I have been running a set of RF parts 3-500z tubes for about six months I have noticed sometimes the output would vary from time to time yesterday powered up all was fine when I keyed up no output on meter I unkeyed then keyed again and output was fine I talked maybe 6 hours I let it cool down and shut off. tonight I turned it on warmed up no output again this time it didn't fix itself. I pulled the tubes 1 had a filament to grid short replaced with backup set all is good. Has anyone else had a tube short like this I never run it on high always low which is approx. 2200v high is like 2900v. Think maybe just a bad tube or did I do something wrong to kill it?