[35Z5]

 BTW, are you the same 35Z5 from ARF?

One in the same.

If only using one primary winding the available current on secondary is going to be half, less what's drawn by the primary now supplying B+ voltage... BUT the tubes draw such a small amount of current it's a non-issue... The secondary is center tapped so you'd only need half, rectify & filtering that 3v would give approx 4.2v DC... Will only require a small amount of resistance added... You could start out on variac, then increase voltage to see what input is necessary to give proper filament voltage... Rather than risk tubes, use something like a #47 dial bulb for load... Also possible to use resistor in primary circuit of transformer, that would limit B+ & filament voltage... 

I may seem cautious but those frail little battery tubes give me the willies... I stay away from them as much as possible...