[Mondial]

The darkened glass around the plate structure is due to electron bombardment. Extra electrons which are not attracted to the plate hit the glass and eventually darken it. It indicates the tube has had many hours of use, but does not necessarily indicate the tube is weak or bad.

The getter spot is on the side under the 6DK6 marking. The edges of the deposit are beginning to lighten, which shows that some gas has been liberated and absorbed. When the spot becomes completely light gray or white, then the getter material has absorbed all the gas it can and the tube is bad.