[Reece]

You'll find a lot of ideas about how to work around these bias cells. They were used in some radios from around 1936 to just before the war, and then pretty much disappeared. They were a good idea for providing a steady negative grid bias, but they can eventually wear out. Some try to rejuvinate them by injecting water, others use a watch cell as Tom C. noted: you'd dig the guts out of the old cell and modify it so that the watch cell could be used in the old holder, with the negative terminal towards the 75 tube grid. Yet another way that's easier and will probably work fine for you is to just replace the 1 meg resistor in that line to the grid with a 10 meg from grid to ground and leave the old cell in there for looks but disconnected.

A grid will collect electrons and bias itself negative but unless there's a way to bleed some of them off it will become too negative and tend to cut off the tube or cause erratic action. The high value resistor will bleed off the excess. This is not the most sophisticated way to get grid bias but it should get the job done.