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Well, a zener diode is basically the simplest kind of voltage regulation circuit, quite useful when a fairly limited range of current supply is needed. I was curious how high a voltage one could find in a single zener, and generally found that common ones top out at 200 volts. Since they are quite inexpensive, putting a bunch of them in series is still quite cheap, and they don't take up much space.
I've used zener regulation in battery eliminator power supplies I've built for 1920s radios and later tube-type portable radios, so I'm generally familiar with their usage for this purpose.
The current in the focus circuit is very low, so not much power dissipation is needed. Using 1.5 watt rated diodes is complete overkill, but there was no real cost difference between these are lower power ones, so I went with these.
I also bought some 4 kV rectifier diodes with the idea of putting several in series to replace the 1X2 focus rectifier tube in case that was also needed. But so far, it seems to be fine with the original vacuum tube rectifier, so I'll leave that in there in the interest of keeping as much of the vintage circuitry going as possible.
I've done a little more circuitry modification this evening to improve the color tracking from dark to light. Basically, with the condition my CRT is in (which doesn't seem all that bad on the tester, but does show a weaker red gun compare to the other two), I found that it was not possible to adjust the green and blue drive low enough or the green background high enough, if I had the screen settings about right for good grey level color balance. So I modified the range of all those controls, and now the color tracking from black to white is reasonably decent.
These changes have slightly hurt the dynamic convergence, so I'll go back and see if that can be improved by readjustment now that the other settings seem to be in the right range. In particular, adjustment of the screen voltages seems to impact convergence (and focus) so it's not surprising that convergence will need some touch up.
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