In 1933 National introduced the FB-7 receiver. This was a cost reduced design based upon National's earlier AGS (Aeronautical Ground System) receiver developed for the CAA (known as the FAA today). The RF preselector was made an optional accessory, the case design was simplified and only one set of coils was sold with the receiver. Even the power supply and tubes were extra cost items. Later, National made available a crystal filter that could be installed by the buyer. It was a popular receiver with the cost conscious ham of the 30s. (Click here to see a 1934 station with an FB-7)
This is a FB-7 (really an FBXA) lineup as it might have existed in 1934. The speaker is the pre HRO / three vertical bar style with the National emblem embossed directly above the speaker opening. The FB-7 did not include an RF stage but National offered the PSK RF preselector pictured here. This unit bolts on the side of the receiver with 1" spacers. This allows the crystal filter contols to be operated. Band changing required the removal/replacement of three coils.
The FB-7 required seven tubes plus one more in the PSK and another in the power supply. The knob on the rear left transformer is the BFO pitch. The "A" designation on this model comes from the use of improved IF transformers while the "X" signifies the crystal filter. The filter is installed on the rear right side. The crystal and phasing control are visible form the top while the switch on the side (between the receiver and the PSK) controls the filter function.