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EF8 / VR123 / 10E/49 RF pentode

This valve measures 85x30mm overall and has 8 side contact (P) base.

This is a pentode but it has four grids. Partition noise is the main noise source in pentodes making it important to reduce the screen grid current. In the EF8 an extra grid (g1') was inserted between the control grid (g1) and screen grid (g2). Typically the grids were renumbered so the screen grid is numbered G3 and the suppressor grid G4. This additional grid is aligned with the screen grid and held at cathode potential. This causes the electrons on their way to the anode to avoid the screen grid. Thus the EF8 is actually a kind of aligned grid (beam) pentode, but instead of aligning the control and screen grids the additional grid was added.

The EF8 was used as the RF amplifier in early Chain Home radar receivers because of its low noise.

Heater voltage 6.3V
Heater current 200mA
Max anode voltage 300V
Max screen voltage 300V
Max anode dissipation 2.5W
Mutual conductance * 1.8mA/V
Anode impedance * 450k ohms

*: At Va=Vg3=250V, Vg2=Vg4=0, Vg1=-2.5V

Pin Function
1 Metallising
2 Heater
3 Heater
4 Cathode
5 Suppressor grid
6 Grid 1'
7 Screen grid
8 Anode
TC Control grid
Above: EF8, below: VR123

Thanks to Jan Heering for information about the grid configuration

This file was last modified 15:25:41, Tuesday September 02, 2014