[Amps] Thoriated tungsten lifetime when idling tspa

[k7fm]

I suppose the knowledge of filaments has advanced since 1942, but there was 
a special reason to "make do" when the Radio Handbook was published.  There 
is an ad in the back for RCA Transmitting Tubes.  It still makes interesting 
reading, though some may debate some of the statements:

HOW TO MAKE TRANSMITTING TUBES LAST LONGER

LONG LIFE - not maximum output - is the keynote of transmitting tube 
operation today in many services where, because of war restrictions, it may 
prove difficult to replace tubes that wear out.  Since care in the use of 
tubes - even far beyond what might ordinarily be considered necessary - 
should pay big dividends in longer life, the following suggestions are 
worthwhile:

For tube types using pure tungsten filament, a reduction of only 5% in the 
filament voltage doubles tube life.  A reduction of 15% increases it almost 
tenfold:  Decrease filament voltage to 80% of normal for standby periods of 
less than two hours.  For longer periods, the tubes should be shut down.

Care should always be taken in starting up tungsten filaments.  Never should 
the filament current exceed, even momentarily, a value of more than 150% of 
normal.

For types using thoriated-tungsten filaments and oxide-coated filaments, the 
filament may be operated on the low side - as much as 5% below normal 
voltage - if the loading is light.  The filament voltage should be increased 
gradually to maintain output.  Toward the end of life, additional service 
may be obtained by operating the filament above its rated voltage.  During 
standby periods of less than 15 minutes, the filament voltage may be 
decreased to 80% of normal to conserve life.

For heater-cathode tube types, where some operating delay can be tolearted, 
it is a good practice to drop the heater voltage as much as 20% during long 
or frequent standby periods.

For all types, reduce dissipation of grids andplates to a minimum to avoid 
overloading and, thus, obtain materially longer life.

ROTATE SPARES - Like auto tires, vacuum tubes should not stand unused for 
long periods. Rotate your spares to assure every bit of service of which 
they are capable.

KEEP THEM COOLER - A good way to increase the life of tubes is to keep them 
cooler.  One method of doing this is to reduce plate voltage and 
dissipation, to the lowest permissible limits.  Another, and often more 
feasible, is to use forced-air cooling.  Use it on tubes, even where not 
specified and increase it above the normal amount where it is specified.


Some of this information may not be consistent with Eimac literature, but is 
probably still good for the old 813 amps.  RCA offers a complete guide for 
all their transmitting tubes for 35 cents.

Colin  K7FM