THE GREAT MULLARD MAGIC BLOG — mica valve spacer

Last time 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;...

Valve lore contains a lot of references to mica but what is a mica?   Well, let's start with the raw material.  Mica is a silicate material having the chemical formula: -  Na2Al4–6Si8O20(OH,F)4.    The only thing we need to remember about this chemical formual is that it provides a material which splits into very thin sheets of a material that is impervious to gas, rigid but brittle and a useful insulator.  Interestingly, the material is named from the Greek word mica which means crumb and also the Greek word micare for glitter.       The mica supports used by Mullard were obtained...