The 164V LF amplifier triode from Mullard dates from 1929 and was introduced at the Olympia show. This example looks to be of later manufacture. The horizontal box anode of the 164V has given way to a vertical mesh anode. The electrodes are still held in a glass bar rather than held between top and bottom mica sheets.
The glass rod to the side is a construction technique from the Azide process days.
The mesh anode side view. Lower right is the getter holder cup.
The identification is etched into the dome of the balloon envelope. The heater is given as 4.0 Volts and the anode voltage is given as 200 Volts. The grid supports are some way from the cathode and the grid wires are shaped to curve round the cathode but be pressed together between the cathode and the supports. The cathode tube can be seen to be oxide coated, but the coating is not smooth and polished.
A closer view of the anode.
A μ-Tracer plot of the characteristic curves for this exhibit. The valve tests to 78% of the published nominal values - so still good as there was a considerable spread of values from valve to valve.
The balloon envelope is 46 mm in diameter and, excluding the B5 base pins, is 93 mm tall.
References: 1931 datasheet & 1040. Type 164V was first introduced in 1929.