Vacuum tube works with cathode (an internal structure which is heated at extremely high temperature to emit electrons) to generate electron flow – i.e. electron current, which is usually measured in mA (number of electrons flow through per second). This is the most important indicator of tube life.

Strong emission = longer tube life. Weak emission = shorter tube life.

Strong emission also means that the tube will perform in the circuit closer to the equipment’s designed specification.

With time, all tubes will have weaker and weaker emission, until eventually they cannot perform adequately in a live circuit.

Other factors will also affect tube life such as vacuum leakage, heater to cathode leaker or internal gas.