Tom Rauch (W8JI) wrote:
If heater voltage is reduced durning operation of metal oxide
cathode
tubes, it can more harmful than running too much voltage. Low catode
temperature can instantly ruin the tube!
Also remember the warm up time specs are for conventional filament
supplies, if you reduce the voltage or severely limit current tube
warm
up time may need to be extended.
I have seen several 8877's with stripped cathodes caused by reducing
filament voltage below the specified range in an attempt to extend
tube
life.
In amateur service, emission failures caused by excessive filament
time
or temperature are almost unheard of. Since the tubes are generally
pushed right to the limit using the rated filament voltage makes
sense.
73 Tom
Thanks for the advice, guys. I am working on an 8877 based amplifier
(originally a commercial ISM RF generator), and the transformer in the
unit wants to run the filiment at 6.0 volts rather than 5.0. Does
anyone know the current draw of the tube at 5.0V? I want to use a
resistor in series with the filiment to get the voltage right ... or
should I put it in the transformer primary?
-Joe KM1P
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