A tube power amp can get away with a good solid power supply (PS) that consists of a big transformer , rectifier and a simple capacitor /inductor/capacitor bank.
If its big enough and of sufficient quality thats all that's really needed.
A preamp though is a different story- its a gain device and if the PS isn't clean, fast and powerful the good (the signal) will get amplified along with the bad (noise and distortion) .
A really good PS for a preamp will have 0.0mV of noise at the voltage output of the PS feeding into the loads of the triode tubes used for gain.
So for example, a Supratek will have 300V DC at the loads and will measure 0.0mV AC with no signal.
There's not many preamps that can achieve this and it takes an evolved and sophisticated design to do this competently and reliably.
Personally , I dont really take a PS for a preamp seriously unless it is regulated.
This means the voltage will always remain constant no matter what the mains voltage is or regardless of the demands of the circuit to produce distortionless amplification.
There's simple and more sophisticated ways of doing regulation, a good solid state regulator can be made with a handful of transistors and components, but being a died in the wool tube guy, I prefer a tube based regulator.
These come in either series or shunt versions, series is simpler.
Tube shunt regulators bleed off a portion of current to earth to maintain a constant current through the preamp circuitry, and from a sonic perspective are superior to series regs.
I've been using shunt regulators in my preamps forever, and they are a big determinant in the sonic signature of the Supratek preamps.
They provide a "blackness" around the amplified signal, with zero noise and distortion.
There's no strain with complex or dynamics passages as the voltage to the circuit is always constant and uninterrupted.
In contrast, an unregulated PS will "sag" with the demands of voltage peaks.
Component choices are also a big factor in performance. Once again, you will hear me say how detrimental electrolytic capacitors are to audio quality, and how so many audio devices these days are filled with them. Its not uncommon for a modern preamplifier to have 50 or more electros in them, with a maximum lifetime of 10,000 hours.
There are 2 electro caps in a Supratek linestage, one for the tube heaters, and one for the remote volume.
Polyprop caps are much better, or even better again, minimal or no cap designs simplify design, performance and reliability.
There is no substitute for transformer size, you can definitely have too small a transformer, but its hard to go too big!
The Supratek preamps have a seperate power supply chassis simply because the PS is too big to fit in one chassis, and there are advantages in getting the transformer and inductors away from the circuitry.
I also use 1.2mm copper chassis for both preamp and PS, its expensive but if you live with a copper chassis for a while you will hear that a steel chassis does have a "sound" that is less neutral than copper. It's almost intangible , but it's there.
There's a good reason the hyper expensive Japanese AudioNote gear is copper chassis based.
Power supply on the left. The middle tube is rectifier, the outside tubes are the shunt regulator tubes.
