The Triotron G4120N is an indirectly heated full wave rectifier. The maximum input voltage is 500 Volts and the rectified current is up to 120 mA. The G4120 rectifier has the same HT rating but is directly heated.
The twin rectifier sections. The cathode tube sits between the two open cylindrical radiating fins.
The pinch and lower mica.
This image chows the anode shape and method of fixing. The insulated heater is enclosed in a ceramic tube within the cathode tube. Clearly this rectifier was designed for a large heater/cathode voltage difference.
A closer view of one heater.
The top mica fits to the inside of the shouldered envelope and carries the anode top supports.
The classic envelope is 48 mm in diameter and, excluding the B4 base pins, is 107 mm tall.
Reference: 1043. Type G4120N was first introduced in 1934.