The KL1 holds a special place in UK valve history as it was the first production indirectly heated valve made. Commercially the valve was less successful and required an excessive seven watts of heater power. The KL1 was prone to heater cathode short circuit problems.
The KL1 predates the B5 five pin base having been developed by the M-OV research lab in 1926. The addition of an insulated side contact on the base gave the cathode connection.
The valve featured a large diameter oxide coated cathode tube, circular grid and anode. The anode has side flanges to aid cooling. Sprouting from the right hand leg of the anode support is a wire support leading to a small metal disc. This is the getter holder.
Looking down on the electrodes the concentric construction is obvious. The central closed off tube is the cathode. The oxide coating does not extend outside the confines of the anode. The grid is supported on either side.
The supports extend upwards from the pinch but there is no top support to define the geometry of the relationship between cathode, grid and anode. The image shows the displacements from purely concentric.
The side view is confused by the reflections from the internal silvering of the bulb. However, the main features stand out. The long cathode tube, the grid extending either side of the anode and the stout fixing rods. A close inspection of the tube below the cathode reveals that it is a separate entity joined to the main tube by spot welded tape.
The anode is spot welded to the support rods in five places. The grid winding seems to distort the side rods and they bend inwards.
The KL1 cathode was heated by radiation from the heater. In this exhibit the heater is intact but sadly detached from its supports. While most receiving valves of this period featured a single thin strand of tungsten as filament, the wire here is coiled and not ultra thin. The diameter of the coil is 0.73 mm and it is 15 mm long and has 17 turns. The cathode tube is about 4 mm. Clearly the aim was to provide a separation to reduce short circuits but the distance would lead to considerable thermal resistance and so require a high heater temperature, and consequently consume more power.
The etched wording includes the Marconi Osram Valves logo, protected by patents and made in England. The KL1 is on the other side of the envelope.
The balloon envelope is 62 mm in diameter, and excluding the B4 base pins is 124 mm tall.
Reference: 1043. Type KL1 was first introduced in 1927.