How do I calculate the capacitor values in a SE amp? Coupling + k bypass

[dubadub]

Those GXSE are "only" rated to 40Hz, should I still aim for 3-5Hz?

 

[Suncalc]

Yes. The f-lo in the coupling equation is the -3dB reference point. Setting it between 3 and 5 Hz means that you'll have minimal loss at 20 to 40 Hz.

 

[20to20]

These high pass filters are simple 1st order filters that have a 6dB slope per octave. So 3dB is 1/2 octave. For 5Hz the rolloff starts at 7.5Hz.

 

[Suncalc]

These high pass filters are simple 1st order filters that have a 6dB slope per octave.

Almost. The coupling capacitor relation is a first order rolloff. However, the cathode bypass is actually the halfway point between the stage gain fully bypassed and the stage gain unbypassed. So it has more of an "S"-curve shape to it. Here is an example using a simple 12AU7 stage.