Tesla 420U Trio

In 1956, Tesla Bratislava introduced a new type of radio receiver for AC and DC mains that used a new series of tubes with 150 mA heater instead of the traditional U21 tubes with 100 mA heater. The 150 mA heating was common in the USA, but no other Tesla device used it, nor did any other European manufacturer. A radio with these tubes consumes less power on 120 V mains than with the older tubes, but on 220 V mains, the power consumption was much higher and the radio got very hot. Later, the owners of these radios had to cope with unavailability of these tubes.