Types and Applications of Vacuum Tubes


It was stated earlier that the diode was used to rectify AC into DC. It acts as a one-way switch for electrons since there is no way for them to travel from anode to cathode. The nature of the device is why it was sometimes called a valve since it controlled the flow of electrons like a water valve controls the flow of water. Vacuum tube diodes were used to rectify AC to DC for many different things. Electric railway needed DC. Large scale electroplating factories needed DC and used rectifiers to supply it.  Before transistors and variable frequency AC drives, rectifiers supplied DC for variable speed DC motors in industrial settings.
diode
Drawing of a vacuum tube diode.
Image credit: http://newitfolder.blogspot.com

diode
Image depicting what happens when the plate(anode) is positive or negative.
Image credit: http://archive.hnsa.org

rectifier
Schematic diagram of a fullwave bridge rectifier using vacuum tubes. Note that here the 4 vacuum tubes are designated V1-V4. V was short for Valve.
Image credit: http://www.rfcafe.com

High current rectification in industrial settings did not generally use a traditional cathode . Different rectifiers were used but two of the main high current ones were the Ignitron Rectifier and the Mercury Arc Rectifier. In an Ignitron Rectifier, the electric current must be started by energizing the cathode. Each time a conduction cycle begins, the cathode must be energized. Ignitrons were normally arranged in large banks to supply very large amounts of HVDC. The Mercury Arc Rectifier differs in that the arc only needs to be started once when it is first turned on. Both devices used a pool of mercury as a cathode. Obviously these devices are no longer produced though some were still used up until recently.

ignitron
This image depicts an Ignitron High Current Rectifier.
Image Credit: www.ominous-valve.com

mar
Old magazine cover depicting the bulb of a mercury arc rectifier.
Image credit: wondersofworldengineering.com

mar
Image of a mercury arc rectifier. The large portion of the bulb was used to condense the mercury vapor
and the 6 smaller bulbs are where the anodes are located. There are 6 anodes for 6 phase AC.
Image Credit: technabob.com

The Triode is made by adding another element to the tube between the cathode and anode. This element is a grid which the flow of electrons must pass through before reaching the anode. The grid controls the electron flow. When the grid is at a negative potential with respect to the cathode, no electrons will flow. When the grid’s voltage rises to 0 volts or above, electrons are permitted to flow. When a weak signal is applied to the grid, the electron flow through the tube mimics the signal but it is much larger. The flow of electrons in the tube is proportional to the signal applied to the grid. In this way, the signal at the output of the tube is an amplified version of the weak signal placed at the grid. The triode amplifier needs a steady DC bias voltage at the anode to operate correctly. Vacuum tube triodes were used as amplifiers for decades and there are still many amplifiers that use them today. Triodes could be found in radios, test equipment, and in old analog and digital computers.

triode
Image of a triode type vacuum tube.
Image Credit:
www.extremetech.com

triode amp
Schematic of a simple triode amplifier.
Image Credit: www.ibiblio.org

triode bias
This depicts the bias of the triode type vacuum tube.
Image Credit: archive.hnsa.org

eniac
The Electronic Numerical Integrator and Computer used thousands of vacuum tubes.
Image Credit: www.maximumpc.com

Triodes were also used as oscillators and regenerators. They were able to convert a DC signal into an AC signal that could reach millions of cycles per second. Old radio transmitters and receivers used triodes in this way. High frequency industrial heating used triodes. Triodes could also be used in industrial control processes by tying the grid to an electrical device needing to be controlled. Also, when the traditional heated cathode is replaced with a photosensitive material, light can control the emission of electrons in the tube.
schematic
Schematic of a regen receiver using one triode. Note the anode of the triode is tied to the receiving end of the circuit by a coil. This
allowed positive feedback into the input which aided in amplifying the received signal.
Image Credit: http://upload.wikimedia.org/wikipedia/commons/a/ac/Regenerative_Receiver.png

regen
Simple regenerative receiver using one triode.
Image Credit: http://antiqueradios.com/albums/Misc/Sept_36_PM_regen_4.jpg

oscillator
Here is an oscillator using a single triode tube.
Image Credit: http://w1ujr.net/hartley-oscillator

Tetrodes and Pentodes came about to combat problems dealing with capacitive effects from the grid and secondary emission from the screen. These devices acted similar to triodes but had better performance at high frequencies. Many other types of tubes were produced. Many were combinations of diodes, triodes, tetrodes, etc.
tetrode

Schematic for a tetrode based amplifier.
Image Credit: www.ibiblio.org

tetrode

GMI-83B power amplifier tetrode.
Image Credit: http://www.nd2x.net

pentode
Schematic diagram for a pentode.
Image Credit: www.allaboutcircuits.com

pentode
Depiction of electron flow through the pentode tube.
Image  Credit: electriciantraining.tpub.com

pentode
Labeled parts inside of a pentode tube.
Image Credit:/www.vacuumtubes.net


Vacuum tubes eventually became phased out by solid state electronic devices. Transistors and diodes replaced the tube in many devices including radios, televisions, computers, etc. Industrial control now uses programmable logic devices and variable frequency drives which utilize transistors. AC to DC
as well as DC to AC conversion is done with solid state electronics. Cathode ray televisions stuck around until the late 90's to early 2000's before LCD and LED televisions became more affordable. Vacuum tubes are still used in HF amplifiers and some other specialty areas like high end audio amplification. Aside from those areas and antique electronics restoration, tubes aren't used much anymore.

 

Index

Intro

What is a Vacuum Tube?

Some History

How they work

Types and Applications

 Bibliography