Cathode Poisoning

This refers to chemical processes occurring at the cathode of a tube amplifier. There are several forms of cathode poisoning, including absorption of gas into the oxide coating, but the most dangerous type to amplifiers is the growth of interface resistance. When a tube cathode is fully heated but no anode current is allowed to flow for long periods of time (several hours), a high-resistance chemical layer can grow between the cathode tube and the oxide coating. This has an effect like an unbypassed cathode resistor; it increases noise and reduces the useful gain of the tube even though the oxide coating may have plenty of life left in it. The worst part is that once formed, this chemical layer cannot be removed.