Standard Resistor Values

[snitch56]

I am in the process of replacing all the old carbon comp resistors with metal film in my Pat 4 preamplifier. I would like to use Vishay (type RN) resistors, the only problem is their “standard” values don’t match the original values (i.e. Original=33k Vishay=33.2k, Original=120K Vishay=121K, Original=330 Vishay=332 etc.)

Is there any rule of thumb on the maximum relative error for a resistor value substitution?

Since the old resistors are 5% tolerance and the Vishay’s are 1% I am assuming swapping values in a reasonable range should not cause any problems.

Thanks in Advance,
Brad

[mesherm]

NTE packages 2% metal film resistors in 5% values.
They come packs of 2 to 6 depending on wattage.

http://www.unicornelex.com/category.php ... 182b3624f1

[EWBrown]

With the Vishay 1% resistors, just choose ones closest to the original 5% or 10% resistor's values. There is enough built-in "wiggle room" in tube circuitry, so that "brain surgery" precision isn't needed Yellow_Light_Colorz_PDT_04

HTH

/ed B in NH

[snitch56]

Thanks for the advice and link. I did find some of those blister pack NTE resistors locally. I checked all the original AB carbon composite resistors (circa 1968) in the preamp and it seems that all the measured values exceed the manufacturer’s specifications by varying amounts.

As resistors age does the resistance increase? By visual inspection it appears that none of the resistors over heated.
Just curious

[mesherm]

It has been documented that carbon comp resistors increase resistance with age whether in a circuit or sitting unused. Metal film resistors typically do not exhibit this aging behaviour. About the only real reason for using carbon comp would be in a high pulse current application. Since the whole center part of the CC resistor conducts, a high current pulse is a lot less likely to burn the resistor out. A metal film or wire wound might not have time to conduct the heat from a current pulse into the ceramic substrate before the conductor burned out. But the pulse scenario is unlikely to occur in tube or solid state analog amp duty.

[ecir38]

It has been documented that carbon comp resistors increase resistance with age whether in a circuit or sitting unused.

Mesherm, would this hold true for carbon film resistors too?

[mesherm]

Carbon film resistors are made in a similar manner as metal film and don't have the same aging charactoristics as carbon composition.
Carbon and metal films are made by vapor depositing carbon or metal onto a ceramic core. Carbon composition resistors are made by pressing a cylinder of carbon powder mixed with ceramic or some other insulating material together, sort of like making pencil "lead". The percentage of carbon powder among other things determines the resistance of the cylinder. I'm sure most people have broken each type of resistor in half. In the CC you see the central core of pressed carbon mixture. In metal and carbon film it is much harder to spot the conductive film since it is on the outside surface only.
The film resistors have only the pure carbon or metal film, not a mixture, and are usually laser trimmed by removing enough film to get the required resistance before they are coated.
The films in the resistors are usually very pure and will not deteriorate with age.

[TomMcNally]

You can trim carbon composition resistors to "precision" values
by filing notches in them. That was an ancient way of tweaking
values. They coated the notch with paint after filing.

[ecir38]

Thank yall for the detail answer and tweak :).

[snitch56]

Thanks for the quick lesson on resistors Mike. If Carbon Comp resistors increase resistance with age why would anybody want to purchase NOS resistors more than 10 years old?

Tom, your advice is a great way to do some “in-situ” adjustments of the resistance to see if it makes any difference in the output signal.

[mesherm]

Thanks for the quick lesson on resistors Mike. If Carbon Comp resistors increase resistance with age why would anybody want to purchase NOS resistors more than 10 years old?

People buy NOS and used resistors, tubes, capacitors (ie Black Beauty, Vitamin Q) etc, for a variety of reasons. Tubes I can agree with but passive devices like caps and resistors I would rather have new. Some guitar amp people say CC resistors and old caps have a certain sound. Differnt strokes fer differnt folks. I personally like the modern metal film and oxide resistors myself and almost never use CC.

[dcriner]

If Carbon Comp resistors increase resistance with age why would anybody want to purchase NOS resistors more than 10 years old?

NOS carbon-comp resistors made by Allen-Bradley were very stable and generally have not increased in value. But those made by other manufacturers are very prone to increasing.

NOS A-B resistors show up on eBay, but they tend to be pricey.

Some people probably think carbon-comp resistors sound better? And people restoring vintage equipment may prefer an original appearance instead of modern-looking components.

[dhuebert]

I looked but didn't see it... I thought Steve Bench had an article on the resistor. If I remember correctly carbon comp resistors value was somewhat dependant on voltage, maybe an inverse relationship? Making me think that carbon comp resistors on the plate of high voltage stages might make even harmonics as the stage gain changes with cathode voltage. This is the only reason I can think of to use them.

Very noisy in gain stages.

Don

[nyazzip]

these resistor trimming comments, especially using a file!? are interesting because recently i worked as a peon temp at motorola, and i spent many a month sitting in front an automated laser trimmer, which robotically "cut" a component on a mini circuitboard until it either functioned within suitable range or was discarded. these were tiny planar type resistors i guess, only 2x2mm or so....the area that the laser cut turned black and left a black dust in the machines after awhile....
i understand this particular component was used to generate carrier waves in some of the radios....
nobody ever explained to me, or could explain to me, what i was doing, nor even spoke english for that matter, so i learned nothing.

[ecir38]

I personally like the modern metal film and oxide resistors myself and almost never use CC.

meshern, by saying this I take it you wouldn't use a carbon film in place of a metal film either.

Do you prefer a certain name brand for the metal oxide and metal film resistors you use or do you this this is just snake oil?

I can see where differant capacitor choices would make a differance but have read in the past where boutique resistors really don't.