Valves as we know, have many different functions and for each application there are certain parameters which ensure the best use in these applications, for example: -
For an audio amplifier - knowledge of the maximal power output and distortion of an output valve for various levels of signal input under variable anode load resistance and grid load resistance is important.
For a radio receiver - knowledge of the cross-modulation factor allows effective choice of frequency changer valves and IF amplifier valves so that effective AGC circuits may be designed.
In order to measure these parameters and others such as equivalent noise resistance, hum level, conversion conductance, power output and harmonic distortion, the Mullard Valve Measurement & Applications Laboratory designed and built in house a range of test rigs which would test these parameters The design of each of these instruments was particularly elegant in that circuit components and input voltages were infinately variable and easily switchable and that obtained test results could be directly read from meters and indicators which were calibrated in the units required without having to resort to complex calculations.
The results of these basic dynamic measurements were used at key stages in the history of a valve type. During development and pre-production trials, samples were tested to ensure the performance of a new valve met the required specification. In production, similar measurements provided information for publication in valve data manuals and application reports. When the valve was an established production line, checks were made on sequential valve batches to ensure that consistency of performance was being maintained.
In today's Mullard archive picture shown below, you can see a Mullard Valve Applications Laboratory physicist in the foreground measuring Equivalent Noise Resistance, on his right, a colleague is working the test rig for measuring Power Output and Distortion: -