[AMRadio] filament voltages

Donald Chester k4kyv at hotmail.com
Tue Aug 17 19:15:03 EDT 2004

>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?

That is a good way to test transmitting tubes.  A good tube will have high 
reserve emission, and it will take a substantial filament voltage reduction 
to reduce emission.  A marginal tube will show a dropoff of emission with 
even slight undervoltage.  A weak tube will often show full emission with 
filament voltage above the rated value, but that proformance is likely to be 
short lived.

For actual operation, be sure to carefully check the PEAK emission of the 
tube.  You can have substantial peak limiting before it shows up on the 
meters in the transmitter, since mechanical meter movements are 
average-reading devices.  Such operation will reduce tube life. Better to 
use a scope or LED peak reading device.

Don k4kyv

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