preface, i'm not an EE or a tech.
the 757 is rated at 28V and .08 amps. i get 350ohms for the filament resistance. this is verified here:
http://www.donsbulbs.com/cgi-bin/r/b.pl/757.html
so, in our circuit, 48V supply going through a 220 ohm resistor and a 757 bulb, we get 18.5V at the...