[old_tv_nut]

It's not the filament you want to kill. You really want to discharge the HV.

Some sets may do this by drawing a lot of cathode current while the raster is still collapsing (somehow the cathode voltage collapses faster than the G1), so you get a momentary bright flash as the raster collapses and no long-term spot. Sets with focus dividers will discharge the HV through the divider.

Motorola's first solid state monochrome chassis had a spot killer that worked when the power switch was turned off, but they didn't think of the case of someone pulling the power cord with the set running. Pulling the power cord was SOP at the end of the production line, and the cathode voltage, derived from the flyback and having no bleed path, stayed up so long that the spot formed many seconds later and burned a spot in the center of the screen after the set was boxed. This was not discovered until sets were unpacked after shipment. A quick fix had to be added to the chassis so that when B+ collapsed, it would pull down the video supply.

In the end, I agree that if a defocused spot is normal operation, you should just enjoy the nostalgia.