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My first thought was that the tube was just a bit gassy, but as you have tested so many (20?) tubes it is unlikely to be a tube problem. Now you say the beam is at zero volts, so no beam current, therefore it is not associated with the front part of the tube. I would check the voltages in the dynode multiplier chain and all the other tube voltages are within the normal range.
The conical shape of the spot is interesting, as it is a picture ( with dimensions) some scanning must be happening of whatever is arriving at the dynode section, I assume the amplitude changes as you adjust the dynode gain control.
If you turn the beam up do you get any sort of a picture?
Best though so far is that one of the electrodes is way off correct voltage and electrons are being dragged of it through the scanning on its way to the dynodes.
does any of this help? or even make sense??
Best
Brian
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