Single ended amplier

[Terry]

... if the OP valve is biased in Class A, how can the anode current double?

Andy, the anode current only remains constant if there is no input signal. It is then determined by the (fixed) bias but, when you apply an input signal, the anode current must increase for positive going half cycles of the input waveform. However, as it must also decrease on the negative going half cycles, the peaks and troughs must average out to the same level.

Because of the smoothing components, as far as the PSU is concerned, the current demand doesn't change for Class A, as you've said.