1949 DuMont - issues when turned on cold

[bigaudioal]

When I turn on my DuMont RA-108 and it is cold, it takes several minutes to come up to full brightness. When it starts out, raster is barely visible. But then slowly, after a few minutes it is full brightness and totally fine. Any idea what might cause this? Once it is up to full brightness, set plays for hours with no issues. If I turn it off and then back on, it comes right back to full brightness. Unless I let it sit for a while and it cools down. Then condition returns. Any ideas?