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 inner grid is fixed to a support rod at the top of each loop. The outer grid has loops fixed to a support rod below the winding. The anode support rod ia also below the loops.
The concentric electrodes with a single strand filament at the centre.
The balloon envelope is 46 mm in diameter and, excluding the special T5 base pins, is 86 mm tall.
Reference: 1003. Type K4 was first introduced in 1924.