Zenith HV is serious stuff compared to other makes with more open tolerances. I once had an arc snap the neck off a 25GP22 in a 20X1C38 all-wood dark walnut console that was working nicely up to that point. I just gave up and put a CCII vert-chas Zenith, CRT and escutcheon in the cabinet and sold it.
I am also sure that my 1967 set did NOT have a spark gap in the CRT socket, leaving it open to this colossal fail.
I never heard of this before. By the time I finally worked at a Zenith dealer, all this vital and golden info like Zeno's TA, was gone.