Tested each tube and number 4 will not work.
Perhaps dodgey connection of one of the other pair, but now the original
pair of 2 tubes at the top work for 1 second.
Cleaning pcb around HV areas and resoldering HV and monitoring solder
points , still 1 second only.
What sort of compact load to represent the duff tube? a few neon bulbs
in series perhaps.
I don't understand the monitoring as connects to midway point of 2 tubes
and no meggy resistors around, just 10s of K.
One SMD diode marked A2 is used elsewhere in the LV circuitry, as is
transtor marked 3FW. 2 perfectly normal pitch spacing SO23 Q marked 7.AJ
or 7AJ are not repeated elsewhere but not obviously HV