I came across this channel a couple of weeks ago
https://www.youtube.com/watch?v=QT7n6zQdk8o
This link is for a walkthrough of the Mullard 3-3 circuit where they explain how the circuit works. They have other circuit videos as well, very useful for beginners to valve circuits.
If you like guitar amplifiers, there's Uncle Doug who has some theory videos as well as repairs.
https://www.youtube.com/watch?v=U6By31V9fDo