The circuit was designed so as to get the maximum possible gain out of the first three transistors (which operated at radio frequencies). The first transistor used as a frequency converter was operated very close to its VCBO of 30 volts (from the 22.5 volt battery). This gives a larger depletion layer between the collector and the base which reduces the parasitic feedback due to the Miller effect and extends the frequency range. The two intermediate frequency amplifier transistors are neutralised to cancel out their parasitic Miller effect feedback which also extends their frequency range. Replacement transistors were supplied with the correct neutralising capacitor of 100-200 pF.