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In Reply to: RE: cathode bypass cap calculation, please help posted by Tre' on May 27, 2024 at 07:48:09
Dear Tre,
Thanks for reply
But I am not very good at math, in my case where the Direct couple second stage 6dj8 connecting a 6.2K cathode resistor, what is the capacitance minimal. It would be grateful using your calculation expression to show.
Sorry for asking, I am a novice
Follow Ups:
Tell me the value of the plate resistor and the value of the following stage's grid resistor and I'll run it for you.
Tre'
Have Fun and Enjoy the Music
"Still Working the Problem"
Dear Tre, as follow
second stage 6dj8 15K plate resistor (8mA idle)，next stage grid leak resistor 220K
171uf
Tre'
Have Fun and Enjoy the Music
"Still Working the Problem"
Dear Tre,
Thanks for your effort
The calculation result is beyond my initial estimation because I thought a triode connecting a higher value of cathode resistor require smaller bypass capacitance.
AS I am not very good at math. Does it mean lower tube internal resistance need more bypass capacitance ?
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"
Thanks Tre, I learn a lot
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