I never meant to imply that leaving a tube component running 24x7 will extend the life of the components or the tubes. Assuming a properly operating tube amplifier or receiver, if you leave the component powered on all of the time, the tubes should reach or exceed their rated life. If you power the unit on and off several times a day, the life of the tubes and other components that are stressed during the voltage surge during startup will be shortened, by how much no one can say.
So if a given power tube, such as an EL34, is rated at 2500 hours life, after this many hours of operation most people would consider the tube's output to be reduced to the point of diminishing its function in an audio circuit. Unless the tube fails internally, where one of the grids can collapse and short out the cathode or the plate structure (rare but it does happen), or the vacuum seal is lost, the tube will continue to limp along in a weakened state for anywhere from several hundred to several thousand more hours. Many times, a tube that tests very weak and is many hundreds of hours past its expected life will still play and will sound every bit as musical as a stronger testing example. Although testing weak, such a tube can becapable of being used for audio applications, due to the transient nature of the music audio signals.
Heat is the one factor that shortens the life of all electronic components, including tubes. Anything you can do to keep get rid of the excess heat generated by the tubes, and other components, and keep the operating temperature down will extend the life of the tubes and the equipment.
If you set the bias of your output tubes higher than recommended, for those amplifiers that have adjustable bias, then they will draw more current when idling and will run hotter. This will shorten the life of the output tubes. If the maximum recommended bias is 50 milliamps per output tube, then don't set the bias current (or in the case of the Dynaco ST-70 amplifier, the 1.5 volt test point voltage equivalent used to indirectly measure and adjust the bias level) any higher than this if you want your tubes to last. Reducing the bias, by 10-15%, will lower the idling current of the tubes and the temperature, while maintaining the desired musical sound quality of the tubes during operation.
If you are using a vintage amplifier or receiver, and take care to see that the line voltage seen by the power supply is no higher than 110 volts, the entire amplifier will run cooler. 108-115Volts AC was the norm when these components were designed. Today. the AC line voltage frequenltly exceeds 123 volts, at least at my house.
Use a small 3 or 4 inch fan to blow air across the power tubes, and through the chassis. The operating temperature of the amplifier will be greatly lowered, and the tubes and all of the components will last much longer. For power tubes, figure on more than doubling the expected life of the tube if you provide additional ventilation cooling via a fan.