This months tube is a rectifier tube, the 5X4:
This is a 'classical' rectifier as used in most tube rectified amplifiers. It has two diodes with a common cathode in one bottle. The rectifier is directly heated which means the filament is also the cathode.
The 5X4 was only made in ST glass as far as I am aware. That's why it is also called 5X4G. It has an Octal base. The pinout is shown on the left. It is typically used with a transformer with a center tapped secondary. The secondary connects to the plates of the rectifier and the center tap serves as ground. B+ is then derived from the center tap of the heater winding, or from either end of the filament. It can also be used in a bridge rectifier arrangement without center tapped secondary, if it is augmented by two additional diodes. These connect from the secondary ends to ground, cathodes to AC and anodes to ground. I mostly use TV damper diodes as rectifiers, that's why I have not used the 5X4 in any of my designs yet.
The reason why I am presenting it here in the tube of the month series is it's electrical equivalence to the very often used 5U4. It only differs in the pin out. 5U4 or 5U4G are often seen in the power amplifiers of 300B amplifiers. Hence they are commanding a hefty price. The 5X4 however is still largely overlooked. It typically costs a third or even less of the price of 5U4s.
Sockets which are wired for the 5U4 can be very easily rewired to accept the 5X4. While the plates of the 5X4 connect to pins 3 and 5, the 5U4 has the plates brought out to pins 4 and 6. The former has the filaments at 7 and 8 and the latter at 2 and 7.
The rectifier socket can even be wired such that it accepts both tube types. All that is needed are 3 jumpers which connect pins 3 with 4, 5 with 6 and 2 with 7.
The first photo above shows Rogers 5X4G made in Canada. Here some more details of this tube:
A Sylvania, made in USA:
A close up of it's ribbon filament:
RCA slightly changed construction details and printing over the years:
A close up of the filament of a RCA tube:
Availability of the 5X4 is still good. If your amp uses 5U4 rectifiers and you have difficulties finding replacements, consider modifying it to accept 5X4.