What are indirect and direct heating tube?
Inside a indirect heating tube, the filament is used to heat a separate electrode called the cathode, and to use the cathode as the source of electron flow in the tube rather than the filament itself. This minimized the introduction of hum when the filament was energized with alternating current. In such tubes, the filament is called a heater to distinguish it as an inactive element. On the other hand, direct heating tube is to use the filament as the source of electron flow. For example, 6080, 6922, and 5AR4 are indirect heating tubes while 300B and 5U4G are directs.
