[Heatercathodeshort]

Not absolutely certain about the exact happening but I would guess that a more intense beam current may bring some of the 'dead' phosphor back into action, rather like being hit by a brick rather than a tennis ball.
Just a pure guess but it is rare to see heavy staining on a PYE D16T with 6kv of mains derived EHT but very noticeable with a receiver operating on around 5KV particularly the Mazda series.
The stain tends to take on a bluish tinge due to what could be described as 'poisoning'.

Just a point but it is very important that the ion trap is always adjusted for maximum brightness and not used to correct focus, corner cutting etc.
Incorrect adjustment can lead to permanent astigmatism and a 'splat' shaped ion burn if the tube is not aluminised.

I have no technical proof for this but can only guess that the intense off beam electron stream being pulled off line damages the tiny hole that the beam has to pass through in the grid of the gun assembly, making it slightly oval after a long period of time. I have seen this several times with the MW22-16 in Bush TV22s. In every case the ion trap has been offset to quite a large degree. Correcting this produces a very watchable picture but it is difficult to focus both vertically and horizontally together. Just my thoughts.

Don't confuse the tiny ring magnet that is fitted to mainly early GEC triode tubes. This is not an ion trap and is usually fitted to tubes that require added correction for corner cutting and overall focus. John.