[AMRadio] filament voltages

Geoff/W5OMR w5omr at w5omr.shacknet.nu
Mon Aug 16 16:41:18 EDT 2004

> Too low of a filament voltage WOULD be detrimental to the tube life, BUT 
> reduced PLATE voltage while maintaining full filament voltage WILL 
> extend the tube's overall lifespan.
> That's an area that I agree with Don completely!
> It's the one place that variac use on start up of an older stored rig 
> can give you problems.  The rectifiers are functionally out of whack.  
> Solid state "temps" would be wiser during that stage.  Or even reform 
> caps with an old heath cap tester.  It has multiple voltages to test 
> them with.

I've got a small variac now on the filaments of my 250TH's, and always
run a variac on the plate transformer primary.  But, just to test the tubes
in the final, I reduced the filament variac down to where the 5v meter was
showing around 3.5v, before any emmission was lost in the 250TH's in
the final of that rig.  So, I'm wondering if there would be any harm in 
dropping the filament voltage to 4.0v (as opposed to 5.0v) if there is no
reduction in emmission of the tubes?
250TH's require 5v @ 10.5 amps, per)

73 = Best Regards,

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