[Amps] Switching Power Supplies

[Tom W8JI]

> The reason I chose a small switcher for my filament supply 
> was to
> provide a stable DC filament voltage over changing input 
> line voltages. The switcher I found was a fully shielded 
> commercial variety that appears to be relatively RFI free, 
> weighs less than the transformer normally used and 
> generates none of the heat an analog regulator produces.

Filament or emission life is virtually never an issue for 
amateur service. All the worry comes from commercial 
applications, where the tube runs 24/ 7/ 52 weeks a year.

In amateur service the primary problems are turning the tube 
filament off and on over and over again, extended periods of 
non use allowing gassing, and just plain bad tubes. In 
amateur service we are lucky to see 500-1000 hours a year. 
It'll be long gone for some other reason before the emission 
rolls out.

In commercial service the tube sits and runs never being 
cycled, usually running at a small fraction of ratings. A 
tube often sees over 8000 hours a year and they might try to 
milk it out for a few years, so emission or filament life is 
a concern.

73 Tom