After passing the knock test, valves were passed to the main test bench and plugged into a test board where a specially designed eight position switch applied successively, appropriate voltages such that values were obtained for the following parameters: -
Control grid current
Mutual conductance ( at 4 - 7 points on the characteristic curve dependant on valve type)
Inter electrode insulation for all electrode combinations
Only those valves whose performance fell within specified limits were then passed into "Factory Stock."
In the photograph below we see Enid Barribal testing valves using one of the Mullard Blackburn 'Test 1' main test boards: -
Mullard Blackburn also had a semi-automated final test board where valves were plugged into sockets mounted on a revolving platter. As each valve passed from station to station, the various tests were automatically applied but the indicating instruments on the test board were replaced by sensitive relays which would energise if a faulty valve was detected hence activating an automatic claw to pull and dump the faulty valve in a reject tray.