Hi Karl,
The way a particular rectifier will sound in a given circuit is 100% dependent on the power supply circuit and amplifier circuit as well. There are generalizations, but they are only that, generalizations.
For example, a power supply designed to deliver a certain target DC voltage to the amplifier using a 5AR4 will *typically* sound more relaxed and less dynamic with a 5V4 or 5U4, due to low voltage.
OTOH, a power supply designed to yield that same voltage with a 5U4 will obviously sound different with a 5AR4. Whether it sounds more aggressive due to increased voltage or not will depend on the power supply topology, amplifier circuit, tubes used, and the output stage's operating class.
Directly heated rectifiers like 5U4s perform very differently at lower AC input voltages than at very high ones. Because the electron path is quite long in a DH rectifier, the electron emission cloud is slow to be attracted to the plates, unless the plate voltage is quite high (thereby increasing the electrostatic attraction). These lethargic electrons congest the area between the cathode and plate with a negatively charged field, and since electrons all have a like negative charge, they repel each other and disrupt the electron flow further. This is called the
space-charge effect.
Because of the space-charge effect, a given rectifier may perform very differently in a lower voltage/lower current application than in a higher current/higher voltage application. An example would be a simple preamp using say, an EF86 front end with a 6DJ8 cathode follower, or a low voltage spud amp, powered by a choke-filtered, capacitor input power supply. Here, the total current draw may be very small, and each leg of the power transformer secondary may only be around 180 volts. The voltage drop across a directly heated rectifier at low plate voltages increases sharply at low plate voltages with only a very small increase in current demand, due to the space-charge effect. This renders the 5U4 unstable at low plate voltages.
An indirectly heated cathode-sleeve rectifier with a very short electron path, like a 6X4 or EZ81, may sound very different (and likely much better) here than a 5U4, even though the output voltage yielded is almost identical. They suffer much less from the space-charge effect due to their very short electron path, but cannot produce a great deal of current the way the 5U4 can.
OTOH, in a scenario where you have a high powered PP amp that requires lots of current at some pretty robust voltages, a 5U4 may be a great choice. At higher applied AC plate voltages, rectifiers like the 5R4 and 5U4 that suffer very little from the space-charge induced instability and reduced conduction that plagues them at low plate voltages.
Mercury vapor rectifiers were developed to eliminate the space-charge effect altogether. The ionized mercury vapor has a positive charge equal to the negative space-charge, thereby counteracting it and providing an electrically neutral path for the electron emission. Mercury vapor rectifiers exhibit a certain fixed voltage drop that remains constant regardless of plate voltage, which gives them a very stable output voltage.
The power tubes make a difference. Power triodes and beam power tetrodes/power pentodes operated as strapped triodes act differently from normally operated beam power tetrodes and power pentodes when the plate voltage is raised or lowered independent of everything else. So, a given rectifier will change the sound more or change it less depending on these things, as well as differently.
Also, with certain beam power tetrodes and power pentodes operating in Class AB, changing to a lower voltage rectifier will lower the screen grid voltage enough to substantially reduce the transconductance of the tube. This reduces the gain and increases distortion.
The power supply topology used, whether capacitor or choke input, will have a great deal on how the rectifier type affects the sound, and how much as well. Whether a cap input supply is inductively or resistively filtered will as well.
These are a few of the very basic basics. There are so many things that determine how a given rectifier will sound in a given circuit that an entire chapter of a book could be written on just it alone.
Eddie