Dealing With Secondary Emission in Tetrodes

6AK7 pentode in metal envelope.

As we saw in earlier posts the diode vacuum tube (or valve) became a triode by inserting a grid, known as the control grid, which allowed it to amplify audio and RF signals. The triode had a problem of interelectrode capacitance which limited the frequency at which it would operate properly. By adding another grid the triode became a tetrode. The second grid, called screen grid, has a positive DC bias but an AC ground via a bypass capacitor.

The tetrode was an improvement over the triode but it had a problem with secondary emission.

When the electrons pass the positively biased screen grid they speed up then when they strike the plate (anode) they may dislodge another electron and send it into the space between the screen grid and the plate. They may be attracted to the screen grid and establish a current flow from the plate to the screen grid known as secondary emission.

Secondary emission can cause a condition known as negative resistance where an increase of plate voltage will result in a decrease plate current. Negative resistance may lead to unstable operations.

Bernhard D. H. Tellegen

In 1926 Bernhard D. H. Tellegen, a Dutch electrical engineer, inserted a third grid following the screen grid and operated it at ground potential or slightly negatively biased. This grid, called a suppresser grid, prevents the electrons that were creating the secondary emission to be attracted back to the screen grid. Thus the pentode, five electrode, tube was born.

Both the tetrode and pentode tubes have high power sensitivity when compared to the triode tubes with the same power output.

Another tube designed to reduce unwanted secondary

Beam Power Tube Illustrated

emission is the beam power tetrode. Instead of a suppressor grid it has beam-focusing plates that are run at ground potential or slightly negative

potential. They follow the screen grid and cause the electrons to flow in a narrow beam to the plate thus pushing any potential secondary emission electrons back to the plate. The beam power tubes are usually used as power amplifiers for RF and Audio. The 6146 was a very

6146

commonRF final amplifier beam power tubes used by ham radio operators in the 1950s.

Many of the linear amplifiers used by amateur radio operators still use vacuum tubes.


 

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