Some versions of the graphite 3-500Z have slightly less gain than the original and would require more drive to obtain the same output. Amperex claimed this on their 3-500Z graphite tube. They were at one time, the best 3-500Z money could buy. Handling abuse like no other. No longer made and any NOS that may be sitting around, is likely to have the gas problems noted in the above post.
If I needed a new one today, I would see what Ameritron is using inside their recent production amplifiers needing that tube. They have probably already taken the time to select a brand with fairly low failure rates in order to prevent redundant warranty issues. People I know, have had excellent luck purchasing 3-500Z tubes directly from Ameritron.
One thing I can say, old school Eimac's typically test at double the breakdown voltage between grid and filament on the hi-pot tester as compared to anything made today. You bet that makes a difference in reducing the chances of an internal arc.
