[Amps] A Filament Query

[TexasRF at aol.com]

Hi Denny and others, 
 
The 60 hour loss of life for each off/on cycle is new information for me.  
What is the mechanism that drives that? Are there methods to mitigate the  
effect?
 
This subject just begs for more discussion.
 
73,
Gerald K5GW
 
 
 
 
 
 
In a message dated 1/15/2011 7:22:57 A.M. Central Standard Time,  
ad4hk2004 at yahoo.com writes:

Roger  and Carl...  I'm aware of the cycling number, which is why after 
mulling  it over in my mind I opted to recommend the 3% drop during the idle 
periods as  opposed to attempting to hit the magic minimum voltage all the 
time...   Certainly your method works as proven by your amps... The problem I  
encountered with attempting to hit the minimum filament voltage is when the  
power company quietly drops the line voltage by 20 volts to save coal - and 
 kill my expensive tube...

A 3% decrease will double the filament life  (5% will triple it)... A 3% 
decrease keeps the hot oxide from passing down/up  through the critical 
temperature (mid 600F range) which is the major part of  the loss of lifetime of 
60 hours, that each off/on cycle does to a tube... A  3% swing (and even 5% 
probably) will not subject the filament to undue stress,  either electrical 
or mechanical as there will not be an "inrush" then the  voltage is raised - 
the filament is still a medium orange hot and has  significant resistance at 
a 3% decrease...

denny /  k8do




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