THE GREAT MULLARD MAGIC BLOG — mullard

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: - Heater current Cathode current Control grid current Screen current Anode current Total emission 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...

And so the story continues.......... a couple of blog entries ago, we left the production process at the sealing stage at which point we had a sealed and functioning valve, however, further processing was required before it was suitable for use for it was required to undergo a process called ageing. Ageing consisted of operating the valve under load under carefully controlled conditions.  Large numbers of valves were plugged into a socket rack like the one in the photo below: - Once the rack was fully loaded, the valve heaters were activated at a 20% higher than nominal rating, so...

In March 1952, if you wanted to spruce up your service department, you could buy a smart, dustproof, tailor-made cover in strong silverised leathercloth for your MHSVT.   It had red piping accents and was only 13/-. Here we see Hortense Balchin who stated that 'My Wilbert always said that a workman must care for his tools and they will go on to care for him." and she went on to demonstrate how clean this cover kept her Wilbert's instrument : -

Now then, everyone tends to think of Hivac when they think of miniature valves for hearing aids but Mullard made them too.   Hearing aid valves were specialised in that they needed a small envelope size, must operate at low filament current and be non microphonic. Due to the National Health hearing aid being introduced in 1948, Mullard developed a series of 10mm subminiature valves to power it ......we have two of these smashing hearing aids available for sale elsewhere on the site - anyhow, I digress......... These 10mm specials  were swiftly superceded by the DF66, DL66 and DL68 which...

In this process, where sealed valves were passed from the last station of the sealing machine, a mechanical tri-claw hand fitted with an asbestos glove (imagine the 'elf n safety implications today!!!) transfered the valve to the first vacant position on the pumping machine. The pumping machine comprised of another rotating table which had a sequence of rubber bungs around the periphery.  Each bung was connected to a vaccuum manifold supplying a high torr vaccuum.  As the valve abutted a bung, the process of pumping commenced.  As the valve rotated around the table, the valve passed through a heated convection...