The 12Z3 is a half wave rectifier and was designed primarily for use in domestic equipment without mains transformers and American mains supplies.
The 14Z3 is electrically the same but on a B8B Loctal base.
Radiomuseum.org quote 1933 for first introduction but Thrower has a date of 1936. Heater voltages aligned with lead acid storage batteries appeared in the mid 1930's. This exhibit has the shouldered envelope whilst the 12Z3 looks to be later with the tubular envelope.
The anode is a folded metal plate. The heater is coiled within a ceramic tube. In a series heater chain the rectifier would have the greatest heater/cathode voltage difference and good insulation was a must.
The Harvey-Burnham name and logo pressed into the plastic base cap.
Side view showing the heater top connection.
The wide glass tube envelope is 38 mm in diameter, and excluding the UX4 base pins is 88 mm tall.
References: Data-sheet, 1040 & 3002. Type 12Z3 was first introduced in 1933. See also1933 adverts.
