Electrode spacing in disc-seal triodes

Four different methods have been investigated for determining the working cathode-grid clearance in disc-seal triodes. The methods involved (a) direct observation with a microscope, (b) capacitance measurements, (c) measurement of current flow and (d) determination of magnetron cut-off conditions. Methods (a), (c) and (d) gave reasonably consistent results. The range of clearances involved was 0.1 to 0.5 mm. Methods (a) and (d) were also used to find the cathode-anode spacing in the same triodes. The capacitance method, though of little use for determining the clearances directly, has been applied to the extension and checking of the clearances determined by the other methods.