Voltage stabilisers have a nominal striking voltage but when permanently connected to a heavy current drawing shunt load may require a much higher applied voltage to ensure that the discharge ignites. This higher voltage could result in the tube and load being driven into overload.
The CV versions made for Government (Military) service are fitted with a priming anode connected to pin 4. For some of the commercial versions this pin connects directly to the main anode. The data-sheets suggest that pin 4 is connected to the anode pins only through a high value resistor - 70 kΩ for example.
Tests on a CV287 indicate that the priming anode has a pronounced effect on the ignition voltage. With the priming anode connected to the HT rail through the 70 kΩ resistor the striking voltage was 185 V. With the priming anode disconnected the striking voltage rose to 210 Volts.