The AC/HL indirectly heated amplifier triode makes use of the 'Shortpath' technique construction seen in Cosmos valves from Metrovick before the merger to form AEI and the transference of valve production to the Mazda name formerly used by BTH.
This valve type, manufactured in the early 1930s and sold under the Mazda label, is the direct successor of the Landmark Cosmos AC/G of 1927/28.
The AC/HL was a very long-lived valve. Many originals survive in working order and during the late 1960s it was claimed that a specimen removed from obsolete BBC studio equipment still met the original specification for a new valve after over 250,000 hours in service! Note that the Type No. AC/HL was also used by other brands of valve, including Lissen.
The cathode assembly passes through the far end of an oval grid. The grid is fixed only at the outer end to a stout single rod. The anode is a thin sheet and is open at the front of the grid support. The channel formed by the anode sheet encloses the cathode and the far end of the grid.
The anode is opened out to form wings that aid cooling.
The Mazda brand name and the AC/HL Type designation on the top of the balloon envelope.
The grid support is the white rod in the centre. The cathode tube and heater can be seen to the left of the grid support.
The balloon envelope is 55 mm in diameter and, excluding the B5 base pins, is 109 mm tall.
References: Private communication, datasheet & 1043. Type AC/HL was first introduced in 1929.