TRF (Tuned Radio Frequency) Radios were popular in the 1920s and early 1930. They quickly became obsolete with the introduction of the modern superheterodyne circuit.
A TRF radio is a radio in which the actual radio frequency signal undergoes several tuned stages of amplification before the signal is detected (that is before the audio frequency signal that we can hear is stripped off the carrier radio frequency). The major flaw with the TRF radio lay in the vacuum tubes available at the time. These tubes were good at amplifying radio frequencies at the lower end of the broadcast dial (550KC) but were poor at amplifying the higher frequencies at the top of the dial. Such a radio will become less sensitive the higher up the dial you tried to tune. It was not possible to increase the amplification (gain) at the higher frequencies without causing severe oscillations (radio squealing). Thus a single tube could not be designed that would adequately and equally amplify all broadcast frequencies. The strategy taken to overcome this problem was to employ low gain amplifier tubes that would not squeal at the higher frequencies. By itself such a tube could not adequately amplify any broadcast radio frequency, but by coupling several of these tubes together in series all broadcast frequencies could be adequately amplified.
Because of this multiple stage radio frequency amplification strategy, TRF radios can usually be identified by two features. They normally contain a multi-section ganged tuning condenser (4 or 5 ganged sections are not uncommon in these radios). Also, such radios contain a row of identical tubes, One for each stage of radio frequency amplification.
The Sparton Equasonne, manufactured by the Sparks Withington Company of Jackson Michigan in the late 20s and very early 30s, is not a typical TRF radio, in that the individual radio frequency amplification stages are actually untuned.
If you wanted to be highly technical on this point, the Equasonne is not a TRF radio at all. The Equasonne circuit was originally called the "technidyne", and was designed by Lester Jones. All tuning happens up-front, in the selector unit, which contains a 4 section ganged tuning condenser, and 4 coupling coils, used to construct 4 loosely coupled tuned circuits. None of these 4 circuits produce any amplification (with the exception of those models which incorporate a single 484/485 type tube into the first tuning cirucit of the tuning selector unit). Amplification happens afterwards as the selected radio frequency signal of interest, (which has hopefully be adequately purified out from all other signals from its journey through the tuned circuits) is passed through 5 stages of untuned amplification in the RF (radio frequency) amplifier unit of the set.
Thus simply stated, a true TRF radio both amplifies and purifies (selects out) the desired radio frequency by passing it stepwise throuh a series of tuned circuits, all tuned to the same frequency. The Equasonne purifies (selects out) the desired frequency first, and then amplifies just this one frequecy later.
Presumably The Sparks Withington Company designed the circuit in this manner to avoid having to pay licensing fees to RCA required to use the patented and more typical type of circuit. Sparton manufactured absolutely all of the components used within its radios, did things its own way, and seemed to be very proud of this fact.
The important point to realize about the selector unit, is that the 4 tuned circuits through which the RF signal passes must be adjusted so that each one is tuned to precisely the same frequency at the same time. Otherwise the signal will be lost, distorted, or weakened as it is passed through these 4 circuits. As each tuning cirucit cannot be manufactured to be 100% identical, small mica trimming capacitors are incorporated in parallel with the tuning condensor section in each circuit to allow fine adjustment of each circuits total capacitance.
In order for the RF Amplifier to work optimally, all 6 tubes used in it should be matched (that is their electrical behaviors while operating should be as identical as possible). 484 and 485 tubes used in these amplifiers are no longer manufactured, are quite rare and quite expensive, so obtaining a matched set today is very difficult. Also, as the tubes start to wear out, they become un-matched. To compensate, in order to operate optimally, the positioning of the 6 tubes within the Equasonne amplifier is critical.
Finally, most Equasonnes only provide for one stage of audio frequency amplification after detection. This is accomplished by a pair of tubes working a push-pull arrangement. Push pull only works optimally when both of these tubes are equally matched. This can be difficult to achieve today as these audio frequency amplifier tubes are even harder to come by and more expensive than the RF amplifier tubes.
An interesting feature of the Equasonne radio is that it is modular. Most models consist of 3 distinct units (tuning selector, RF amplifier, and power converter). Some models incorporate an extra stage of audio frequency amplification, that is contained in a 4th unit. Thus the chassis of an Equasonne comes apart into several large discrete functional units. This makes the Equasonne an easy set to work on, and a simple circuit to understand. Apart from external cabinet differences, Equasonnes do differ from each other electrically. There were at least 3, possibly 4 slightly different versions of the RF amplifier unit, 3 versions of the selector unit, and numerous power converters. The different models are different combinations of the different versions of these 3 parts. The major difference between models seems to be the power converter unit. As already mentioned, some models featured a fourth unit (extra audio amplification stage). A couple of models also incorporated a turntable.
Wonder what ever became of the Sparks Withington (SPARTON) company that manufactured your equasonne all those years ago? During their radio making days they certainly made some of the grooviest and most sought after radios ever built! Today they seem to be alive and well, and although they no longer appear to manufacture radios, they still manufacture high end electronic components, they even have a web site!