Electronics > Beginners

Why weren't Vacuum Tubes designed for higher currents

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T3sl4co1l:

--- Quote from: ejeffrey on August 13, 2019, 05:14:01 pm ---I was wondering about this and thought maybe you could make a vacuum tube that used electron multipliers dynodes like a PMT to increase the anode current beyond what a heated cathode could apply.  Yes: a "grid controlled electron multiplier" is a thing https://ieeexplore.ieee.org/document/1472778

--- End quote ---

Neat, I wonder what it looked like.

Secondary-emission tubes have a long pedigree:
http://www.r-type.org/exhib/aai0191.htm
https://www.radiomuseum.org/tubes/tube_1630.html

They were generally unpopular due to the higher operating voltage, current consumption (secondary emission counts twice!), and noise (in addition to pentode partition noise, there's multiplier noise too), as far as I know.

I remember something else about high power vacuum devices, but can't recall exactly what they were calling them.  Offhand I see keywords like "triggered vacuum switch", which appears to be a vacuum ignitron as it were (so, operation will be limited by sputtering of the electrodes providing the plasma).  These are often discussed in context of mains distribution handling (surge arresting, arc extinguishing, etc.; but probably not carrying load current, or turning off, for which a recloser is usually needed, AFAIK).  So, on the order of 50kV and about as many amperes peak.

Tim

ZeroResistance:
What kind of heat losses is one looking at in vacuum tubes. If a tube is rated for 100kW, how much % of that would be wasted in heat?

tszaboo:
If you place a conductor (wire) in vacum, the only way it can cool down is through conduction and emission. So your wires in the tube would get very warm if the current is high. Also, I dont have the number to back this, but high voltage low current is probably more efficient as high current low voltage. Then if you have a load witch requires high current, for example as 8 Ohm speaker, then you just run the current through a transformer. Which was the standard way of making these amplifiers.

ZeroResistance:
I read that

"vacuum tube current is limited by space charge"

I understand that an electron cloud forms on the cathode but is space charge the effect of the electron cloud that is near the cathode repelling back the electrons emitted from the cathode?
How then do high current vacuum tubes mitigate this effect?

bsdphk:
Some of the best articles about this can be found in Bell Systems Technical Journal (can be found on archive.org), in particular there are some good ones in the years after second world war and up to the transistor taking over.

The answer is that there are a large number of limiting factors, including the already mentioned cathode material issues, but many other more or less obscure effects.

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