Why is a negative feedback in the amplifier desirable

Negative feedback and vacuum tube life cycle

No, whether or not a vacuum tube is used on a negative feedback circuit does not directly affect the life of the tube. As long as the tube is not misused, it should be fine. It may be easier to design a circuit with feedback to make sure the tubes run within a few parameters, but this is just one more thing to consider when designing a circuit without feedback.

The main cause of tube failure, aside from direct physical damage, is cathode wear. The cathode has a special coating to reduce the work function. This allows for reasonable cathode currents at lower temperatures than usual. However, this coating degrades over time, especially as electrons leave the surface of the cathode. In very broad terms, any cathode coating is good for a certain number of total coulombs, although there are many more than that. Feeback or no feedback alone does not mean higher or lower average cathode current. Typically these tubes are biased at the recommended operating points, so both circuit types would likely have roughly the same average cathode current.

Super cat

While the extent to which a tube wears out depends primarily on the number of coulombs pushed through it, I would expect the amount of wear and tear a tube can withstand before being ready for use in a becomes unsuitable for a particular circuit, one function is the tolerance of the circuit to changes in pipe parameters. If a circuit attenuates a signal by a factor of four and then feeds it to an open gain amplifier with a gain of 20, a 25% change in gain would affect the output by 25%. However, if the circuit used negative feedback instead to reduce gain, the output would be far less affected.