In this method, the cathode itself is a heater or heating element or filament. Hence, a small amount of heat energy will provides enough energy for the free electrons to escape from the cathode. The free electrons that are escaped from the cathode will enter into vacuum. These free electrons in the vacuum are attracted towards the anode.
In the directly heated cathode , the amount of heat energy required to emit the free electrons is less compared to the indirectly heated cathode. If the heat is supplied indirectly to the cathode, the cathode is said to be indirectly heated.
In the indirectly heated cathode, there is no electrical connection between the heater and the cathode. When the heat is supplied to the heater, it gains heat energy. The heat energy gained by the heater is supplied to the cathode.
Thus, heat is indirectly supplied to the cathode. When the free electrons in the cathode gain enough energy in the form of heat, they break the bonding with the cathode and jumps into vacuum. The number of free electrons escaped from the cathode depends on the amount of heat applied to the cathode and the work function of the cathode. If large amount of heat energy is supplied to the cathode, large number of free electrons is emitted from the cathode. Similarly, if small amount of heat energy is supplied to the cathode, less number of free electrons is emitted from the cathode.
Work function is the minimum amount of heat energy required to remove the free electrons from the metal. Low work function metals require less amount of heat energy to emit the free electrons. On the other hand, high work function metals require large amount of heat energy to emit the free electrons. If no voltage is applied to the control grid and positive voltage is applied to the plate, the vacuum triode behaves like normal vacuum diode, because control grid will not shows any effect on the free electrons emitted from the cathode.
If voltage is applied to the control grid, it produces electric field. In this case, no voltage is applied to the control grid. Hence, control grid will not produce the electric field to attract or repel the free electrons.
Therefore, the free electrons emitted from the cathode will easily moves towards the anode or plate from the openings of control grid. If negative voltage is applied to the control grid without changing the positive plate voltage, no electric current flows in the vacuum triode, because the control grid opposes or repels the free electrons that try to move towards the anode.
Because of this supply of negative voltage, the control grid becomes negatively charged. Hence, it produces negative electric field. On the other hand, free electrons emitted from the cathode are also negative charged.
Hence, free electrons also produce negative electric field. We know that, if two like or same charges are placed close to each other they get repelled. Hence, the control grid opposes or repels the free electrons emitted from the cathode. However, a small number of free electrons overcome the negative electric field of the grid and move towards the anode.
If the negative voltage applied to the control grid is increased, no electrons will move towards the anode. Hence, no electric current flows in the vacuum triode. If positive voltage is applied to the control grid without changing the positive plate voltage, electric current flows in the vacuum triode, because the control grid attracts large number of free electrons.
This is depicted by the red dot here on the data sheet's plate characteristics , which show plate current versus plate-to-cathode voltage. Fundamentals of Guitar Amplifier System Design - design your amp using a structured, professional methodology.
Each curve represents a different grid voltage, from -5V in the lower right to 0V in the upper left. The blue dot shows that if the grid voltage is reduced to -4V, no plate current flows, a condition known as cutoff. Moreover, a 2V change in grid voltage results in a 2. The control grid exerts control over current flow. The green dot indicates that with a -3V grid, the plate current is 0.
If the plate voltage changes by 2V, to V or V, plate current barely changes. Grid voltage has much more control over plate current than plate voltage because the grid is closer to the cathode. Its electric field effectively screens the cathode from the electric field extending from the more distant plate.
This phenomenon forms the basis for guitar amplifier preamps: a small input voltage swing at the grid creates a change in plate current that can be used to create a much greater output voltage swing. This is the essence of voltage amplification. New - Our Latest Book! A deep dive into the electronic design of the ultimate rig-of-death! From TV front to the 5E and 5F Series narrow panels, from system design concepts to individual stage operation, an all-new examination of Bassman electronics.
The triode valve or triode vacuum tube takes the basic concept of the diode and moves it on a significant stage further. A third electrode called a grid or more correctly a control grid is placed between the cathode and anode of the basic diode and by applying a potential to the grid, it is possible to repel or attract the electrons being emitted from the cathode and in this way affect the flow between cathode and anode of the triode vacuum tube.
When there is no voltage on the grid of the triode valve or triode vacuum tube, the current between the cathode and anode will be at its maximum. Placing a negative voltage onto the grid will have the effect of repelling some electrons back to the cathode and thereby reducing the number of electrons travelling to the anode.
When a triode valve is used within in a circuit, a resistor is placed in the anode circuit. The control grid typically consists of a helical wire that is placed between the cathode and the anode. In some instances a mesh may be used. The control grid in the triode valve or triode tube serves as an imperfect electrostatic shield allowing some, but not all of the electrostatic flux from the anode to leak through to the cathode.
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