Thyratrons: How they work, what they are used for, and how to read their datasheets
Pins (for 6D4)

1 - grid (the control element)
3, 4 - heater (connect 6.3V AC or DC there)
5 - cathode
7 - plate (anode)


Connect your main voltage between the plate (+) and cathode (-) and keep the grid at a negative voltage, when you want the tube to start conducting, make the grid voltage less negative (the datasheet says that the tube starts conducting when the grid is at -6V - -26V depending on the plate voltage).

Thyratrons work by initiating a gas discharge in a tube. They can have hot (as is the case with 6D4) or cold cathodes. Cold cathode tubes are less sensitive and require higher plate voltages (almost enough to initiate the discharge in the gas, then you connect a positive votage to the grid and the discharge starts). Hot cathode tubes require heater power but are more sensitive and can be used with lower voltages (some tubes can be turned on by about 15V).