This month I'd like to introduce another full wave rectifier tube, the 5U4.
The 5U4 is probably one of the widest used directly heated rectifier tubes and is still in production today.274B which was presented last month. But it can handle more current, especially more peak current which enables the use of larger load capacitors directly after the rectifier. Although choke input filters are the better solution and are easy on the rectifier, cost saving measures forced amplifier designers to work with cap input filters and larger capacitors instead. While the 274B allows 4uF load capacitance, the 5U4 can handle 10 times that, 40uF. But the actual limiting factor is not just this cap value but the steady state peak plate current the tube can handle. As long as this value is not exceeded, the actual capacitance is of secondary importance. The 5U4 can handle a whopping 1A of peak current. This of course comes at a cost and requires a heftier filament. The 5U4 needs 3A filament current. See the General Electric datasheet for detailed technical parameters. Like the 274B the 5U4 also has a 4 pin counterpart which is the 5Z3. There is also a variant which is technically the same, also has an octal base but a different pinout. That is the 5X4. The 5U4 was made by all the major tube manufacturers and is still being made today among others by ELROG.
Here we have a 5U4G made by RCA.
Next we have a National Union:
A Raytheon 5U4G in plain military box:
Another RCA, also in military packaging: