[3] Directly heated cathode circuits connect the cathode bias resistor to the center tap of the filament transformer secondary or to the center tap of a low resistance connected across the filament.
The capacitor makes the gain of the stage, at the signal frequencies, essentially the same as if the cathode was connected directly to the circuit return.
[6] In some designs, the degenerative (negative) feedback caused by the cathode resistor may be desirable.
[7] In class A push-pull circuits a pair of tubes driven by identical signals 180 degrees out of phase may share a common unbypassed cathode resistor.
He said that fixed bias, unlike cathode bias, does not provide a margin for error that protects the system from inevitable differences between vacuum tubes nor does it protect against run-away conditions caused by tube or circuit malfunctions.