As long as electric guitars have been around, they've been backed up by tube-based amplifiers. Even today, after the invention of solid-state amps, tubes are still the preferred choice of many musicians—not only guitarists, but bassists, keyboardists, and players of other electric instruments as well. Along with their warm, resounding and distortion-loving tone, the beauty of tube amps is the ability to swap out the tubes to customize the sound.
Power tubes are one of three types of tubes you might find in your amp. When compared to the other two—rectifier and preamp tubes—the power tube's effect has less to do with tone and more to do with how high it lets you crank up the amplifier's volume. That makes your power tubes especially important when you're dealing with big, high-powered amp heads on monstrous speaker stacks.
This isn't to say that their effect on tone is negligible, though. Preamp and power tubes follow each other directly, and the overall sound of your amp is something of a combination between the two. Think of the power tube as a filter and a booster: it takes the tone created in the preamp tube and puts on its own finishing touches while kicking up the power high enough to drive your cabinet. What all of this boils down to is that you should be looking for a power tube that will complement your amp's preamp tube while also delivering enough ""oomph"" to make your speakers sing.
Figuring out which power tubes will hit that sweet spot is a matter of trial and error, but there are a few especially popular tubes that make a good starting point. For the 6550 type, Tung-Sol has a good reputation. Svetlana makes a great 6L6, and JJ Electronics and Mullard have excellent EL84s and EL34s, respectively. Whether you go with those suggestions or try something different, once you've got a handle on which tubes are right for your amp, all that's left to do is stock up and hit the stage.