filament type gas rectifier question

[Opcom]

Here is a full wave xenon rectifier for 725PIV @ 6.4A DC.

The data sheet says the filament voltage phase should be one way or another relative to a particular anode voltage to make best performance. It says to check this with the tube out of the socket and with some load.

What do they mean by this? Out of the socket?

Do they mean that during a half-cycle, the side of the filament that is next to the negative plate should be on the positive side of the filament voltage?

seems to split hairs but at 6 amperes I guess it makes a difference. The anodes are the top and bottom rings. the filament is coiled inside the center shield and pin one is filament+shield.