Found in an 1868 Mullard documentation this circuit has a dynamic range of 10 Hz to 25,000 Hz. Modern equivalent transistors can be used.
This is the typical circuit of an FET amplifier stage in the common source configuration. The circuit is...
Pressing and releasing S, the capacitor C1 charges and discharge slowly through the oscillator...
For the 7 to 10 MHz band L1 is formed by 3 turns of 22 to 28 AWG wire in a ferrite core 15 to 20...