Of the three different tube types that you might have in your amplifier, none are more important than the preamp tubes. These tubes are like miniature sound generators, taking the signal coming from your instrument and effects units and turning it into the tone that will eventually be played by the speakers. Because they're the chief performers, you'll have preamp tubes in any tube amp, including hybrid models which may have solid-state rectifiers and power sections.
With the responsibility to shape and refine the tone and gain of the sound, preamp tubes essentially represent the voice of the amplifier. This makes your tube choice critical to the final sound—but, like the strings of a guitar or the skins of drum set, the perfect tubes will always come down to personal preference. If you're a veteran performer, you've probably gone through plenty of tubes before finding your favorites. If you're a novice, consider doing some experimentation to figure out which tubes work best for you.
Your tube-based amplifier might have one preamp tube or several, depending on the power level and how it's set up. As a rule of thumb, the most important tube to the overall sound will be the first one in line, closest to the amp's input. If you're looking to customize the sound of your amp, then that initial tube should be the first one on your swap list. Of course, every tube has an impact, and if your amp has multiple tubes, using different combinations can give you a huge variety of potential sounds.
Whether you're doing a routine tube replacement or working on fine-tuning the sound of your amplifier, the preamp tubes are an essential part of every tube amp. You might swap to preferred tubes for different venues or even different songs—whatever it takes to get that perfect tone and gain. There are plenty of preamp tubes at your disposal from a variety of brands, each with its own unique character.