The PHILCO Phorum

Excuse such a simple question, but why is it that on all of my radios the dial bulb fires up bright and then immediately dims to where it barely serves its purpose. They all use a 6.3 volt, 150ma that I'm supposing is correct. These are all 1940's table top Philcos.
Thanks
Alan

The following link should explain much. Simple yet eloquent design. Richard
http://www.angelfire.com/electronic/funw...AA5-1.html