He starts with a comparison to a solid-state FET, which may be most useful to "assist the solid-state-expert" but the discussion gets less technical from there and ends up recognizing some of the superstition that has persisted to muddy the waters into the 21st century.
I could go a little further on this little topic and it may clear up a little superstition if there is any interest.
Try the direct links on this page to each PDF:
https://www.gitec-forum-eng.de/the-book/
He goes into good detail on the magnetic pickups, and also has quite a bit on the vacuum tubes and high voltage amplifiers that go with them.
For tubes a good place to start would be the "Input Tube":
https://www.gitec-forum-eng.de/wp-content/uploads/2019/02/po...
He starts with a comparison to a solid-state FET, which may be most useful to "assist the solid-state-expert" but the discussion gets less technical from there and ends up recognizing some of the superstition that has persisted to muddy the waters into the 21st century.
I could go a little further on this little topic and it may clear up a little superstition if there is any interest.