The 1A7GT is a heptode frequency changer. The American term is a pentagrid converter. The 1A7GT is designed for battery superhet radio receiver use. The local oscillator is formed between the cathode and grid 2. Grid 1 being the oscillator control grid. The pulsating electron flow is screened by Grid 3 from the oscillator and grid 5 from the anode. In this screened space is the mixer control grid g4. For maximum isolation between oscillator and signal circuits the mixer control grid is taken to the top cap.
As this is a low power device the anode is left bright. Looking at the base of the anode the grids can be seen. The inner grid is wound tightly. The second grid is a pair of stout wires. Grid three and five are spiral wires and can be seen to be bridged together. The fourth grid is not exactly mid way between the screen grids and the connection can be seen on the right.
The top mica and above can be seen the grid spacing and thickness of the supports. Note that the second grid support has no notches as it carries no grid wires. The filament is a single strand passing through the centre.
The wide glass tube envelope is 29 mm in diameter, and excluding the IO base pins is 70 mm tall.
References: Data-sheet, 1040 & 1043. Type 1A7GT was first introduced in 1937.