My present understandig: When the anode voltage is applied to the cold tube, the cathode during heating up is operated in the saturation region. i.e. it cannot emit as many electrons as could be drawn off. It is said, that this can damage the cathode.
Why else would we find many devices where a relay does just that: Switch on anode voltage only after the cathode is properly heated? Just because of nice to have or to avoid start-up hum?
Regards, H.
Is it not funny, that in many old tube radios you find ECH81, where the triode part is deaf, whereas the heptode part still works fine? One may assume, that the triode (oscillator for AM) was mainly left without current, because people preferably listened to FM.
Why do tubes go?
Obviously what goes is the cathode. Obviously it does not burn, like the wires in light bulbs. It loses the ability to emit. Now why is that?
Regards, H.
Tubes with pure tungsten filaments don't have to be preheated. They don't have any coating or other material alloyed with it to damage. Tungsten filament tubes can be run in emission-limited mode with no problem; some are even made to be used this way. Examples are the 1236 and 2AS15. Tungsten filaments are also used in vacuum gauge tubes because they aren't hurt by being exposed to air (when cold).
Pure tungsten filaments are used in very few tubes because they are inefficient and have short life because of their high operating temperature (they glow bright white).
Tubes that we are likely to encounter with pure tungsten filaments are old > John Byrns wrote:
issue,
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