The Mullard valve factories had numerous test schedules for valves produced but today, we are going to look at the production test that EVERY valve that went into stock received which was known colloquially throughout the Mullard organisation at ' Test I'.
It is interesting to note that, with the exception of the knock test, the test schedule is identical to that used by Mullard Magic before any of our valves go on sale.
The Test I schedule was conducted by two girls, however, by mid 1953, for some popular valve types, Test I was automated meaning that approximately 1500 valves per hour could be tested in this way by a single girl and this is what was done: -
KNOCK TEST - in this test, a valve was supplied with appropriate voltages whilst seated in a standard valve base and then treated to several sharp raps with a rubber hammer by the operator. A milli-ammeter in the anode circuit would indicate faults such as low or no emission, open circuits, short circuit or intermittent operation. An IF component abstracted from the anode current was amplified, rectified and passed to a loudspeaker and neon network where variations in anode current would cause characteristic sounds or indication flashes.
Having survived the 'Ben Nock' test, the valves under test were passed to another test bench where they were seated in banks of valve bases and preheated before being subjected to additional quantitative tests. which included:.................
INSULATION RESISTANCE - in this test, with the cathode maintained at a positive potential, measurements of leakage between the cathode and other electrodes were made under specified voltage conditions. For indirectly heated valves, the heater to cathode insulation was also checked.
EMISSION - in this test, all electrodes excepting the cathode were strapped to form essentially a diode and an AC supply at a specified voltage was applied and the total rectified current measured.
MUTUAL CONDUCTANCE - in this test, the anode voltage was maintained at a specified value and three different values of grid bias were aplied to the control grid and the corresponding anode current values were noted and checked against the slope of the theoretical Vg - Ia curve.
VACUUM - in this test, the valve was operated at nominal working paraeters and the vacuum was tested by measuring the reverse grid current or 'gas' flow using a series connected micro-ammeter in the grid circuit.
VISUAL INSPECTION - in this test, each valve was inspected under a 4x bench magnifier in daylight balanced (3200K) light for general cleanliness and internal appearance.
In the following photograph , you can see a valve test station at Mullard Backburn, on the left station is the 'knock test' board, the centre station, the characteristics test board and at the right station, the microphony test board: -