This interesting looking set is a fairly rare survivor of a batch of 40,000 given away free in exchange for coupons from packets of cigarettes in 1930. The cigarettes in question were "Best Dark Virginia", a brand made by Godfrey Phillips Ltd., and in order to qualify for a free radio, it was necessary to spend something over £12 which would have purchased 500 packets of ten.
At that time the price of receivers was very high being the outcome of a protection scheme run by a cartel of British manufacturers. Rules abounded...valves had to be British, royalties had to be paid to the Marconi Company, the BBC etc. and the sale of cheap foreign imports was outlawed.
Maybe the "sale" of cheap foreign imports was banned but giving them away "free" wasn't, so this loophole was exploited in order to sell more cigarettes! I understand the receivers were made by KB but cheap imported valves were supplied by the tobacco company.
The KB shown here is one of the earliest bakelite-cased models and is an extremely neat design using a TRF circuit around a couple of 2-volt valves. The lid which carries the embryonic loudspeaker hinges up to reveal the tuning and reaction controls etc. Unfortunately although the receiver's physical size is tiny, the accompanying batteries were pretty standard and had to be employed externally to the set. This changed the neat receiver into a bit of a "Heath Robinson" affair with its trailing leads and ugly collection of batteries.
|TRF sets were very simple in design. A stage of aerial amplication and detection was followed by an audio amplifier. In order to drive the loudspeaker from the tiny aerial signal a huge amount of amplification is needed. For example, if you require 100milliwatts to drive an 8 ohm loudspeaker you need about 1 volt across its coil. A typical aerial signal from a local transmitter might be 10mvolts. You will therefore need a combined amplification from the two valves of 100. For an aerial signal from a more distant station of 1millivolt you need a gain of 1000. If the gain of the audio amplifier is 10 then the gain of the RF amplifier must be between 10 and 100. This is achieved by applying positive feedback to the point just before the valve oscillates. The front panel controls of the KB include a Reaction Control (on the right) which is turned clockwise and set to the point immediately before the RF amplifier oscillates.|