[Amps] Purpose of cathode resistor?

[R.Measures]

On Dec 28, 2005, at 9:33 AM, Vic K2VCO wrote:

> Bill Turner wrote:
>> 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.)
>
> My non-expert guess:  if the fuse blows, it prevents the filament
> transformer secondary from rising to a very high positive voltage,

The problem is negative potential.

> possibly exceeding it's insulation rating.

The fly in the pie is that 250v fuses don't simply open in 3kV circuit, 
during a glitch they create a metal vapour arc that does not limit 
current until after they explode.  Thus, during a glitch, the cathode 
can rise to c. 2900v negative.  Using a component at 10-times its max V 
rating is probably not good engineering practice.
> -- 
> 73,
> Vic, K2VCO
> Fresno CA
> http://www.qsl.net/k2vco
> _______________________________________________
> Amps mailing list
> Amps at contesting.com
> http://lists.contesting.com/mailman/listinfo/amps
>
>

Richard L. Measures, AG6K, 805.386.3734.  www.somis.org