I have posted a video on Youtube explaining how a tube amp works or valves, illustrating the operation of each of the stages in the simplest possible way. This is a beautiful 1950s Pathfinder amp that I got my hands on recently. Like most amplifiers, its design is simple but functional, as well as PERFECT for a didactic explanation of its operation.
Tube amplifier operation video
I leave you with the video, in it I have published the schematic diagram of the circuit with all its details, from the rectifier to the power amplification stage. I hope it is to your liking and usefulness.
Errata: In the video I present the power supply rectifier as a half wave rectifier, being a full wave rectifier. Thanks to friend Taoiste, who pointed out to me that it is a double diode.