The 4-THA is a triode hexode frequency changer designed for AC sets that were fitted with mains transformers, for AC/DC (universal) sets the functionally identical 203THA with a 200 mA heater (for serial heaters) was also issued.
The triode is employed as the local oscillator and the hexode for the mixing process, the input signal being coupled to the control grid via the top cap for maximum separation of the circuits.
The top cap connection to the copper supports of the hexode. Th screen grids are strapped together at the top.
The pinch with the rest of the structure hidden by the coating.
Using the microscope the rectangular cathode, triode grid and multi pass heater can be seen.
The classic envelope is 44 mm in diameter and, excluding the B7 base pins, is 119 mm tall.
References: 1043. Type 4-THA was first introduced in 1938.