The ATS70 UK Army transmitting tetrode dates to the mid 1920's. The graphite anode has a maximum dissipation of 70 Watts and the valve is designed to operate at up to 20 MHz. The L4 base is a special low loss type. The top cap is offset to one side.
The screen grid is close wound and extends beyond the anode cylinder. The screen grid is supported by a collar clamped to the pinch stem. The top of the screen is closed by a metal disc.
The control grid is an open helix visible at the top of the anode.
A closer view of the top of the anode, screen grid and control grid.
An alternative view. Note the wire wrapped round the anode support. The filament can just be seen attached to the central rod.
The military arrow on the glass.
The lower end of the anode. The control grid is difficult to see because of the screen grid and the distortion of the glass.
The wide glass tube envelope is 61 mm in diameter, and excluding the L4 base pins is 150 mm tall.