Vacuum tube or Valves

Only the general image below shows true 'valves'. Many of the other interesting tubes here are technically not all valves. Only the Miniature valve, thyratron, VFD, magnetron, X-ray tube, magic-eye and vidicon tubes have thermionic emission from heated cathodes.

Miniature valve

The very small vacuum tube shown above is just over 1" (2.5cm) long. Its tube construction is similar to a small neon indicator lamp and was taken from an old 1960's tape recorder.  It appears to have two grids making it a Tetrode. A thin directly heated wire cathode is used, as was common on battery operated valves. The 'mouse-over' image show the heater/cathode wire glowing when supplied with 3V.

Thyratron

This version of a mercury arc rectifier has been superseded by the solid state device shown above, known as a thyristor or SCR (silicon controlled rectifier). The Thyratron was used in applications of high current DC control, such as motor controllers. 

Photo Multiplier Tube

This device uses secondary emission of electrons to amplify small signals produced by low levels of photons.  Photons hitting the sensitive coating on the front of the tube knock out electrons which then collide with charged plates called dynodes.  This creates secondary emission of electrons. A cascade of electrons reaches the anode resulting in a larger output signal.

Geiger - Muller Tube

This radiation detector contains gas at low pressure, usually argon. Other gasses are added to quench the the discharge. Alcohol and halides have been used for this purpose. The window is made from mica as Alpha particles can pass through this.  There is a high voltage applied between the centre pin, which is just visible through the mica window, and the metal casing.  If ionizing radiation or high energy particles hit a gas molecule in the tube electrons can be knocked out of the atom. When this happens an avalanche of electrons occurs through the gas atoms due to the applied voltage. This creates a conductive path between the pin and casing. The resultant current can be detected and can be amplified to produce a click on a speaker.  Interestingly, if viewed in a dark room, it is possible to observe the faint flash of light emitted by the gas in the tube when ionization occurs.  This is shown in the second image below.  This picture is taken from the front looking in through the mica window.  A discharge glow can be seen from the centre pin (cathode) to the wall of the tube (anode).  A small alpha-source is positioned at the left side of the mica window. 

Photo Tubes

These are photon detector tubes which are based on the photo-electric effect. They are gas filled and coated to readily emit  electrons when hit by photons. The tube in the foreground is a Mullard 90CG photo tube.

- on the photo-electric effect

Vidicon Tubes

These tubes were used in video cameras to convert images into electrical signals. They have been replaced since the early 1990's with the advent of the Charged Coupled Device (CCD) silicon chip.  The tubes employ an electron gun and electromagnetic deflection as in a television. Instead of the electron beam being scanned over a phosphor coated screen, the vidicon electron beam scans a light sensitive coating. The coating's electrical resistance varies with light intensity. The beam current is therefore modulated by the light on the coating.   The optical shutter seen on one of the tubes below allows a colour signal to be formed using Red, Blue and Green filtering.  The deflection and focusing coils which normally completely surround the tubes have been removed.

Helium Neon Laser

These low power visible red lasers have mainly been replaced with solid-state laser diodes. Power supplies of around 6kV are required to operate this type of laser.  They tend to stop lasing with age due to the helium escaping from the glass envelope.  In this state the neon remains and the internal discharge will continue to glow but no laser light is emitted.  The red laser beam can be seen emerging through the partial mirror in the right hand image.

More detail can be seen on the tube below. Laser light emerges from the partial mirror on the left (cathode end). The yellow strip on the back of this tube contains a conductor connected to the anode. This aids with starting when the voltage is applied.

Nixi Tubes

Nixi tubes were commonly used as numerical displays in early electronic equipment. They have a glass envelope filled with neon gas at low pressure. The gas around the metal numerals glows when a voltage is applied to them.  The tube on the right (Dekatron Counter tube) also uses neon gas as an indicator but small pins were illuminated by the neon glow to indicate position or count. These were often used in early counter / timers

Vacuum Fluorescent display

These are commonly found in consumer electronic products. In this example the 7-segment figure 8 is clear. However, many displays have complete words, icons and symbols. They operate like a valve, in that there is a heated cathode, control grid and anodes.  The anode is coated with a fluorescing material. Many colours can be used in one display.  A number of fine wires run across the front of the display. These are heated by a small current and form cathodes. The electrons emitted can travel to the anode if permitted to pass by the charge on the mesh grids in front of the fluorescing anodes. 

Although these displays were popular in the 80's and 90's they are becoming less so now due to their power consumption and higher voltage drive circuits. The complete display is evacuated. The dark patch on the bottom right is 'getter' to preserve the high vacuum during the displays lifetime. 

The 'mouse-over' alternative image shows the display with some numerals on. 'Click' on the image for a close up .

Radar Magnetron

Primary Radar Magnetron without magnet. The heater and cathode connections are out of sight at the top of the white plastic case. 

  label 

Early Cold cathode X-ray tube

An example of an early cold cathode or Crookes X-ray tube is shown here. This one is likely to date form around 1920.  It is manufactured in England by Cuthbert Andrews. These tubes rely on an imperfect vacuum to operate. The small tube on the top is designed to release gas molecules into the tube. The image on the right shows the tube operating at 25kV. The green glow is from the gas fluorescing and the blue glow at the top is from gas molecules in the tube.  This is the lowest voltage the tube will run at and X-rays are generated in this condition. 

Modern X-ray Tube and Housing Assembly

The X-ray head assembly below is a Varian  B-112 containing a M-113 tube. The unit is oil filled for electrical insulation and to aid heat removal from the tube and stator.  The tube has dual filament cathodes and a rotating molybdenum alloy anode target. It has a maximum acceleration voltage of 39kV as it is specifically designed for Mammography. 

The black connector on the left has feeds for the cathode heaters and the stator windings. The larger connector below is the high voltage feed to the tube anode. The X-ray window can be seen on the right. 

  - on the above assembly's construction.

Corotron  High voltage regulator

This type of high voltage regulator behaves like a zenor diode, but uses a gas discharge to create a path of low resistance once a certain voltage is reached. This example regulated a cathode ray tube supply at 14kV. The outer metal tube is the return path for the high voltage supply.

The construction is illustrated below. (1.) High voltage positive terminal  (2.) Insulation (3.) Anode and cathode  (4.) Gas at low pressure.

Magic Eye

This is an early indicator valve known as a 'Magic Eye' used to indicate signal strength.  A grid voltage controls the beam pattern landing on the anode. The anode surface is coated with fluorescent paint which glows green when hit by electrons.

6M1             Image on anode                    Symbol