The documentation on which we get the diagram shown is from 1962 and the ultra-sensitive circuit uses only one valve. As always, the problematic component, besides the valve is the transformer, but there are several alternatives, because in this circuit, the current is very low. The indicator is a common neon lamp. The valve is a 6J7 that has a top (helmet) attached to the grid and where the sensor plate will be placed, as shown in figure 2.
Figure 1 - diagram
Figure 2 - Sensor disk placed on the valve.
