Fun fact! One of the early patents for what would become Class D did rely on tubes
Class-D was originally patented with tubes, but didn't really work so good until fast solid-state switching devices came around.
The concept patents go back a long ways. Class-A, Class-B, Class-C, Class-D is not really a huge separation in time. Class-D only seems recent because it works so well with new Solid-State. Cheap, Reliable, lowest THD of any amp in history, efficient. Class-d is 90% or the entire audio amplifier market these days.
Older tube based patents
1959
Patent US3011025 - High power amplifier - Google Patents
1930
Patent US1874159 - Electric amplifying circuits - Google Patents
