Third-harmonic tuning of e.h.t. transformers

A detailed analysis of third-harmonic tuning of e.h.t. transformers is given, using a simple, approximate equivalent circuit. It is shown that, if the leakage inductance is tuned to about the 2.8th harmonic of the flyback pulse by the stray capacitance. (a) Ringing of the e.h.t. winding during the scan is eliminated. (b) The e.h.t. voltage pulse is maximized, the increase being 55%. (c) The voltage pulse applied to the driver tubes may be increased or decreased; the maximum possible reduction is 25%. A theoretical transformer should therefore have a pulse step-up of 1.55/0.75, i.e. 2.07 times the turns ratio; practical transformers rarely give a step-up of more than 1.5 times the turns ratio. The explanation is in the approximate nature of the equivalent circuit used for the analysis. The outline of an analysis using a complete equivalent circuit is given.