[Arran]

It's strange that I thought that I posted about this earlier but the post isn't here, no matter. These bias cells were introduced by Mallory back in the 1930s, in my opinion they were sort of a solution looking for a problem that nobody really had. In any event these are basically small carbon zinc cells, that put out about 1.4 volts originally, and since they were connected in a control grid circuit for bias they basically lasted their shelf life since there was no current draw, so the same would be true of a modern cell in the same circuit. There are three ways of handling this, one is that you use a modern watch battery as a replacement, but your have to reverse the connections on the bias cell holder, and either gut the bias cell and use the outer shell as an adapter for the watch battery or short across it in some way such as with a short piece of wire running from the cup section of the holder, or by wrapping the old bias cell in aluminum foil. The reason you have to reverse the connections on the holder is that on a watch battery the outside of the case is positive whilst the center is negative, whereas bias cells are more like a conventional battery, at least this is one of the better holders with the cup rather then one of those flaky ones that holds the cell in place with a stiff "U" shaped piece of wire. Another method is to rewire the tube inserting a resistor and capacitor between the cathode and common negative so that the tube will now be self biasing, this will take some research to select the right values, but flipping through some Riders schematics should help, including other Philco ones. The third method would be to boil the bias cell in water in the hope that some water will get inside and re-activate the electrolyte, or to drill a small hole in the cell and inject some distilled water, I don't know how successful or long lasting this method would be.
Regards
Arran