This is a small “signal booster” for the 6 meter and 10 meter amateur bands, what we would call a “preselector” today. The advertising states that it is not a converter and not an RF amplifier, but a device to match the aerial, to the receiver front end, and to provide some gain as well.

The name comes from K for the Kingsley Radio Company, and S9 to improve your signal strength to Strength 9. This was advertised in Australasian Radio World magazine in 1947, September, October and November. It was an attempt to stimulate the use of new VHF amateur bands, and also to provide some business with the manufacturing wind down after WW2. The articles were written by H. K. LOVE VK3KU, the owner of Kingsley Radio.

The device is contained with in a small steel box, 6 inches x 8 inches x 5.5 inches deep. It is black wrinkle painted and the legends are engraved. It contains one 6AK5 valve and two tuned circuits. There are two plug in coil boxes, one for the 6 meter band, and one for the 10 meter band. Note that the boxes are identical to the RAAF World War Two AR7 receiver, which had four per plug in band box, and the handles are the same as well.


There are four terminals on the front panel, two for the aerial INPUT and two for OUTPUT to the receiver. There are also three wires coming out the back through a grommet, and these are for connection to the receiver power supply. There is a ground wire, a 6.3 volts AC wire, and a HT wire, which requires 150 to 250 volts DC. There are four controls. The ON/OFF switch, connects the INPUT and OUTPUT terminals together, effectively bypassing the KS/9 completely, and also removes the HT voltage. The AERIAL control tunes the aerial circuit. The RECEIVER control tunes the OUTPUT circuit. The SENSITIVITY control adjusts the screen voltage to vary the gain.

The circuit is quite simple. The input coil has a 10K resistor across it to make it broad. There is a 100 pF air variable capacitor in series with a 5 pF coupling capacitor. The output circuit is the same. The 6AK5 has normal cathode bias, and normal screen and plate supplies. The screen voltage is varied with a 30K potentiometer. Each coil box contains two isolated coils, each with a tuning slug, and shielded from each other.


The restoration was minimal, as there was little wrong with the device, only one high resistor, and the capacitors were fine. The two perspex strips holding the terminals were broken and were replaced. The steel case needed repainting in black wrinkle. I removed some ham modifications, and then aligned it correctly. The literature claims 30 dB gain, but I found that if the coils are peaked to get this gain, the unit becomes unstable, and oscillates. I set the coils for a flat response across the band, and this gave me a stable and uniform 15dB gain, across the 6 meter band (50 to 54 mHz), and the same on the 10 meter band (28 to 29.7 mHz). Be careful when removing the device from the case. Set the AERIAL and RECEIVE controls to fully meshed (setting 10), otherwise they may foul the case and bend the plates.

The photographs and sketches in the original advertisements, show two vertical handles, and a Kingsley oval nameplate. Then later advertisements show only one slanting handle. Then later advertisements again show no nameplate at all, just the engraving.

An interesting device from the post war era. It was available in kit form, or wired and tested for 6 pounds 6 shillings.


Australasian Radio World magazine 1947, September, October and November

Ray Robinson VK2NO

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