Heater voltage.

[ppppenguin]

If you have an assortment of wirewound resistors in your junkbox there's a fair chance you can cobble together something that will work. Prove it by measuring the heater voltage when the radio is working. If the result is too untidy then buy something that gives a neat result. Your 100W lamp looks like it will work perfectly well as the dropper.

You could steampunk the set and put a lampholder on the top.

[This last comment is a joke. The mere thought of doing that to a nice radio should get me lynched]