Before the late 1950s, every amplifier available to musicians had a power supply that used rectifier tubes. That changed with the invention of the transistor and the development of solid-state and hybrid amps, but these tubes live on in vintage, boutique and all-tube amps. Today, there are plenty of casual and professional musicians who continue to prefer the unmistakeable classic character that comes with the traditional rectifier tube. Unlike preamp and power tubes, rectifier tubes don't actually process the audio signal directly. Instead, their job is to convert AC power from the wall outlet into the DC current that your amp's other tubes run on.
When it comes to your sound, this means that the effect of changing rectifier tubes is a bit more nuanced than it is with the other two types. The characteristics of power and preamp tubes actually change—in some cases, significantly—at different voltages. As a result, your choice between a 350-volt and a 400-volt rectifier tube, for example, will actually alter the performance of the other tubes. Another important characteristic of rectifier tubes is that they have their own internal impedance, which reins in their voltage briefly when a load is first put on them. When you play, that sounds creates a sagging effect on the attack of a note, followed up by a swell of volume as the tube kicks its output back up. This effect gives an amp with rectifier tubes a unique quality almost like a built-in compression effect, softening your lead-in and boosting the rest of the note. Different rectifier tubes will express this effect differently, so it's a good idea to read up on what other guitarists have to say about each tube to find out how the ones you're considering will sound. The most popular rectifier tubes for medium to large amplifiers tend to be the GZ34 type, which are liked for their build quality, efficiency and strong power levels. The 5Y3, on the other hand, shines in amps smaller than 15 watts. Any of these classic tubes is a solid choice for any amp that it fits, whether it's a genuine vintage head or a modern boutique combo amp.