A friend has an 811H with 3 year old 572B's. One shorted?and causes the fuse to blow immediately. Tube replaced and all is well.
Measuring with a DVOM shows no shorts at all. I do know some tubes will short after they get warm/hot but this one blows the fuse immediately (no time to get hot) but does not measure any short circuit.
I've now talked to two other hams on the air that say the exact same thing happened to them.
Bob