[Amps] Negative current with a zener screen grid regulator

[N1BUG]

Hey amp gurus,

I have been going around and around with a recently rebuilt VHF 
amplifier that is refusing to work correctly. To help me think this 
through, I want to be sure I am understanding how some portions of 
it are SUPPOSED to work, or the theory of them.

I have a 4CX1500B tube. Screen voltage is regulated by a string of 
five high power zener diodes with the screen voltage tapped at the 
third or fourth zener from the bottom of the series string. I think 
I understand how this works with positive screen current but can 
someone nudge me in the direction of understanding what happens when 
the tube draws negative screen current? My belief is that current 
through the zeners (or SOME of them?) INCREASES in by the amount of 
negative screen grid current.

Is that correct?

Other than exceeding the power ratings of the diodes, are there any 
special concerns here? Is there risk of the thing dropping out of 
regulation and allowing voltage on the screen to rise significantly?

I feel sheepish asking this, as I ran this amp for many years with 
negative screen current and it obviously worked. But now that I'm 
having problems with it I want to understand things better.

I will ask my other question separately as it is not particularly 
related to this one.

Thanks,
Paul N1BUG