In most applications these can also be used as a direct replacement for the 811A. The 572B has a much more robust anode design so can stand higher power outputs for longer periods without the risk of anode meltdown/short circuit.
These are brand new Thomson CSF tubes and have proven to date to have excellent performance and really low failure rates.
The valves are tested and supplied with individual data on request.
No, not the cheapest you will find but in a year or two they will still be going strong. You get what you pay for.