How does a vacuum tube get hot?

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ISX
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Main Question or Discussion Point

Old CRT tv's use vacuum tubes and they get hot, but how does the glass get hot when the inside is under a vacuum and therefore shouldn't transfer any heat?
 

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  • #2
Born2bwire
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Radiation is probably the biggest contributor. There is still a very small amount of gas in the tube.
 
  • #3
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Sunlight feels warm, right? Even after 93 million miles (150 million km) of vacuum.
 
  • #4
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Heat travels in three ways: conduction, convection, and radiation.

The heat from the hot, glowing filament is radiated outwards, just as the sun radiates infrared radiation, just as the heat from a bonfire reaches you hundreds of feet away.

The current passing through the tube also heats the cathode and anode. (Pass too much current through the tube and the anode will glow red ... *not* good for the life of the tube!!)
 
  • #5
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Adding to what twang said, the cathode and anode are both physically attached to the tube, so you get conduction as well as radiation.
 
  • #6
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Dale is correct ... and to add another detail, some tubes will glow various colors when they're in operation because they have quite a bit of gas inside.

See here: http://thetubestore.com/blueglow.html

That gas will *also* carry heat to the glass envelope of the tube.
 
  • #7
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I hate to tell you this but vacuum tubes have an internal heater, additional to the business electrodes (anode and cathode). This is usually run at 6 or 12 volts and works on resistive heating.
 
  • #8
rcgldr
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Infrared doesn't pass through glass well (which is why glass is used in greenhouses), so the heat can only be transferred out via conduction at the glass surface, and the result is hot glass, even with a near vacuum inside. The purpose of the heater inside a vacuum tube is to provide a source of free electrons.

Also I had the impression that most of the older vacuum tubes operated around 100 volts or so.
 
Last edited:
  • #9
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The vacuum tube plate dissipation (plate voltage times plate current) is often greater than the heater (or filament) power dissipation. Most of this heat is radiated out to the glass and cooled by air convection.

Bob S
 
  • #10
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The vacuum tube plate dissipation (plate voltage times plate current) is often greater than the heater (or filament) power dissipation.
Good point, but typical lineup in valve equipment is 1 to 5 signal stages followed by output stage where this may well be true.

For example just looking at random in Ball

An ECF80

Heater wattage = 6.3 volts x 430 milliamps = 2.7 watts
Anode watts = 180 vols x 5 milliamps = 0.9 watts
 
  • #11
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The ECF80 is a dual pentode-triode tube. The pentode section plate dissipation is rated at 170 volts x 10 mA = 1.7 watts. the triode section plate dissipation is rated at 100 volts x 14 mA = 1.4 watts, for a total of 3.1 watts. The heater is 6.3 volts x 430 mA = 2.7 watts, as pointed out above..

Bob S
 

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