[rfeenstra]

Hi Meghan.  here is a rundown of  what each tube does.  The 25Z5 is a rectifier that converts the 120 volt AC input to DC voltage that the circuits can use.  The 6A7 is a dual purpose tube that is both a radio frequency amplifier and the superhetrodyne oscillator  to convert the incoming radio frequency to the intermediate frequency (IF frequency) of 470 khz.  The 78 tube amplifies the IF signal to a useable value.  The 75 tube then extracts the audio from the IF signal using the diode part of the tube.  That signal is then amplified in the second part of the tube (a triode).  The audio signal from that amplifier is fed to the 43 tube which amplifies the audio signal sufficiently to power the speaker through the output transformer.

Since all 5 tubes as well as the resistor marked 36 in the schematic are in series,  all must be in place.  If you pull any one tube, or one is burned out, the entire radio will be dead since none of the tubes will then light.  There is no "overflow concept" involved.

Good luck and have fun!