Some amplifiers use the 572B tube and upgrading the AL-811 or AL-811H from weak 811A tubes is performed by Ham radio operators. Years ago, reliable NOS Cetron 572B tubes were sold at reasonable costs. A few years ago $75 NOS Cetron 572Bs were easily available. Now NOS Cetyron 572B are $200 each at RF Parts company and about $120 each on eBay if found at all. And, lots of three or four NOS Cetron 572B tubes are extremely rare. The Chinese Shuguang 572B tubes are as low as $42 each new from China. Is it worth the risk for a Chinese 572B? Early manufacture Chinese 572B tubes were a disaster. Many arced over at the plate insulators. RF Parts had to check each tube before shipping. Rejection rates were high. Chinese manufactures solved the insulator arc over issues with RF Parts help. Next, Chinese 572B tubes were noted to not last more than a few years. This did not look good for RF Parts 572B selling at premium prices, yet out of warrantee. I suspect the Chinese 572b has been further improved and is a long lasting tube now. I have not heard of any failures after 2 years of late. Interesting the Chinese 3-500Z tubes sold 10 years ago are still producing full power today. I am not sure what the new Chinese 572B tube changes have benn, but it is likely filament design related. None of the Chinese 572B tubes were noted for low vacuum issues. I suspect the thoria content in the 572B tubes has been improved and/or perhaps early Chinese 572Bs did not have either a carborized filament or enough of a carborized coating. You can thank RF Parts for the 572B improvements. For the new Chinese 572B tubes on ebay selling for $42, ask if new manufacture vs being returns from RF Parts. I would take a risk on the Chinese 572B tubes now from RF Parts vs the $200 Cetron 572B tubes. Not sure if the $42 tubes on ebay are newer high quality tubes. Some Ebay sellers state directly tested by Shuguang at $60 per tube.