that is the typical failure mode?

complete loss of HV, blooming,other? I gather testing them is not vialbe due to the HV nature (I have a hickok 533 and a B&K 600) so subbing is the starndard check?

I think blooming is the most common symptom for that tube and I subbing is the best way unless I guess a scope check of the circuit if u don't have a spare to swap.

They get gassy and glow blue inside, the filament opens up resulting in no hv or I have seen they get weak and cause blooming/dim picture. The hickok should give you a good idea. Check it at correct filament voltage then back it off a volt for 3 volt tubes and half a volt for 1 series, it should hold fair to good emission.

I have seen a few have the filament break loose and the tube shorts. This can overload the high voltage circuitry. This usually happens on early1B3's. they had a spiral filament that was hung top to bottom.
Later tubes have the filament wound around a support rod.

its going back to trying to get a handle on my zenith with a 3a3 HV tube.

The HV measured as the CRT anode varies what seems like a lot (25kv to 20kv) based on the scene on the set (I presume beam current).

I have my strongest HOT in it and have checked all the drives to that tube, all seems in order.

What chassis is this Zenith and what regulator tube?

20x1c38

6hs5