In January of 1948, a group of engineers at Mullard Research Laboratories, Salfords were given the brief to design and bring to production within two years (24 months), 'a valve testing apparatus suitable for use by dealers and service engineers'. The aim was to showcase this exciting new post war development at the British Industries Fair Radio Show in late 1950 at the Castle Bromwich, Birmingham halls. The target was met and sure enough, on Mullard's stand at Site 64, the 'valve testing apparatus' was proudly presented to the waiting market.
The design brief and the problems in meeting this requirement specification was interesting, the new tester had to be 'modern', compact, accurate, give a fast stable result - automatically, be easy to use by non-skilled personnel and finally, most important, be fitted with safeguards to prevent damage to tester and valve under test in the event of incorrect operation. All this added up to a tall order indeed.
The 'modernity' aspect was provided by housing the new tester in a hammertone silver finished case having a low aspect, 'crouching' on the bench. Difficulties with sourcing suitable moving coil meters for test indication allayed to the 'no damage on misuse' design brief lead to the novel concept of using a cathode ray tube for test indication - a twin edged sword - for although there was no meter to burn out, there was no way to get a discrete quantitative estimation of measured test parameters, however, it was felt that service departments only required a go/no-go indication anyway.
It is quite amusing now looking back that much was made of the new tester's bench footprint of 16 inches square and a height of only 12 inches. Not a bad result, however, this new tester drew much from the Mullard Master Test Board of 1935 in that similarly, it depended upon the use of punched paxolin cards to be inserted into a gate switch to set the test parameters for a specific valve under test - with over 8000 valve types in service and a requirement for at least 250 cards to cover the most common, storage for the associated cards was both bulky and commensurately heavy!!
We will look at the Mullard High Speed Tester in more detail in a future blog but I should like to finish by explaining - in Mullard's own words - a little about what this device could do:-
"A non technical person could learn to use the new tester safely and effectively after 10 minutes of instruction. After inserting the correct card for the device under test and utilising a simple arrangement of switch and push buttons can, under working conditions test for faults with: inter electrode insulation, continuity of electrodes, degree of emission, grid current, and heater cathode insulation with the position of a moving spot on the cathode ray tube indicating whether or not the valve on test is or is not within it's proper operating limits."
Interesting though, that many preferred this little number by a competitor, similarly priced but slightly lower such that the savings would allow a pretty nice austerity lunch for two, pass the salt old bean, my Brown Windsor soup is a little insipid. Sir Stafford Cripps would certainly approve.