THE GREAT MULLARD MAGIC BLOG — VALVE MANUFACTURE

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...

Today, I would like to raise the topic of SCREENS.  As we discussed last time, internal carbonising only decreases the risk of secondary emission but cannot prevent this unwanted electron straying.     As we have noted, secondary emission can play havoc with a valve's preformance, particularly in multi-electrode cages due to unwanted  coupling effects.   For some multi-electrode cages, metallic screens are fitted to keep the electrons on their true path.  Below, we have a photo of just such a deflection screen shown on the electrode cage of a dismantled EF91 pentode where you can clearly see a metallic...