Hi Pete,
as I already tried to explain in a previous message, the biggest stress
moment for the filament is when a cold one is powered and nothing exist to
limit the inrush current. Filament size and shape is temporary but
consistently modified (a bit frightening to see with big size long
filaments) and this is the main reason for filament failures, although
failures of this kind are not so common. Someone believe that saturated core
transformers are enough as a cure to prevent high filament inrush currents
but this method is actually far from beeing really safe.
Compared with commercial amplifiers, the amateur Amps generally suffer a
precocious death because of other matters than exausted or broken filament
but this is not a reason to enhance those risks switching too frequently
between ON and OFF.
There is nothing against to keep warm the filament with a smaller current in
a stand by condition ( TV sets using cathode ray cinescopes already use this
technique for more than 20 years) but I think this is finally not so much
more useful if compared with a really limited inrush current or a symple
"leave it on", and amateurs have have to develop their own circuits since
hardly a manufacturer will ever include something like that for amateur
products.
73,
Mauri I4JMY
>I wonder if anyone has ever tried reducing filament voltage on a
> ham amp during standby conditions. Any reason not to?
>
> 73, Pete N4ZR
> Happy Holidays!
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