[Amps] Thunderbolt questions

[Bill Russell]

I have a Johnson Thunderbolt hf amp and need to replace the doorknob caps 
in the output stage. It uses 2 500 mmf at 20kv in parallel. Can I use 2 
640mmf as replacements without causing problems. The amp uses 2 4-400A's.
Next question.....
The amp uses 4 VR tubes in series for screen requlation. There are 2 
vr150's and 2 vr105's. Can I replace them with zener diodes of the same 
voltage and if so, what size zeners should I use.
Thanks,

Bill Russell
KC2IFR