Two Gas Discharge Tube in parallel; a fast acting gas acting tube and a gas discharge tube
Anything placed directly in parallel with the GDT, that clamps at a lower voltage, will prevent the GDT from striking. With a fast-rising voltage, the "slow" GDT needs to reach 525 V to strike, or needs to be held above 325 v for some microseconds.

So going back to a TVS: If you make a fast, high-voltage clamp at 250 V, it needs to tolerate this voltage for some microseconds... which is many Joules. With a lower-voltage device in parallel, the TVS gets the whole surge with no help from the GDT. (Until the TVS fails, then the GDT will operate).

The usual solution is to put something between the GDT and the TVS, so that the TVS can clamp and protect the downstream circuit, while not limiting the voltage rise on the GDT. This could be:


a resistor, sized to limit the current in the TVS while V_GDT is 325 or 525 volts, before it strikes.
an inductor for the same purpose
a few metres of coaxial cable, to give the GDT time to strike, if it's that kind of signal








simulate this circuit &ndash; Schematic created using CircuitLab