The Mazda 6M1 tuning indicator was a replacement type and not current in the mid 1950s and first use was around 1950.
The construction follows the usual pattern with the triode amplifier above the pinch and the display viewed through an end window. The screen is circular and the central electrode is a small black button. The full display range is given for a 22.5 Volt alteration in grid voltage.
The triode anode connects to the display control electrode.
The display control electrode is above the cathode and partly hidden behind the central black cap. In use the target glows green almost as a complete circle with -22.5 Volts on the triode grid. This represents maximum AGC from a strong signal. Weaker signals produce less AGC voltage and the bright area opens back either side of a centre line to show a darker shadow. Tuning is complete when the shadow is at a minimum.
The wide glass envelope is 28 mm in diameter and, excluding the IO base pins, is 69 mm tall.
Reference: 1040. Type 6M1 was first introduced in 1937.