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The Screen in an EF80

The Radio Constructor, July, 1961.
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This information has been distilled from the In Your Workshop feature.

Valves employed in television IF stages normally have an internal screen, which should not be confused with the screen-grid (g2). In the EF80 the internal screen connects to pin 6 and the screen-grid to pin 8

The internal screen in a television IF pentode like the EF80 screens the connections within the valve whereas the screen grid (g2) screens the control grid from the anode within the electrodes themselves. The extra screening is required because of the high mutual conductance (gm) of these IF valves and thus high gain.

Part of the EF80 valve. The lead inside the valve from pin 6 connects to the metal screen support and, thence, to the cylindrical mesh screen which surrounds the electrodes and the strip metal screen which runs alongside the pin 7 anode lead. All connections shown are made by spot welds. The assembly illustrated here is typical.

The wire from pin 6 connects to the mesh screen around the anode. Also a piece of sheet metal or metal mesh is connected from the cylindrical mesh screen and extends down to the base of the valve between the pins. This strip effectively screens the anode wire from pin 7 from the control grid wire diagonally opposite from pin 2.

The metal screen resting on the pressed glass foot of the valve is close to the valveholder and if that holder has a metal centre spigot the screen action is almost complete to the earthed valveholder spigot. In this way the control grid circuit remains screened from the anode circuit from beneath the chassis all the way to the working space inside the anode cavity.

This screening of the grid and anode circuits helps prevent instability in the operation of the IF amplifier. Additional screening may be provided with the addition of a metal screening can around the outside of the valve.

An almost unbroken screening effect is given by the combination of the centre spigot of the valveholder and the internal screen in the valve. The valve-holder is shown here in side view with all tags except 6 and 7 removed.
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