[R-390] Resistors

[Bob kb8tq]

Hi

Here’s a summary article showing the basic megohm X megahertz issue with carbon comps. 
It does not go into the why. 

http://g3ynh.info/zdocs/comps/refs/EPD_Botos.pdf <http://g3ynh.info/zdocs/comps/refs/EPD_Botos.pdf>

Very simply put, in a carbon comp, the stray C bridges across small segments of the resistor.
Rather than showing up as “pure C” that a tank circuit would resonate out, it stays resistive, 
but at a lower value. It’s not the same effect as end cap capacitance in a metal film resistor. 
Since tube tanks may have quite high values of load resistance, anything at those impedances
could be an issue. If you get down to tens or hundreds of ohms, you will not see a problem at R-390
sort of frequencies. 

I’ll keep digging for the paper. I last read it in 1973 ….

Bob

> On Sep 25, 2017, at 1:53 PM, Charles Steinmetz <csteinmetz at yandex.com> wrote:
> 
> Bob wrote:
> 
>> The gotcha is that the 270K resistor in the circuit at 30 MHz is actually presenting 150K. The circuit was designed
>> for 150K rather than the 270K. It shows 270K since that’s what is marked on the carbon comp resistor. If you are
>> going to replace them, you need to take that into account.
> 
> That differential is more than 10x greater than any reactance effect I've ever observed in an actual, new-condition CC resistor.  There may be CCs that measure like that, but not quality resistors in new condition.
> 
> In the actual CC resistors I've tested, a 270k CC will operate within 5% of nominal through HF into VHF.  Note that most CCs in boatanchors were originally rated +/- 10%, with a few rated +/-5%.  Also, note that a 390/390A will work just fine (and meet spec) with resistors in most positions at +/- 50% (which is even further from nominal than the 270/150 example given, so even if someone did happen to get a 270k CC that exhibited 150k at 30MHz, the radio would almost certainly still meet spec.  But, as noted above, things are not *nearly* that bad in real life.)
> 
> Best regards,
> 
> Charles
> 
> 
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