GEC Tuneon neon tuning indicator

This valve measures 115x16mm overall and has a 4 pin base

The Tuneon consists of a neon filled tube containing three electrodes, 2 short and 1 long. It is intended for use as a visual indication of the correct tuning point in an AVC receiver. On the passage of a small current through the tube a luminous glow appears on the long electrode. If the tube is connected in a suitable circuit correct tuning is indicated when the glow is of maximum length.