The necessary connection to ground (the chassis, which is not really ground, but a common termination point) is made through the volume control. This completes the detector circuit so that electrons flow from the volume control side of the IF secondary through the volume control to the chassis, and then to the cathode of the diode and then to the plate of the diode, and back on to the IF secondary. No current flows in the other direction due to the presence of the diode. The amount of current in this circuit of course depends upon how big of an amplidude modulation was imposed upon the original signal at any one point, as well as the over all signal strength.

The amplitude fluctuations are of course tapped off at the volume control via the movable slider, and are sent to the grid of the amplifier. The fluctuations of this negatively oriented current are also sent through a resistor of large size. A condenser on the other side of this resistor filters the modulations (retains an average charge) so that they build up to an average signal strength, which becomes the AVC voltage.

Also, regarding the additional negative bias of the IF tubes for standard broadcast, this may be to "even out" the response of the radio so that the broadcast band matches the short wave band, which does not use the RF pre-amplifier. Short wave is all over the map as far as signal strength, but if you have several local stations, you may want them to be somewhat similar in strength to broadcast stations. As to why an RF amplifier was used on the broadcast section, broadcast stations can be crowded close together, and a radio is usually used mostly for broadcast reception. An RF pre-amplifier increases the selectivity of the set to some degree even if it is not tunable. It can also be used to filter out harmonics, etc. Short wave can have crowded stations, but most broadcast listeners are less critical of short wave. If they really want to tune in short wave stations better, they will probably purchase a radio specifically suited for that purpose, or perhaps a better all-wave radio.

As I said before, the RF amplifier is not used for short wave due to the lack of a tuner for this section, which was probably omitted to save cost. Think of the short wave in this radio as simply an added bonus. This radio, however, is of higher quality. You should not think of it as basic. Remember that there were 5 tube radios produced with short wave bands, as well as 5 tube radios with only a broadcast band. Your radio is a step above the 5 tube.

Thomas