[HBR] Using vintage Carbon Comp resistors (Don)

[Marvin Match]

> Is the problem a change in the component value over time while in storage or
> does the age reduce the reliability in some way even after installation in a
> circuit? Will it continue to degrade in situ?

The problem with the old carbon comps is that over time they absorb 
water, which alters their value. Cooking them removes the water, 
which brings them (somewhat) back to value (maybe). In a tube circuit 
there's enough heat being splashed around that if the equipment gets 
used it's not much of a problem.
 
> What application in a receivers circuit are carbon comp resistors superior
> to other types i.e. noise, 

Carbon comps are usually noisier than carbon film or metal film...

> stability,

Carbon comps are less stable, in the way we've discussed...

> non-reactive, etc.?

Carbon comps are less reactive than SOME of the modern types. 

The modern types have a metal or carbon film sprayed over a glass or 
ceramic rod, then (some, depending upon manufacturing process) are 
trimmed to value or just raised in value by cutting a spiral groove 
in the film. This turns it into a tiny inductor.

The real question is "Does any of this matter?". The answer is 
"Probably not".

It's important from an engineering standpoint to understand the 
characteristics of the parts in question, then in each application 
you will be armed with the information and decide if any of this 
poses an issue.

One at a time:

Noise - our tubes are several orders of magnitude noisier, so it 
doesn't matter.

Stability - we don't use them in timing circuits, they don't drift 
much as they heat up, so it's not usually an issue. In most of our 
tube circuits the part could be 30% off value and you'd be hard-
pessed to notice.

Reactive - they're not, so it doesn't even enter into the equation. A 
film resistor with the spiral groove cut into it IS reactive, but in 
our application this would actually be an advantage in most places. 
When you get into the microwave frequencies the inductive nature of 
the modern film resistors will bite you (in this case the carbon comp 
is superior) but I've never seen a microwave HBR!

I have no problem using carbon comp resistors. They've been "good 
enough" for at least 50 years. Just be aware of all the 
characteristics.

Marvin
KA7TPH