[Milsurplus] [ARC5] ARC-5 Receiver Question

[Ian Wilson]

The 6SK7 has a grid-plate capacitance of 0.003pF. It is very easy to get
much large coupling capacitances between the grid and plate with poor
layout/grounding, etc.

Adding an external, grounded, screen between the socket pins makes a
noticeable difference to an IF amplifier that is prone to oscillation.

Although the numbers are small, the voltage gain of the tube multiplies
the intrinsic Cgp (Miller effect, as John points out). For a 455kHz stage the
voltage gain can easily be 200, so the effective grid-plate capacitance will
be 0.6pF.

In addition to being a source of instability, a gain-dependent capacitance
such as this causes the tuning to change with AGC.

73, ian K3IMW