[Amps] Surge resistor

[Vic K2VCO]

Rick Stealey wrote:
> Tom, W8JI, says:
>> The small glitch resistors in your amp don't do a darned thing
> 
> I am in the process of finishing up my 8877 amp, and the plans I am 
> using show a 0.6 ohm 1 watter and a 50 ohm 50 watter in series.  So in 
> the event of a flashover (milliseconds ?) the 0.5 ohmer is supposed to 
> blow open?  I can't see the value of the 50 watter.  In case of 
> extremely high plate current, such as hitting it with full drive and no 
> load, say 2 amps of plate current flows and even then the big resistor 
> only drops 100 volts, and heats up 200 watts. I wouldn't expect a 
> wirewound power resistor to fail immediately under these circumstances, 
> and I can't see how it is protecting anything.
> What am I missing here?  Is the 50 watter the wrong type?

The 50 ohm surge resistor is designed to reduce the very high current 
that would occur if there were an arc in the tube as a result of gas, or 
a 'whisker' on a metallic structure in the tube, for example.  This 
current could be hundreds of amps (the power is stored in the filter 
capacitors). A good surge resistor will allow a small arc to occur, 
which may clear the gas or whisker, and then the amp will continue to 
work normally. Without the resistor, the huge current may destroy the 
tube, RF choke, etc.
-- 
73,
Vic, K2VCO
Fresno CA
http://www.qsl.net/k2vco