L1 is the aerial coil, and L2 the diode coil, and a matched pair of TRF inductors is required here. Pin numbering is for the Denco (Clacton) valve type coils, “Blue” Range 2 for the aerial position, and “Yellow” Range 2 for the diode stage. VC1/VC2 maybe a 2 x 310pF or 2 x 365pF ganged capacitor, with integral trimmers. Separate 50pF trimmers may be added if these are not present.
It is probable that other matched coils, such as those used in a discarded valve receiver, would be found satisfactory here.
Numerous general purpose RF FETs will be found suitable in this circuit. The constructional layout should separate L1 and L2, in the way explained for earlier RF amplifiers. Coupling between L2 and L1 causes the stage to oscillate, so that proper reception is impossible.
Where the amplifier has an input volume control which will be used, VR1 can be omitted. Audio output from the tuner is at a much higher level, than that from a pick-up or microphone. It may thus be convenient to retain VR1 so that output can easily be set at a suitable level.
Operation is from a separate 9v battery, so that the tuner can be connected to any amplifier without needing to draw power from the latter. It would in many cases be a straightforward matter to arrange that current for the tuner is derived from the amplifier.
Such a tuner will normally be used with a transistor amplifier. It can, of course, operate with valve amplifiers, but it is essential to refer to the earlier notes on safety, in such circumstances.
The aerial may be indoors or outdoors, depending on signal pick-up required, freedom or otherwise from local interference, and similar points. The cores of L1 and L2, and trimmers for VC1 and VC2, are initially set in approximately similar positions. Subsequently adjust the trimmers at a quite high frequency in the band covered, and the coil cores at a quite low frequency, for best results. As these settings interact to some extent, repeat the adjustments a few times.
