[R-390] Resistors

[Bob kb8tq]

Hi

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.

Bob
> On Sep 25, 2017, at 12:16 PM, Charles Steinmetz <csteinmetz at yandex.com> wrote:
> 
> Bob wrote:
> 
>> The “problem” with carbon comps at RF is that they don’t really go inductive or capacitive. The
>> physics of the resistor is such that they stay resistive, but drop in value.   *   *   *
> 
>> The effect is similar to stray capacitance (since internal capacitance is what causes it). Large
>> values are impacted at lower frequencies compared to small values.
> 
> The two statements above are inconsistent with each other.  The effect *is* due to reactance, but the phase angle of the AC signal through the resistor doesn't change as much as it would with a pure RC circuit. This is because the reactance does not change at 10x per decade (as it would with a pure RC circuit) -- which is how distributed capacitance works.
> 
> This is essentially the same as saying that the net capacitance in CC resistors is due to a bunch of really bad (lossy) capacitors.
> 
> If someone were to raise this feature of CC resistors as a reason not to replace them with MF resistors -- i.e., the MFs do not exhibit the faults of the CCs, those faults were "designed into" our HF boatanchors, therefore the radios won't work properly with MFs -- that is simply a bridge too far.  The reactive effect on CCs at VHF and below is negligible, just as the inductance of MFs is (in both cases, we're talking about sane resistor values for RF work).  [*NOTE* -- I am *not* suggesting that Bob was making this argument.  I'm merely anticipating another old wives' tale I've seen repeated far, far too often in the BA community.]
> 
> Best regards,
> 
> Charles
> 
> 
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