I came across this site written by Max Robinson whilst looking for theory on Intermodulation Distortion for Part 7, Chapter 55 of my T & M Theory and Repair Book !
https://www.angelfire.com/electronic/funwithtubes/
I can whole–heartedly recommend Max's Website, there is an enormous amount of very useful information and constructional ideas, including notes on how to make replacement valve–type i.f. transformers, amongst other things !
He has also written a complementary site, "Fun with Transistors".
Enjoy, there's tons and tons to read !
Chris Williams