Most of the power supplies found in audio amplifiers follow conventional design practices, and common arrangements or topologies can usually be found when looking at various amplifier designs. There are two or three power supply topologies which are seen again and again in tube amplifiers. They are regarded as almost standard, being more or less copied from the popular early amps. However, it is worth noting that these designs are not compulsory. There are many ways to arrange the same transformer and rectifier, depending on the output voltage we need.
For example, the circuit in Fig A below is an ideal half wave rectifier producing about +100VDC from 70VAC transformer (remember, VDC = Vrmc x square root of 2). Thus, we know that there must be almost 100VDC across the capacitor. If we make the negative end of capacitor zero volts (ground) then other end must be +100VDC.
Alternatively, we could ground the positive end of the capacitor. There must still be 100V across the capacitor, so its negative end must now be -100VDC, as is shown on Fig B. Both circuits Fig A and Fig B work identically of course, all we have done it moved our frame of reference. We could also alter the direction of current flow by reversing the diode and capacitor, in which case we could devise the circuits as is shown on Fig C and Fig D.
Although A and C considered to be "normal" arrangements (perhaps because they are little more intuitive when drawn as a schematic) there is no reason why B and D could not be used to achieve the same results. this ability to shift our frame of reference and look at how a circuit works rather than what it looks like when compared with someone else's circuit is essential.
Depending on where we place the ground reference, rectifiers can be made to produce positive or negative DC voltages.