Tuned radio frequency receiver

They typically have a long, low appearance, with a flip-up lid for access to the vacuum tubes and tuned circuits.

That feedback can cause instability and oscillation that frustrate reception and produce squealing or howling noises in the speaker.

In 1922, Louis Alan Hazeltine invented the technique of neutralization that uses additional circuitry to partially cancel the effect of the interelectrode capacitance.

"[2] "Perfect neutralization cannot be maintained in practice over a wide band of frequencies because leakage inductances and stray capacities" are not completely canceled.

The classic TRF receivers of the 1920s and 30s usually consisted of three sections: Each tuned RF stage consists of an amplifying device, a triode (or in later sets a tetrode) vacuum tube, and a tuned circuit which performs the filtering function.

Terman characterizes the TRF's disadvantages as "poor selectivity and low sensitivity in proportion to the number of tubes employed.

The problem of achieving constant sensitivity and bandwidth over a range of frequencies arises only in one circuit (the first stage) and is therefore considerably simplified.

A superheterodyne receiver only needs to track the RF and LO stages; the onerous selectivity requirements are confined to the IF amplifier which is fixed-tuned.

[7][8] This produced audible heterodynes, shrieks and howls, in other nearby receivers tuned to the same frequency, bringing criticism from neighbors.

[7][8] Britain,[9] and eventually the US, passed regulations that prohibited receivers from radiating spurious signals, which favored the TRF.

Although the TRF design has been largely superseded by the superheterodyne receiver, with the advent of semiconductor electronics in the 1960s the design was "resurrected" and used in some simple integrated radio receivers for hobbyist radio projects, kits, and low-end consumer products.

This 1920s TRF radio manufactured by Signal is constructed on a breadboard
Tuning a TRF receiver, like this 5 tube Neutrodyne set from 1924 with two stages of RF amplification, was a complicated process. The three tuned circuits, controlled by the 3 large knobs, had to be tuned in unison to the new station. So tuning in a station was a process of successive approximation. Once a station was found, the numbers on the dials were written down, so it could be found again.
Tuning all 3 stages of a TRF set in unison. This 1925 Grebe Synchrophase receiver has thumbwheels instead of knobs which can be turned with a finger, so a third hand is not needed.
Block diagram of TRF receiver
Leutz 9-tube receiver from 1927 clearly shows the component parts of a TRF set. Each RF stage is in a separate compartment. Within each compartment can be seen (from top) : the triode tube, the interstage coupling coil, and the capacitor attached to its front panel tuning dial. The compartments contain (from left) : the 4 RF stages, the detector stage, and the 4 tube audio amplifier. The capacitors could be linked to a common shaft and tuned together, or tuned separately.