It is well known Chinese 811A and 572B tubes fail early. The Cetron 572B are known to operate well after 25 years. I do not have information of actual vacuum tube use in hours of operation, but something is amiss with these Chinese 811A & 572B tubes. The Chinese 3-500Z is noted to be going strong after being introduced 10 years ago. No doubt Chinese tube factories can make a high quality tube. Some have been in business since 1950. It is not vacuum loss, melted plates due to an undersized plate (dissipation capacity) or internal shorts causing the failures. The 811A and 572B filaments are a thoriated tungsten design. Tubes were also built of pure non-thoriated tungsten. It is easy to see the difference due to the pure tungsten filaments operate bright white like an incandescent bulb. Thoriated tungsten filaments operate at a yellow/white color. The thoria is added so the tungsten filament does not have to operate as hot as pure tungsten filaments and aids in the electron cloud produced in the tube at reduced filament temperatures. A thoriated filament will not function long term as the thoria becomes depleted if not controlled. To control thoria loss, the filament must be coated with a carbon film. The carbon film is created with a small shot of a carbon gas such as acetylene inside the tube when under vacuum at -5 to -6 Torr. After the carbon coats the filament part of the vacuum is loss. The tube needs to be re-evacuated after the carbon coating and this takes time. If a tube is not carbon coated at the filament, it takes double the time to finish a tube. I have not any hard facts and no one has at least in the USA regarding the early tube failures. But, I am guessing the Chinese 811A & 572B tube filaments are missing the carbon coating. Any other suggestions, I hope to hear from you.