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Sorry for your disappointment. It must be great.
When you turn the brightness up, the beam current goes up. This would tend to diminish the high voltage except that the shunt regulator would relinquish current to keep the load hence high voltage constant. The fact that the arc intensifies with increased beam current is a clue.
What I believe may be the case is the internal conductive aquadag coating is cracked and the increased beam current is causing an arc to the part of the coating which may have broken away from the 2nd Anode (HV connection) The arc is caused by the increased current within the tube.
When the arc begins, does the picture shape distort slightly?
That is my guess. The fact the arc increases intensity with increased beam current is important. I expect you did not see an arc in the neck and the arc seemed confined to the bell?
And are you sure it was in the tube and not in the nearby HV components?
Terry
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