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RE: cathode bypass cap calculation, please help

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To cover 20Hz with no amplitude loss or phase shift the 3db down point of the high pass filter needs to be at 2Hz.

A cathode bypass cap is bypassing the impedance of the tube's cathode (Zk) in parallel with the value of the cathode resistor (Rk).

The "total" cathode impedance Zk is cathode resistance of the triode (Zk`) in parallel with cathode resistor Rk.

Triode cathode resistance is Zk`= (Rl+rp)/(mu+1)

Where mu is amplification factor and rp is anode resistance in concrete operating point. Rl is load resitance for AC, and this is anode resistor (in preamp stage) or reflected primary load of the OPT (Za) in output stage. Note that actual Rl can be Ra//Rg, or anode resistor in parallel with second stage grid resistor.

The final formula for Zk (Zk`//Rk) is: Zk = (rp + Rl) / ((mu + 1 + (rp + Rl)/Rk)

And our capacitor for chosen -3dB frequency is: Ck = 1 / (2Pi * f * Zk)

Tre'




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