EF8 / VR123 / 10E/49 RF pentode
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.
*: At Va=Vg3=250V, Vg2=Vg4=0, Vg1=-2.5V
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