Some winter reading....
http://www.w8ji.com/qst_the_care_and_feeding_of_a_3-500zg_filament.htm
Sent from my iPad
On Dec 21, 2015, at 1:17, John Lyles <jtml@losalamos.com> wrote:
> 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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