WARNING: Tube/Valve amplifiers use potentially LETHAL HIGH VOLTAGES.
Building, troubleshooting and testing of these amplifiers should only be
performed by someone who is thoroughly familiar with
the safety precautions around high voltages.
You may also want to get a copy of the MIT Classic "Magnetic Circuits and Tranformers" (the best) and the 3rd edition of Reuben Lee's "Electronic Circuits and Tranformers