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 screen plate between the anode lead as well as the grid and cathode connections.