This is a simple schematic of a Geiger clicker. It power a Geiger tube with 350-400V and extract/amplify it’s “clicky radiation sound” making it earable.
The first transistor on the left is an NPN BC546. It regulate the second transistor, it’s complementary PNP BC557 working point. This second transistor is an oscillator drived by the first one. The third is a MPSA44 and is a fast switching high-voltage NPN. This part is critical. You must use the MPSA44 other transistor probably will not work. It drives a current tru a 20mH coil. This coil must be capable to resist to 2-3A impulsive. Select one with low series resistance. The lowest the best. This inductance could be hard to find so a good idea is to wind one by yourself. The 1N4007 could be replaced by UF4007 diodes or better shottky high voltage diodes.
The feedback network that regulates the output voltage is 30Mohm series 400V zener diode. The 30Mohm could be made with 3x10Mohm 1/4w carbon resistors and the 400V zener could be a series of 4x 100V zeners. The last transistor is the led/speaker driver. A 2n2222 or a bc547 will work well.