Introduction 1. Whilst good/bad testing on a semi-production basis will undoubtedly be the major use for this tester, it is certain that the instrument will find considerable use in laboratories and service departments where engineers and skilled personnel will be available, and where more precise details of valve performance can be used to advantage. To this end, additional facilities on the tester, enable Ia/Va, Ia/Vs and Ia/Vg characteristics to be plotted over a wide range of voltages, these being readily available from the calibrated panel controls. Principles of Operation 2. The tester is basically designed to check the valve according to its static characteristics which would normally require the provision of the requisite range of variable dc supplies. The difficulty lies in the regulation problems involved in the supply of the wide range of dc anode and screen voltages, on which the loading might vary from a fraction of a mA to over 100 mA, dependent on the type of valve being tested, and the nature of the test being performed. Such a requirement could of course be met by the provision of a number of regulated power supplies, which would render the instrument cumbersome and expensive, whilst a large amount of metering, would not only mean additional expense, but also make the instrument difficult to use, and would not entirely overcome the problem. 3. It can be shown, however, that if alternating electrode voltages are applied in their correct proportions, an amplifying valve can (by virtue of its property of self rectification) be caused to give dc anode and screen currents which for all practical purposes bear a constant relationship to those obtained from its dc static characteristics. 4. This immediately simplifies the problems of power supply to the valve under test. The design of transformers to give negligible regulation errors over the range of secondary currents involved is comparatively simple, whilst the range of electrode voltages may be simply provided by a predetermined secondary tappings selected by calibrated switches, thus minimising to a very large extent problems of size, weight and cost, and eliminating the necessity for separate metering.