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