[morzh]

There's another kicker there.
The divider works only when all the tubes are inserted. When one is missing, the cap charges to the full divider input voltage. So when you inserting that tube live, the rest sometimes go ... poof! This is why putting a zener in parallel to that cap is a good idea.
The zener should be a bit higher voltage than the one that powers the filaments so it does not conduct and does not load the divider. But if a tube goes it will limit the volts to somewhat higher but still safe for the filaments value.