VALVE TYPES & MANUFACTURING TECHNIQUES (viii) - FILAMENT PREPARATION
Posted by STEVE M on
Previously, we looked at wire production and although this is used for numerous valve componentry preparation, the biggest use by length has to be filaments so, today, we are going to look at filament preparation.
Tungsten wire is the base substrate for filament wire with some wires being just 8 microns thick or 3/10000 of an inch or 1/10 the thickness of a human hair. The wire is then coated with a admixture of strontium and barium carbonate using the process of cathetophoresis where an electrostatic field induced into the wire attracts colloidal carbonates suspended in a polar carrier solvent. The 'wet' wire is then dried in a tunnel furnace leaving a carbonate deposit.
Once the wire is coated it is 'filament tabbed' where the filament is supported at one end whilst having a nickel tab attached at one end and then a shorter tab and spring wire then fitted to the first end. In the next two photos taken from Mullard standard operating procedures you see the process flow and finished tabbed filament: -
In the filament tabber, the coated wire is passed through the machine where after a set length has unravelled, a small hammer breaks the coating after which a compressed air jet blows away the unwanted portion and a pressure pad then does a final clean. At the next machine station, an 8mm wide but 8 micron thick nickel tape is wound around the cleaned area of the wire then spot welded onto it before finally being clipped to the required length.
The continuous length of tabbed wire is sliced into separate filaments by a tool which cuts the tabs at a point 1mm from the end at which the spring wire is welded after which a mechanical grab places the completed filaments on a strip of belted cardboard from which they can be taken and inserted into the electrode cage build.
The long tab at one end of the filament is the connection that will be welded to the bottom filament support with the shorter tab being the connection that will be welded onto the upper filament support thus keeping the filament rigid and aligned.
