[Radioroslyn]

That's a good question. I guess the good thing is the D cells are pretty cheap as you can get 2 for a buck at the Dollar store. It might even be 4 for a buck. Unfortunately I don't know of a holder that is wired in parallel most are wired in series so you won't end up with 1.5v in the end. I guess you could rewire the filament in series but most of the time the output tube filament is rated at a higher current than the rest so you'd need to put in a resistor to balance the current.
GL
Terry