Beam 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.

Vacuum Tube / Thermionic Valves Includes:
Basics     How does a tube work     Vacuum tube electrodes     Diode valve / tube     Triode     Tetrode     Beam Tetrode     Pentode     Equivalents     Pin connections     Numbering systems     Valve sockets / bases    

The beam tetrode is a form of tetrode valve that enables the ‘kink’ in the curve of the standard tetrode to be overcome.

The beam tetrode can be considered to be a special form of tetrode in which the design has been modified by beam plates held a low potential so that secondary emission from the anode is reduced to acceptable levels.

6L6 & 807 beam tetrode valves / tubes

The beam tetrode was a very successful form of valve or vacuum tube. It provided a similar level of performance to that of the pentode without infringing the patents of the companies that held the patents for the pentode. As such the beam tetrode was widely used, especially for RF and audio power applications.

Beam tetrode valve basics

The beam tetrode consists of a basic tetrode valve or vacuum tube in which beam forming plates are added between the screen grid and the anode.

The distinctive features of the beam tetrode are that the control and screen grid use helices of wire are wound on the same pitch with the wires align so that the screen wires lie in the shadow of the control grid wires.

Beam tetrode internal construction

The tetrode vacuum tube also has a flat cathode and beam deflecting plates. Also there is a relatively large screen-grid to anode distance.

The side deflecting or beam plates are electrically connected to the cathode and together with the flat shape of the cathode they serve to form the electrons passing between the cathode and anode into a beam with only a small lateral spread.

Beam tetrode circuit

The beam tetrode circuit is very much the same as the tetrode circuit would, have been. As the beam plates are connected internally within the valve / vacuum tube, there is little external difference to the circuit. Even the circuit diagram is the same except for showing the beam plates within the valve or tube envelope.

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

In the circuit using the tetrode vacuum tube / valve, the grid and cathode are biased in the normal way. The screen grid is taken to a high voltage via a relatively high value resistor to prevent it from drawing too much current. The screen grid is also decoupled to ground to ensure that it isolates the anode and grid circuits from each other.

Beam tetrode advantages

The advantage of the beam tetrode over the basic tetrode is that it eliminates the significant discontinuity or kink in the characteristic curve.

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

The significant discontinuity within the tetrode vacuum tube is caused by what is termed secondary emission. It results from electrons hitting the anode with significant force and bouncing off.

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