• WARNING: Tube/Valve amplifiers use potentially LETHAL HIGH VOLTAGES.
    Building, troubleshooting and testing of these amplifiers should only be
    performed by someone who is thoroughly familiar with
    the safety precautions around high voltages.

Cathode bypass cap ?

plate resistance

Yes, the effective tube plate resistance goes way up. Add another (u + 1) * Rk ohms to the "stated" value.

For example, a 12AX7 at 1mA, 150V will have an rp of about 60k. If the common cathode amplifier stage uses a 1k ohm Rk, then add another 101k for a total effective rp of 161k.

The bypass caps preserve gain and keep the output impedance lower. But often at the expense of sound.