Tuned Radio Frequency Receiver is an old type of radio receiver, also known as TRF receiver in short. It has one or more tuned radio frequency amplifier stages serially amplifying signals received from the antenna. The output from the last RF amplifier is fed to a detector or demodulator, which extracts the audio signal. Simplest form of detector is a diode. Detected signal is further amplified using audio amplifiers. TRF receivers were popular about a century back and was tedious to operate. While tuning a station, each RF amplifier stage had to be tuned separately to the station's frequency. Later gang capacitors with multiple rotors controlled by a single tuning knob was used for tuning. Still it was difficult to achieve accurate tuning of all the RF amplifier stages together. Final output was fed to a loudspeaker or earphone.
My very first radio projects were all TRF, and I've still got several in my shack. When built right, they are not as tedious to operate as one would think. If the RF stages are correctly built, the bandwidth can be quite narrow, and one-knob tuning is possible. However, I'd like to point out that your definition of "TRF" is a bit too tight. They do not need RF amps, or Audio amps to be TRF. They do need at least one tuned RF stage prior to the detector, and a detector of some sort but that is about it. The audio stage is handy but not required. All of my early radios use high impedance earpieces, and worked quite well. A LOT of us started that way, especially on the AM Broadcast Band. If it were my survey, I'd broaden the questions to allow tube amps or no amps at all, instead of the "one transistor and progressed to four transistor TRF".
I restored a 1925 TRF broadcast receiver. It is a bit tedious to tune, but develops skill after a short time of experience. It has no frequency calibration and tuning is non-linear. The three sections RF, Detector and Amplifier are interactive which makes for fun. Magic radio from 1925.
You are right. As I started late, I started with one transistor radio. As you pointed out, it had only one stage prior to the detector, which was an OA 79 semiconductor diode in those days. I started using tubes quite late, only after I had become a ham, almost a decade later and then after an initial trial of a 5W solid state QRP. That was a @VU2VWN (SK) QRP which was very popular in this region at that time. In younger days, I had no access to tubes and moreover was scared to deal with high voltage circuits. All initial hobby electronics projects were with battery operated semiconductor circuits. Occasionally I used a 'battery eliminator', which was often called just an 'eliminator' here. I still remember the 50 Hz hum produced by those cheap eliminators in those days. My father who was not a ham nor an electronics person, had 'homebrewed' a simple radio with just an earphone and a diode, without the need for a battery. One end would go to an 'aerial' and the other end to the ground, if I remember his description correctly. It used to work well for him as he was staying within 500 m of the local AM broadcast transmitter in those days. 73
I missed all those boat anchor equipment. The very first radio which we had at home and which later became my RX for amateur radio operations including working a few Ws in those days on CW, was a Philips Prestige BC receiver. It was a superheterodyne receiver with 6 tubes or so, including a 'Magic Eye' tuning indicator. I was very fond of staring at the Magic Eye in my younger days. With short wave reception the gap between the upper and lower bands used to fluctuate, with what we now call as 'QSB'! 73
