Philips ‘Colette’ Portable Radio (1956)

Not long after WW2 Philips, a leading tube manufacturer in the Netherlands, iin their famous Natuurkundig Laboratorium (‘NatLab’, in English: Physics Laboratory) developed and perfected a series of miniature low-power tubes employing direct heating instead of indirect, specifically for use in portable radios. These tubes identified by the first letter D in their type code (rather than E for 6.3 V indirect heating) operate at a filament (heater) voltage of just 1.4 VDC and 50 mA current consumption (typical), with the cathode doubling as the heater. Four of these small tubes could be connected in series for running off a standard car battery, which was 6.2 volts at that time.