Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

How does a vacuum tube get hot?

  1. Apr 17, 2010 #1


    User Avatar

    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?
  2. jcsd
  3. Apr 17, 2010 #2


    User Avatar
    Science Advisor
    Gold Member

    Radiation is probably the biggest contributor. There is still a very small amount of gas in the tube.
  4. Apr 17, 2010 #3
    Sunlight feels warm, right? Even after 93 million miles (150 million km) of vacuum.
  5. Apr 17, 2010 #4
    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!!)
  6. Apr 17, 2010 #5


    Staff: Mentor

    Adding to what twang said, the cathode and anode are both physically attached to the tube, so you get conduction as well as radiation.
  7. Apr 17, 2010 #6
    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.
  8. Apr 17, 2010 #7
    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.
  9. Apr 17, 2010 #8


    User Avatar
    Homework Helper

    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: Apr 17, 2010
  10. Apr 17, 2010 #9
    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
  11. Apr 17, 2010 #10
    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
  12. Apr 17, 2010 #11
    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
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook