Yesterday, we looked at what mica actually is, today we will chat in detail about the mica discs used in valve manufacture. As you look at the electrode cage of most valves, you will see that the individual support rods and elements of the cage are supported in their relative positions by two or more discs of mica. These discs are shaped and pierced by a series of precise holes which act as location points for the components to be supported. The function of a mica disc is threefold; to insulate the electrodes from each other; to maintain correct electrode spacing; to hold the electrodes rigid within the envelope. Below you can see examples of a typical mica in this quaint Mullard photograph:-
At Mullard, the mica washers as supplied from India or Canada are stamped to shape and punched with the necessary holes to produce the mica disc. Very close tolerences of these holes is a must and hence samples of each batch of mica discs were inspected using a Shadowgraph - a magnifying projector whose screen is fitted either with a comparator overlay or with a measuring vernier. The magnification used was 30 times and the tolerence demanded was +/- 0.05mm. The acceptance criteria was >95% pass. If this specification was not met then the whole batch was rejected. The next stage of inspection was a 100% visual inspection where operators would reject any broken, bent or mis-shaped mica discs.
Post inspection, the discs were coated with an aqueous solution of Magnesium Oxide - once dried this forms a white powder layer on the mica disc which has very high insulating properties and served to break up the smooth mica surface increasing potential leakage path lengths and repel getter deposition as a valve ages.
Finally, the mica discs are placed in glass flasks which are evacuated and sealed until they were required for use by the valve assembly department. Here we see Dilys Dyke studiously sealing mica discs into evacuated bulbs for storage at Mullard Blackburn: -