tubes age 89
> possibility that the structure is able to dissipate
> heat just as effectively at the higher voltage for the
> shorter period of time as it would at AC.
> Or does RMS power explain it exactly in practice?
Again the thermal time constant determines this. The temperature the filament
reaches will be determined by the RMS value of the waveform applied as long
as the period of the waveform is much shorter than the thermal time constant.
The thermal time constant of many small receiving tubes is on the order of 20
seconds.
> Just how do they establish the rated voltage/power?
From the needed temperature for proper electron emission... At low filament
temperatures, a tube's emission is solely determined by the filament
temperature (the principle behind temperature limited diodes used as noise
generators for testing receiver noise figure). With oxide coated cathodes, like
those used in the 6080 and most modern tubes, cathode emission is far more
than adequate for normal operation. So the cathode temperature is chosen as
a compromise of high electron emission versus filament life and excessive
"boil-off" of the oxide emitters. Because tube filaments operate at lower
temperatures than do the filaments in incandescent lamps, tube life versus
filament voltage is not quite as sensitive as it is in lamps. However the
relationship between lamp life and lamp voltage is well documented and is a
good thing to understand. Chicago Miniature Lamp, Inc. gives the following
relationship:
12
Life at Operating Voltage | Rated Voltage |
--------------------------------- = | -------------------------|
Life at Rated Voltage | Operating Voltage|
[Doing equations in ASCII is difficult, but the second ratio is raised to the 12th
power. The voltages are all expressed as RMS. Read this in a fixed width font.]
Basically this says that raising the voltage by 10 percent shortens the life to
less than a third the normal life. Likewise, dropping the voltage by 10 percent
more than triples the lamp life. [As an electrician, I am sure Joe realizes that
130 volt bulbs last far longer than do 120 volt bulbs.] This is also why those 24
volt bulbs are better replaced by 28 volt bulbs in the R-390 series. This
improvement in life is not without its costs though. The rated light output drops
with operating voltage. In the case of a 10 percent reduction in voltage, the light
output is only about 70 percent of what it was at full voltage. The light is
somewhat more yellow due to the lower filament temperature. But as I said
earlier, this equation does not directly apply to tube filament voltages with their
lower operating temperatures (at least with oxide coated cathodes; it holds
fairly well with high-power directly heated filaments). However it does show that
(14 pages)
(10 pages)
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