[Ron Ramirez]

We need Ohm's law plus power calculations to find the answer.

Using the calculator found here:
http://www.sengpielaudio.com/calculator-ohm.htm

I punched in 115 volts and 60 watts, the values given on the original Philco Service Bulletin for the 40-150 (and 40-155). It gave me a total current of roughly 522 mA, (0.522 amps).

So let's say we want to drop 125 volts to 115, a drop of 10 volts. Since R = E/I, that means R = 10/0.522 = 19.2 ohms.

Now to find the power the resistor will dissipate: P = I squared * R = (0.522 * 0.522)19.2 = 0.273 * 19.2 = 5.23 watts, so you would want to use a 10 watt resistor at a minimum; 25 would be better.

And as Tom mentioned, Mouser sells the resistors with built-in heat sinks that are designed for chassis mounting so the chassis helps act as a heat sink.