Tetrode Valve / Vacuum Tube

The tetrode valve or tetrode vacuum tube has a total of four active elements within the envelope, having a further grid called the screen grid beyond that used by a triode.

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The triode valve or vacuum tube worked well but had some drawbacks, especially in terms of gain and stability. Feedback between the anode and control grid circuits caused by internal capacitance lead to instability.

The tetrode vacuum tube or tetrode valve gave a considerable improvement in performance. It added a second grid called the screen grid into the envelope. This improved the gain of the valve, but more importantly, it also acted as a screen, isolating the input from the output.

Tetrode vacuum tube basics

There are several types of tetrode vacuum tube, but the one that is virtually always referred to is the screen grid class of tetrode vacuum tube.

This type of tube or valve uses a second grid called the screen grid that is placed between the main control grid and the anode.

This type of tetrode vacuum tube arose because the traditional triodes made in the early 1900s were very difficult to use at frequency above 50 or 100 kHz. In these radio frequency amplifiers, both the anode and grid circuits were tuned. The capacitance between the anode and grid lead to significant levels of feedback and as a result these circuits were very difficult to use. At frequencies above around 100kHz the levels of gain had to be severely limited to prevent oscillation.

The screen grid placed between the control grid and the anode was decoupled to ground, thereby reducing the anode control grid capacitance to very low values and enabling the circuits to operate with much higher levels of gain.

Tetrode vacuum tube / valve circuit symbol
Circuit symbol for the tetrode vacuum tube / valve

In order the electrodes within the tetrode valve are:

  • Cathode:   The cathode is heated by the heater element to a temperature at which the electrons can be emitted into the vacuum of the glass envelope.
  • Control grid:   The control grid of the tetrose valve or tube is normally maintained at a voltage which is negative to the cathode. It is fundamentally used to control the flow of electrons between the cathode and anode.
  • Screen grid:   The introduction of the screen grid what gives rise to the difference between the tetrode and the triode. It provides improved performance for the tetrode providing screening between the anode and grid, thereby reducing the capacitance between them and hence the feedback.. The screen grid is operated at a fixed positive potential, but below that of the anode.
  • Anode:   The anode or plate is operated at a high potential and it is the electrode to which electrons are ultimately drawn in the pentode or any other valve.

Tetrode valve circuit

In operation a tetrode valve or tetrode vacuum tube has the same connections as that of the triode valve, but the screen grid is normally held at a high voltage via a high value resistor and the grid is decoupled to ground with a capacitor.

Typical tetrode vacuum tube / valve circuit
Typical tetrode vacuum tube / valve circuit

Limitations of the tetrode valve

The basic tetrode has very limited use these days. Although it provides a significant improvement in isolation between the anode and the grid circuits, it has one major drawback.

Characteristic curves for tetrode, beam tetrode and pentode
Characteristic curves for tetrode, beam tetrode and pentode

It is found that the anode characteristic has a major kink. This results from the interchange of secondary emission electrons between the anode and the screen grid.

In view of this limitation of the tetrode valve, and the fact that much superior high performance valves or tubes are available in the form of beam tetrodes and pentodes, the basic tetrode is seldom used.

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