So in the left image the heaters are in parallel, in the right they are in series. How do we know which he has?...
Left, right... what image are you looking at?
In "my" image, each tube's two heaters are parallel, but the three tubes are in series. Follow the "V2P3" etc annotations. So 18+V 0.3A, right? DC, so more figuring needed for AC.