new theory:
The Condensator must collapse, i.e. the Electrodes will shortcut by producing a spark!
This means that the distance between the electrodes, so the gap between the 2 tubes that form the condensator determines what Voltage must be in the resonant circuit!
The Resonant Circuit is a High Voltage circuit. Try operating the Resonant Circuit at around 5000 Voltage (Could be lower in? water, experiment!).
The collapse will cause the resonancy!