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The other thing that can happen is that the glass can "brown" due to soft X-rays from the beam. Picture 1 shows a 16AP4 (which was operated at about 15 kV) with a brown area where the tube has been scanned. I had to enhance the colors on the photo to get this very slight "browning" to show up. If I can dig out my 3NP4 projection tube (25-30kV), I will post it later.
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