This information on this page was kindly contributed by Nigel Hughes.
How to Drive a TRF Radio
The most popular form of design for radios of the early 1930s was the so-called TRF (Tuned Radio Frequency). Radios then were mostly battery powered and, to save the precious and expensive batteries, they were only switched on for important programmes. Tuning in these radios is far from just a matter of switching on and turning the tuning dial. In fact, the listeners of the time were advised to switch on some ten or twenty minutes before the programme they required, so that they could tune in properly.
An early TRF will probably have two tuning dials, one for the aerial circuit and one for the detector, both usually only marked from 1 to 100, a wavechange switch, a reaction control and perhaps a volume control. There may also be an adjustable aerial coupling condenser. The reaction control is unlike anything on the superhet radios which became normal after about 1933. It works by providing variable positive feedback round the detector valve. Positive feedback means that the amplified signal at the anode of the valve is fed back to the grid to add to the input signal. If the positive feedback is strong enough, the circuit will oscillate on its own, but, by varying the amount of feedback with the reaction control so that the valve is just short of oscillation, it is possible to get very high gain and selectivity. Control of reaction is usually by a small variable condenser connected between the anode of the detector and a winding on the grid tuned circuit.
When trying out a TRF radio for the first time, switch on without an aerial connected, put each of the tuning dials and the aerial coupling condenser at the half way point, set the volume control, if there is one, to maximum and the reaction to minimum (usually fully anticlockwise). Now, slowly turn the reaction control clockwise. You should hear a "plop", "squeak" or "breathing noise" as the detector breaks into oscillation. If you have an RF oscilloscope, you will be able to see the RF appear at the anode. Set the reaction just short of the oscillation point. Now connect an aerial. If you are lucky, you may hear a station. Turn both tuning dials together for maximum signal. As you move the tuning dials towards higher frequencies, you will have to reduce the reaction setting to maintain stability. You will find the detector tuning is much sharper than the aerial circuit. Once you have found a station, you can try the aerial coupling trimmer, if fitted. Reducing the aerial coupling will give greater selectivity and reduced sensitivity. This can sometimes help to reduce interference from strong local stations.
You can now explore across the bands and try to identify the stations. I usually prepare a log sheet for each radio which tells me the best settings of the tuning dials for my favourite stations. Don't be surprised if you cannot stop breakthrough from very strong stations. I find that Droitwich, only 40 miles away, is audible right across the long wave band on my two TRFs. If your set has a frame aerial, or if you can connect it to one, you can angle the aerial to reject the strong station, as long as it is not in line with the one you want to hear.
Yes. Driving a TRF is fun!
TRF Circuit Operation
The aerial is connected to a coupling coil L1 via an adjustable coupling condenser. This enables a compromise to be selected between sensitivity and selectivity of the first stage. L1 is inductively coupled to L2 which, with the "First Tuner" variable condenser, forms the input tuned circuit. Additional coils and switches are provided on multiband receivers. The first stage amplifier is shown as a screen grid valve with screen voltage variable for RF gain or volume control. In its anode is the second tuned circuit L3, coupled to the detector grid coil L5. Also coupled to L5 is the reaction coupling coil L4. The second tuner may alternatively be in the detector grid circuit. Later TRFs used ganged tuning condensers and, sometimes, bandpass coupled tuned circuits, but the principles remain the same. In very early days, before the invention of the screen grid valve allowed valves to achieve real RF gain, the aerial would go straight to the detector circuit.
The detector is of the "leaky grid" type. Cg is a small RF coupling condenser and Rg is a high value grid leak of about 2 megohms. The detector derives its bias voltage by drawing a small amount of grid current on positive modulation peaks. Negative bias voltage is stored on the coupling condenser.
RF passes through the detector and is amplified. The RF Choke in the anode prevents RF from feeding into the AF transformer which normally couples to a further stage of AF amplification. RF from the anode is coupled to the feedback coil L4 and thence to the grid coil L5 by the variable reaction condenser. This condenser provides enough positive feedback to permit the stage to go into self-oscillation, but is adjusted to just below that point. Detector sensitivity and selectivity is then at a maximum. In practice, a "Q" of several hundred is achieved and with the reaction just short of oscillation, the "Q" is so high that the bandwidth becomes very narrow, reducing reproduction of high notes.
Text and Diagram Copyright © 2000 Nigel Hughes.