Cathode ‘poisoning occurs due to absorption of remaining air molecules (after gettaring) and other gasses released from the elements over time. The resultant loss of emission usually determines the ‘end of life’ of the tube. I have successfully rejuvenated many tubes by ‘boiling’ off the cathode gasses by running the cathode very bright (doubling the filament voltage) for about 20 seconds. The gas molecules are then ‘floating’ around in the tube…only to be rapidly reabsorbed when the cathode is next heated. Re-firing the ‘getta’ material (silver stuff) will then absorb these molecules. To re-fire the getta material, place in microwave oven…switch off quickly after the flash otherwise the glass envelope will crack due to the tremendous heat generated. I wait for the flash with my finger on the door-open button. No more than a couple of seconds.

Then test the tube emission again…you will be amazed.

Only use tubes that are otherwise useless…you have nothing to lose should it all go pear-shaped. Also recommended…do it when the misses has gone to bed.