The PAS preamps, as do almost all vacuum tube preamps, need two separate rectifiers (and two separate secondary transformer voltages) because at least two completely different DC voltages are required. They require a high voltage DC supply to run the signal part of the tubes and a low voltage to run the tube heaters (the part that glows red hot). Some tube units just use low voltage AC to run the tube heaters, but a DC supply for this is usually quieter.
At the time the PAS was designed, a tube rectifier was needed for the high voltage DC supply, while the low voltage heater supply could be done with the selenium rectifier (a predecessor to useful diodes).
The selenium rectifier changes the AC from the transformer to DC, the big axial lead capacitors in the bracket below smooth out the rippling DC from the selenium rectifier to a more uniform DC for the heater circuits.
By the way, one failing of the PAS selenium rectifier is that it becomes resistive with age. Thus its normal plus and minus 12V DC will droop down to 10V or less as the years go by. This partially turns off the tubes, causing them to conduct less, and the sound will get pretty muddy. A quick fix is to install a pair of 1N4004 diodes or similar in parallel with the selenium rectifier and replace the old paper electrolytics with new ones. Note that the new capacitors will be much smaller than the originals (modern electronic progress at work) so fitting the parts in place will be a challenge.
You are welcome to call me to get more advice on smoothing these pretty rough details above.
Frank
