in tube amps can persist and be quite hard to find, even for experienced people. My brother's Silvertone 1484 is still crackling sometimes, but there is no rhyme or reason to it, which makes it very hard to troubleshoot. I spent 9 months working on it and restoring it and am not eager to work on it again, because its a PITA to work on, but I'm not satisfied until things are right. Since I replaced everything in this one except the transformers, its not as easy as replacing a part to fix the noise. Good luck on yours.

[admin]

Greg: It looks like success. Apparently, it was a plate load resistor on the Normal channel. The question is, should I replace the others or stop at this one and keep it as stock as possible. It is a 1967 and not a rarity at this point.

[soundmasterg]

I'd replace them all Peter. Like I was saying earlier, the old carbon comps weren't as tight of a tolerance as new ones, and they also absorb moisture over the years, so if one failed, the others will within a couple years. If you replace them with carbon comps, then most people won't even notice the difference, and the ones that do notice will know better than to diss the amp because a couple resistors have been replaced.