After ageing, the valves were ready for a series of intensive tests during which any valve which failed specification was rejected. The first of these tests and the first hurdle at which the valve could be failed was known as the "Knock Test."
To conduct a Knock Test, the valve was plugged into a test board and the appropriate voltages were applied to all electrodes. The valve was then sharply tapped several times with a rubber hammer and any hidden faults such as intermittent short circuits or loose connections were indicated by readings on a series of meters or the lighting of neon lamp indicators. The object of this test was to weed out any valves which were hopelessly defective and which had the potential to damage the very delicate instruments utilised for the final tests which we will be discussing in my next blog entry.