The video shows HV tests with Philips 18504 Geiger tube. It demonstrate HV stability and current consumption under low and high radiation load. As you can see, the HV stays within plateau limits even under very high count load. No drops and no over-voltage on the tube.To perform the same experiment you need to build 10 Giga ohm voltage divider for the multimeter. Such high impedance exclude any effect on HV converter and allows to measure HV correctly.
Technical specifications of Philips 18504:
Initializing Voltage: 275V-325V
Beginning of Plateau: 425V
Ending of Plateau: 650V
The kit was configured for 500V output with jumper. High voltage was measured on the point after 1N4937 diodes multiplier before 4.7M load resistor.
18504 tube has mica window that allows to demonstrate very intensive radiation load count when Am-241 sample placed closely to mica. The distance between mica surface and Am-241 was less than 1.00mm. High speed count under this condition is identical to 1200uSv/h Gamma radiation load, or at least it simulates extremely high load count conditions.