[AMRadio] filament voltages

Donald Chester k4kyv at hotmail.com
Tue Aug 17 19:08:30 EDT 2004

>> > >Of course, we know that running voltages that are too high will kill 
> > >life expectancy of the tube, by why is running too little voltage bad 
>for the
> > >tube, if it's developing full emmission at a lower voltage?
> >

If the lower filament voltage does not affect the emission of the cathode at 
peak plate current, it will not harm the tube.  But even if the average 
plate current seems normal, but the lower filament voltage limits the peak 
even slightly, tube life will suffer.

The procedure with an AM transmitter is to modulate the transmitter to its 
maximum positive peak capability, with full filament voltage.  Then reduce 
the filament voltage until you begin to notice a dropoff of the positive 
peaks.  Run the filament voltage back up just past the point where positive 
peaks are fully restored.  The point is to run the filament just above the 
first indication that the cathode is being starved of emission.


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