[Amps] weird grid current

[Gary Patterson]

I have a 23cm four tube 7289 grounded grid cavity amp.  The four tubes are commonly biased.  It is generating good output.  The strange thing is that on key down for a couple of seconds,  the grid current starts at a reasonable value then goes to zero.  At zero,  I still have full output.  I realize its thermal changes that are varying the grid current,  but dont understand full output with no grid current.  Also,  if the amp is miss tuned,  the grid current will go "negative"  (grid meter reads pinned below zero the wrong way).   How can this be??

W4AF