Hello Albert,

I don't see the rationale behind this. With rising grid voltage larger rings
on the cathode disk take part in the cathode current. Moreover, the increasing
electric field strength in space just outside the already exposed smaller
rings makes it easier for electrons there to escape from the cathode. So I
would still expect a monotonically increasing intensity (or better, cathode
current).

Yep, as I mentioned... my rational may be (and probably is) flawed.
Back when I was still struggling with that 464 and romancing the chance there could be some level of recovery to that CRT, I read a lot of things in a lot of places... In many of them I encountered widespread use of the term "Double Peaking" but in none of them I found explanation about the mechanism that originates the behavior.
I agree that by my own rational, the intensity should always increase (maybe not at an even, or linear fashion, but always increase).

BTW the potentials discussion in the CRT Circuit Concepts book seems
to ignore space charge, while space charge is nearly always discussed in the
context of ordinary tubes. I have no idea whether or not space charge plays a
role here.

You may be right... I`m not familiar with the term "space charge".

The Tekscopes issue
http://w140.com/tekwiki/images/6/6e/Tekscope_1969_V1_N3_Jun_1969.pdf , page
14, mentions this: Gassy CRT tubes may be identified by their "double peaking"
characteristic. No mention of worn cathode material.

Gassy tubes or worn cathode... Well... In my way of seeing it, apart from tubes (or CRTs) that have a manufacturing defect or a crack, I tend to generalize that gases develop on a tube as they wear out... so I think it's kind of difficult to tell apart which of the abnormal behaviors are due to the presence of gas, or from the cathode itself being worn out so, I plead guilty for using "Cathode Wear" for everything.

A little side comment... with so many people in this group with access to things as exquisite as Electron Microscopes, high vacuum pumps and etc... I wonder how come that nobody has come with means to open the worn out tubes, replace or recoat the cathodes, and seal them back again.
I see there are still some plants that do this kind of thing for specialized tubes...

Rgrds,

Fabio