A nice write up of the PSU in the All American Five years radios. I think it was standard to put the lamp across that heater section, according to the write up earlier sets had a resistor as well as the lamp so that if/when the lamp burned out the rectifier did not take too much strain, Later sets did away with the resistor and damage to the rectifier woukd occur a short while after.
Frank
http://www.angelfire.com/electronic/...bes/AA5-1.html