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Re: [Amps] Purpose of cathode resistor?

To: Vic K2VCO <>
Subject: Re: [Amps] Purpose of cathode resistor?
From: R.Measures <>
Date: Wed, 28 Dec 2005 10:33:40 -0800
List-post: <>
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:
>> 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
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Richard L. Measures, AG6K, 805.386.3734.

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