This is National Radio Schools booklet number 21FR-2 "The Vacuum Tube as an A.C. Generator in Television and Radio Circuits". This is not light reading. This intensely technical reading was published in 1937. It covers crystals, grounding, load variations, plate voltage, and most everything else. It centers around the Hartley Audio Frequency Oscillator.The Hartley uses a tuned tank circuit with a tapped inductor in the feedback loop. That means it's inductively coupled. One side connects to the amplifier input and the other to the amplifier output. It uses a tuning capacitor. The Hartley and the Colpitts are two hobbist Oscillator circuits you'll come across a lot in educational texts. the advantage is that they are simple. the disadvantage is that you cant output a pure sinewave for testing, it's loaded with harmonics.
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