Vacuum tube circuits
Vacuum tubes do not last for ever ! The filaments of antique tubes must be heated to approximately 1,400 degrees Celsius in order to emit suffucient electrons, whilst modern coated cathodes need only to be heated to about 600 degrees Celsius. This means that a lot of heat is generated, thus risk of movement from physical expansion.
Some people say target life for a new tube is 2,000 hours, others as low as 500 hours; suffice to say, it can vary substantially ! We have in inventory limited quantities of 10,000 hour rated JAN Mil. Spec. tubes. Life span depends a lot on the use in the circuit, consistent and properly rated filament voltage, and general lack of abuse such as vibration, and switching on and off in rapid succession.
Some vacuum tubes must work much harder than others and generate considerable amounts of heat. As a general rule, tubes at the entry point, or beginning of a circuit, where currents tend to be lower, work less hard and generate less heat than later and output stages. Single output and push-pull (parallel) output tubes work with high voltages and high currents, so they have a harder life, and tend to last less long than pre-amp and intermediate stage tubes.
All mechanical contacts do not age well electrically; check tube pins closely to see they are straight, not loose, and if corrosion is present.