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12a_valves.md

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12. Valves (or tubes)

Definitions

  • Diode Valve is a valve having two electrodes (Cathode and anode)
  • Triode Valve is a valve having three electrodes (Cathode, anode and control grid)
  • Tetrode Valve is a valve having four electrodes (Cathode, anode, control grid and screen grid)
  • Pentode is a valve with five electrodes (Cathode, anode, control grid, screen grid and suppressor grid)

Diagrams

  • Actual valve amplifier (audio)

  • Triode

  • Tetrode

  • Pentode

Graphs

  • Grid voltage

Notes

Diodes

  • By heating a metal, electrons on the surface of the metal become very agitated and electrons will escape from its surface.
  • A diode is constructed with two electrodes: anode and cathode (heater not included)
  • Cathode is tied to ground.
  • High voltage is applied to the anode of the tube (also called plate)
  • Cathode is coated with Barium Oxide
  • Cathode is heated by a filament (made of heating wire)
  • Liberated negative electrons are attracted to the anode because it is at a positive potential
  • Like a semiconductor diode, reversing the polarity of the diode blocks current flow since electrons will not escape from a positive cathode

Triodes

  • To amplifiy, there has to be a means of controlling the flow of electrons from the cathode to the anode.
  • This is achieved by adding a wire mesh (grid) between the cathode and the anode
  • The grid is widely spaced so it does not "physically" impede the electron stream
  • When a negative signal is applied to the grid, it will repel some of the negative electrons and therefore reducing electron flow from cathode to anode.
  • Conversely, applying a positive signal to the gride causes more electrons to be attracted, and thus, more electrons reach the anode
  • Thus, since applying a small current can control a larger current flow, the valve can thus amplify the signal.
  • Such a tube with an anode, cathode and grid is called a triode.

Tetrodes

Issues with triodes

  • The space between the cathode, anode and grid of a triode gives rise to interelectrode capacitance.

    • This makes it not suitable for high frequency applications
    • This also gives rises to feedback, which could lead to instability

  • Triode interelectrode capacitance:

The solution:

  • Placing another grid in between the anode and cathode helps reduce the interelectrode capacitance (series capacitance)

    • This grid is called the screen grid.
    • Usually tied to a high voltage in respect to the cathode.
      • This enhances the attraction of electrons from the cathode to the anode as well.
      • Leads to a higher gain.

  • Tetrode configuration:

Pentodes

  • You may be asking: "Why do we need so many different kinds of tubes?"
Issues with tetrodes:
  • Remember how the screen grid helps enhance the attraction of electrons to the anode?
  • At times, the electrons can have so much energy that when it hits the anode, that they dislodge electrons from its surface.
    • This is called "secondary emission"
    • Has a detrimental effect to gain.
    • Impedes flow of current in the tube
The solution:

  • Add another grid in between the cathode and anode =P

    • Tied to cathode.
    • Since cathode is more negative in respect to the anode, secondary emission of electrons is reduced.

  • Part of amplifier circuit, using a pentode:

Example circuit (TODO: re-write or remove)

Advantages:

  • A valve will withstand a certain amount of 'mistreatment' during tuning
  • A valve is more easily matched to the aerial circuit
  • A poor SWR will tolerated by a valve

Disadvantages:

  • Requires a high voltage power supply
  • Fragile; requires care when handling!
  • Precautions necessary to avoid electric shock
  • Valves wear out and require replacement which is likely to be expensive
  • Less convenient for 'mobile' operation