[Amps] 3-500z ratings

[Vic K2VCO]

The Eimac data sheet for the 3-500z
<http://home.c2i.net/clank/ham/pa/3-500z.html>
says that maximum plate current is 400 MA. This implies that exceeding 
this would be bad for the tube.

But...if the plate and grid dissipation are kept within limits, how 
would it hurt the tube to run it at, say 450 or 500 MA?

I am not saying that I doubt the data sheet. Maximum means maximum. What 
I don't understand is why there is this current limit as long as the 
plate dissipation is not exceeded.
-- 
73,
Vic, K2VCO
Fresno CA
http://www.qsl.net/k2vco