Just how good does a tube amplifier sound? Ask a dozen guitarists this question, and while the answers may be different, the message ought to be the same: tube amps are the kings of tone. If you're here looking for some new tubes, you're likely on board with that assessment. Fortunately, you'll find plenty of tubes to choose from so your amp can keep on giving you that amazing sound.
Preamp tubes are the first ones in the signal path, taking the raw electrical information coming from your guitar and effects pedals and converting it into a tone that the speakers will be able to understand. The preamp tubes are almost exclusively responsible for the way your amp will sound, so this is where you want to start if your goal is to swap out tubes to adjust that tone and gain. If your amplifier has more than one preamp tube, take a look at its wiring chart: the one that has the strongest effect is the very first one in line.
Power tubes are the second stage in your amplifier. When the signal gets here, it's already been shaped by the preamp section, and the power section's job is to give it a big volume boost. Make no mistake, though: the power tubes still make an impact on the sound. Changing your power tubes will make subtle changes to the overall sound of the amplifier, as well as adjusting its potential output volume.
The third type of tube that may be in your amplifier is a rectifier tube. You don't see these tubes as often as the others, but if your favorite amp is an antique or a hardcore all-tube model, you probably have them. What makes rectifier tubes different from preamp and power tubes is that the electricity going through them isn't part of the audio signal—instead, rectifier tubes are part of the power supply for the amp, handling the current being sent to the preamp and power sections. So, how does a rectifier tube affect your sound? There are two answers. First, different rectifier tubes produce different voltages, and a preamp or power tube will sound different depending on the voltage it's getting. Secondly, rectifier tubes drop their voltage when loaded quickly. This creates a softening effect on the attack, which leads to a surge of sound afterward. When you're experimenting to find your ideal rectifier, the sweet spot will depend on both of these characteristics.
Re-tubing is always a chance to try something new with your amp. With a change of the tubes, an amplifier can sound completely different—so don't be afraid to explore different combinations!