Great website that covers the basics of tube radio repair. For a one time fee of $11.58 USD you can watch videos on Tube repair and theory from a technician that has over 40 years experience.
Learn how they work. Ray makes great high quality videos with lots of great tips on tube radio repair.
Here is the intro video.
If you are interested in following his great courses and videos he has a members only area that is password protected. Their is a one time fee of $11.58 USD which is a great bargain. he is going to try and produce a video every week, dpending on how much interest their is in Tube type radio repair.
Learn the basics and theory of tube type radio repair at http://www.radio-workshop.co.uk/join.shtml