TUBE OF THE MONTH - THE 5U4

This month Thomas Mayer introduces another full-wave rectifier tube, the 5U4. The 5U4 is probably one of the most widely used directly heated rectifier tubes and is still in production today.

It shares the same octal pinout with the 274B. However, it can handle more current, especially more peak current, which allows the use of larger load capacitors just after the rectifier. Although choke-input filters are a better solution and save the rectifier, cost-saving measures forced amplifier designers to use cap-input filters and larger capacitors instead. While the 274B allows a load capacitance of 4uF, the 5U4 can handle 10 times that or 40uF. But the real limiting factor is not just this capacitor value, but the steady-state peak plate current that the tube can handle. 

As long as this value is not exceeded, the actual capacitance is of secondary importance. The 5U4 can handle a peak current of 1 A. Of course, this comes at a price and requires a stronger filament. The 5U4 requires a 3A filament current. Detailed technical parameters can be found in the General Electric data sheet. Like the 274B, the 5U4 has a 4-pole counterpart, namely the 5Z3. There is also a variant that is technically the same, also has an octal socket, but a different pinout. This is the 5X4. The 5U4 was manufactured by all major tube manufacturers and is still manufactured by ELROG, among others.