[Amps] Purpose of cathode resistor?

[Bill Turner]

I have a question for all the amplifier experts out there.

In this schematic:

http://members.ispwest.com/dezrat1242/cathode.jpg

there is a 250 ohm resistor connected across the fuse/zener. What is 
the purpose of that resistor, and why is that particular value 
chosen? (I have also seen 200 ohms used.)

I can think of two possible reasons, but I'd like to hear from the pros.

Thanks in advance,

73, Bill W6WRT