Philips EF8 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.

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