U18/20, FW4-500, 45IU, AU1, CV1264, FW4-500, NU12, FW4-800, CV31, 4/100BU, RV200/600 B4 BASED RECTIFIER FOR PX4 AMPLIFIER VARIOUS MANUFACTURERS
The U18/20 full wave rectifier is a much higher power device than most B4 based full wave rectifiers having a rating of 250 mA at 500 V and it's tape filament is directly heated. Just what is needed in the delivery of SAG FREE B+ in high quality audio valve amplification. This device is directly heated having a suspended tape filament which imbues it with instantaneous current delivery right from switch on - just like it's close relative the U52. In fact it looks exactly like a U52 except it has a B4 base - same bulbous ST envelope, same characteristically rounded low loss brown micanol base BUT B4 pin out.
Other specifications include a maximum reservoir capacitor of 16 µF with a minimum effective series resistance is 200 Ohms. In size terms the ST envelope has an approximate diameter of 48mm and exclusive of B4 base pins a length of 120mm.
These devices are not offered frequently and are sought by discerning users whom wish to keep their prized amplifiers using contemperaneous B4 based rectification rather than change to Octal based units.