[Amps] A Filament Query

[Dennis OConnor]

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