[R-390] Replacing carbon comps with metal film resistors

[Gary Geissinger]

Andy,
Your experience agrees with mine. I have always used metal film resistors in professional audio gear. They give excellent in low noise circuits.   Then I got into spacecraft design and metal film resistors were all that I could use.
For high resistance values used in high voltage circuits metal film resistors have an unfortunate property. Due to the internal stress the metal in the film can migrate and raise the resistance.  Might not be an issue in a tuned circuit, but I might choose a different resistor type exposed to over a KV or two.  On the other hand they may be more stable than some other resistors in that case.
Gary WA0SPM


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On Tuesday, July 30, 2019, 6:14 AM, James A. (Andy) Moorer <jamminpower at earthlink.net> wrote:

Every time I have done this test (in the context of pro audio), the 
metal films won hands down. My problem with using them for boatanchors 
is that they are hard to find in high wattages (above 1/2 W) and high 
resistances (above maybe 470K). They are out there, but just hard to 
get. And waaaay more expensive than comps.
-A


-- 
James A. (Andy) Moorer
www.jamminpower.com

On 7/29/2019 11:50 PM, Jacques Fortin wrote:
> Hi Perrier, Bob, and everyone else
>
> This is an experiment I performed a lot of times before, but just to check again....
>
> The Q swamping resistors inside several R-390A IF transformers are 39K, 10%, 1/2W, carbon composition...
> I opened my parts drawer for resistors of that value, and waked up my old trusty Hewlett-Packard 4192A LF Impedance Analyser.
> And there are the values I measured for various resistors types at 455kHz (all those being rated at 1/2W) with the minimum connecting lead lengths by using the HP 16047C Test Fixture.
>
> But just before you look at those results....
> I hope that everyone remembers that in the equivalent serial representation, an impedance is a value of pure resistance, plus or minus a reactance.
> Meaning, Z = R ± j X
> If the measured reactance is positive, you have an inductance in series with the resistive part, and if it is negative, you have capacitance in series with the resistive part.
> In theory, a perfect resistor will exhibit a measured reactance of ZERO, or, expressed otherwise, smaller is the measured reactance, more the part is close to the perfect model.
>
> Ohmite Carbon Composition:
> 40100 -j 1370
>
> Philips Carbon Film:
> 38710 -j 950
>
> Sanyo Carbon Film:
> 38850 -j 780
>
> Corning Glass Works (CGW) Metal Oxide Film:
> 39470 -j 870
>
> Philips Metal Film:
> 38980 -j 400
>
> In this test, at 455kHz, the metal film resistor wins.
> But the surprise is that the Carbon Composition turned out to be the more "imperfect" of the lot !
>
> 73, Jacques, VE2JFE
>
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