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RE: cathode bypass cap calculation, please help

That is part of it and the higher value cathode resistor is part of it.

The plate resistance is part of the equation when determining the impedance of the cathode. (Plate resistance plus plate load) divided by (amplification factor plus 1) = the impedance of the cathode itself. That value is in parallel with the value of the cathode resistor.

So the lower the plate resistance is the lower the impedance of the cathode (all other things being equal).

And the higher the cathode resistor value the higher the total impedance will be that needs to be bypassed but it will always be lower than the lower of the two that are in parallel (the impedance of the cathode itself and the value of the cathode resistor).

I should add that the lower the value of impedance that needs to be by passed, the higher the capacitance value that is needed. (for a given desired frequency)

I have ran into a lot of builders (that should know better) who think the cathode bypass cap is just bypassing the value of the cathode resistor but that is not how it works. It is the value of the cathode resistor in parallel with the impedance of the cathode itself that needs to be bypassed. Equation for parallel resistances (resistor number 1 times resistor number 2) divided by (resistor number 1 plus resistor number 2) Also works for parallel impedances. Note, A resistor is (generally speaking) a non-reactive part so it's impedance is the same as it's DC resistance.

Tre'
Have Fun and Enjoy the Music
"Still Working the Problem"

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