Thorium is added at the 1% level to pure tungsten, to improve efficiency
of electron emission from the filament of direct heated tubes. It allows
a reduction of the temperature of a filament and reduced heating power,
which all contribute to longer life over tungsten filaments. It is not
added to the other metal parts of tubes.
It is true that the number of turn-on events is a factor in the lifetime
of large tubes. Without overloading or misusing the tube,
filament emission lifetime should be the primary end of life event. When
the filament is energized and deenergized repeatedly, even with careful
current limiting and ramping of voltage, mechanical changes occur to the
'basket' and lead to sagging and growth in diameter at the bottom. Given
enough of these events, there is a finite possibility of shorting
filament to control grid in large modern high gain tubes (where the
spacing is already very close). Hence, it is better to leave them
operating, and reduce the voltage when going to standby, but keeping
some heating, sometimes called black heat mode.
Operating a high power tube below it's rating doesn't have an effect on
lifetime, unless the filament is operated far below design which can
result in damage. On the contrary, operating below rating usually does
contribute to lifetime extension, especially if the filament voltage is
managed so that emission is not excessive, far beyond the peak plate and
cathode current desired in the operating point.
I have some amplifiers that run over 100 kW at HF, using modern
tetrodes, and they routinely exceed 30,000 hours life before
replacement. They are used for peak current, but the average power is
quite lower than their rating.
73
John
K5PRO
On 12/17/2015 7:59 PM, Charles H wrote:
On this reflector, I have poo-pooed the idea of baby-ing your radio especially
by turning it off often, lowering xmit power, etc.
However, I read that with ordinary tubes, the chemistry of the cathodes and
filaments are affected by life-of-use, so that tubes do
"wear out" with use. Tubes can be rated in terms of "hours of use" with
the type in a KWM-2 rated at around 2,000 hours of useful function.
Thorium in the chemical make-up of the metal parts in a tube, maybe a
metal or ceramic tube (usually added to higher power tubes) can extend
tube life to about 100,000 hours.
The conclusion is that "hours of use" of course do matter for hams intending to
use their tube-type radios for thousands of hours. The hours of use can be limited by
simply turning the tube off when having only stand-by function.
However, there is some suggestion in the literature indicating that
every "turn on" of a tube (or an incandescent
light bulb) involves rapid heating of the elements inside which could,
immediately or over time, deform elements and causing sudden failure
(given that the metal elements do heat
unevenly--due to resistance in the metal--if only for a very short
duration of time). Repetition of on-and-off cycles are thus a factor in
lower tube life.
Other than these factors, it appears in the literature that operating a tube,
particularly high power transmitting tubes, at its rated values can extend life
IF it is
otherwise not sometimes operated below or above its ratings. It appears
that, for example, driving an RF amplifying tube at its full rated power
does not materially decrease
its life. Reducing drive appears not to extend such tubes' life.
It is all a balancing act. 73, Charly
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