NOS Carbon Composite resistors unused - what to expect

[pjd3]

Hello,

I came across a moderately large supply of NOS carbon composite resistors, actually the basement stock of well known EE and audio engineer Scott Kent.

I was gifted with some things from his wife after he passes away.

What I'm curious about is what to expect from using old but unused CC resistors for guitar tube amp builds. I've measured a significant amount of these resistors and to my surprise they seemed to have increased in resistance as CC resistors are known to do but, I assumed through the years that this was only in circuit used ones that had experienced years of heat and electron flow. It seems to be that CC resistors will increase in value even it not used. (about all of them were on or near the high side of 20% tolerance and many exceeding that).

Is the increasing value of resistance over time the only fallout in CC resistors or are there any other undesirable changes that plague these CC resistors over alot of time? I'm not sure exactly how old these are but alot of his equipment, even test equipment was from as far back as the 60's. He was a smart EE that was happy to maintain old equipment of all sorts, even designing his own units for various things.

So thats it really, I'm just curious if I'm OK using these old CC resistors for new guitar tube amp builds for rock and roll gigging. I probably wouldn't use these if I was building a HI-Fi amp for listening but, these would be for Fender and Marshall type amps for my classic rock local bands.

thanks for the input! I constantly grow in respect for the collective and individual knowledge resource found here.

Best,
Phil Donovan

[pjd3]

I meant to mention that carbon composite resistors seem to be a primary suspect for "Rice Krispy" popping and crackling in tube amps, and was wondering if this obvious breakdown of these resistors is just relegated to those that have lived the rigors of hot high voltage circuits or, if this symptom is something that would arise in NOS CC resistors as a sole result of age, even it they were never used.

thanks !

Phil Donovan

[Stevem]

If they are unused and not new defective ( as I deal with at my job ) as in frying Bacon sounds or high levels of hiss from the start,then they will be fine to use.

[Matthews Guitars]

Carbon composition resistors, if not hermetically sealed types, can be normally expected to slowly experience moisture uptake and increase in resistance and also get noisier.

I have four Ohmite Little Devils resistor organizers full of vintage CC resistors and you can bet that 90 percent of them have drifted high and are out of tolerance.

Since they're CC resistors, they find very limited use.

As per the Iskra and Piher Carbon Film resistors that Marshall used in the audio path of their amps in the 60s and 70s, carbon film resistors should be used in the audio circuits,
and leave the carbon comp resistors to the voltage divider and current limiting applications. However, carbon resistors tend to actually catch fire if they're used as screen resistors on
a power tube that experiences a hard short. That's a hazard! The smart money is on flameproof metal oxide film resistors for that application!


I would not use CC resistors in the plate, grid, or cathode circuits of any preamp tube. Use carbon or metal film instead.

Fender used CCs for most purposes, Allen-Bradleys mostly, because they were cheap. And the amps don't have enough gain for the noise to be objectionable.

[CraigGa]

Not wanting to hijack the thread but seeing as we're talking about resistors!

https://www.hificollective.co.uk/compon ... stors.html

Craig

[sluckey]

I have four Ohmite Little Devils resistor organizers full of vintage CC resistors and you can bet that 90 percent of them have drifted high and are out of tolerance.

Since they're CC resistors, they find very limited use.

Same here, except my CCs are in IRC boxes. Every bin full of NOS MILSPEC 5% high reliability AB and Ohmite resistors that came from the FAA Depot during the '80s. No telling how long they had been in stock at the Depot. I bet there is not a handful that are in tolerance. The 2 watters are closer to tolerance but still not close enough for me to use. Behind those IRC boxes are even more CCs with not so common values, stuff like 110Ω, 13K, or 43K. They're just taking up space!

I've considered putting the whole set on eBay. Heck, the IRC boxes sell for crazy money. I just can't bring myself to do so, knowing the status of those pretty resistors.

[Matthews Guitars]

Not wanting to hijack the thread but seeing as we're talking about resistors!

https://www.hificollective.co.uk/compon ... stors.html

Craig

$62.89 PER resistor?

I get the audiophile thing. I AM one. But only to a certain point, and it's far short of the Audio Note level of "Madness lies far beyond the point of diminishing returns, and we are THERE."

That was amusing. And amazing. What's more amazing is, they'll make money.

[Matthews Guitars]

Oh, I wanted to add this.

Regarding moisture in carbon comp resistors:

Have you ever seen an old carbon comp resistor start to boil when you solder it? That's the absorbed moisture boiling out of it!

If you really want to use those old CC resistors, first measure their value. Then bake them in your oven at 220 degrees F for an hour. Don't be surprised if they look different when you take them out.

Then measure them.

Their value may have changed. Now, if you want to keep them at that value, varnish them. Seal them so moisture stays out.

[TUBEDUDE]

Then put them in little bags with the values marked on them. Put all of the little bags in a large bag. Throw the large bag in the dumpster and use metal films.

[Stevem]

Its my understanding that CC type resistors will not absorb moisture when they just sit , they absorb when they are heated and cooled repeatedly in a hot invironment like a tube amp.

This expansion and contraction makes them porous over time .

[sluckey]

The resistance value of a CC depends on how tightly the carbon molecules are packed. Pack them tight for low values. Pack them more loosely for higher values.

I have another theory... Just as the universe is constantly expanding, so are the carbon molecules in a CC resistor, causing the value to drift higher. In another million years you can use them for insulators.

[gktamps]

I have boxes of NOS CC resistors and of all I've tested (many) all have drifted up, suggesting that they do indeed absorb moisture even when not in use.

[pjd3]

Well this turned out to be an informative thread.

Think I'll take a small handful of different value resistors of the old CC's I have and sit them in an oven for an hour or two at around 200 deg and see if the value (before and after) has changed. What I've done is when a value is found drifted very high, I take a really high value like 2Meg or 2.2 Meg and strap those across the lower resistor of interest to lower the value enough to get it into very close tolerance. Whether I would be introducing some other artifact I'm not sure but, it didn't seem like a super crazy thing to do, not for distorting guitar tube amps. What I can't figure out is if in doing that the CC inherent noise has been increased or decreased.

I could start looking at that, where I finally have some home equipment that may help to assess things like that. Ha, my ears maybe too?

Best, and thanks,
Phil Donovan

[pjd3]

OOoooowwhh, 60 dollar resistors, I would never win a war for the purchase of even one of those, ever.

But, for a hi fi tube amp build coming up, those Auricap true copper caps are really conjuring up the snake oil mystic for me. I may have to collapse into surrender for the .22uF coupling caps at $35 bucks a piece. Dumb F(*Y I am.

Phil Donovan

[mhuss]

The only really good reason to use CCs is when working on an old amp, in order to satisfy a nut job authentic-original fetishist.

There is some evidence that CCs produce 'good' harmonic distortion, but only when used in high voltage audio amplification, i.e., plate resistors.

I like 1 w carbon film for general use, as they are a decent compromise, and are actually rated for higher voltages (unlike, say, 1/2w metal films). I will use metal films in lower voltage noise-sensitive locations, like the 1M grid resistors on most amp input stages.

But to each their own.