The K4 was introduced by the Bower Electric Company in mid-1924. The K4 is a four-electrode valve and was sold under the Thorpe trade name. The valve has two grids but the second grid is not a screen grid, the screen grid was not introduced into valves until 1927. The inner grid is a space charge grid, employed to enable the space charge to be reduced so that the anode voltage can be reduced.
The K4 was intended to be used in Unidyne circuits, where low anode voltages were employed. The Unidyne or Solodyne receiver was popular in 1924. The name implies a single low voltage battery, used for both filament and anode supplies. With subsequent developments of the triode, the four electrode valves were surpassed in performance.
The electrode assembly of the K4 is mounted horizontally. The anode is a 14 turn wire spiral with very close spacing. The bright emitting filament is a single strand running through the centre of the anode. The filament is designed to run at 4.0 Volts at 0.42 Amps. The two grids are also wire spirals but are of open spacing.
All of the circular electrodes are mounted concentrically. Additionally the anode and both grids are fixed at every turn to their own stout wire supports.
The balloon envelope is 43 mm in diameter and, excluding the special T5 base pins, is 87 mm tall.
Reference: 1003. Type K4 was first introduced in 1924.