Nearly every tube amp has a “sweet spot” — a point at which the tubes are cooking but not pushed too hard, the guitar sustains, has good dynamics, and a full, rich tone. But it’s not the same for every amp; some amps are meant to have everything dimed, and sound their best really pushed. Other amps sound best maybe halfway up on the gain knob, and two-thirds up on the volume. Of course, there are other factors that play into this as well: how hot your pickups are, any pedals in the chain, and more. But our focus here is the amp itself, and finding where it performs best.
Here’s a quick method for finding the sweet spot(s) for your amp. You might want to stand back or wear hearing protection if you have a loud amp! There are two aspects to this: volume and gain.
First, start with the volume knob. Start with the amp’s volume almost off, so you just barely hear sound. It will sound thin and kind of choked, because at that low volume, almost no signal is getting through to the power tubes. Now start slowly turning it up, pausing to listen as you do so. You’ll notice the sound getting more full bodied. However, at a certain point, the tone won’t get thicker or more full-range, it will just get louder. The point at which the amp first achieves its full-bodied tone is a “sweet spot” for volume. (There may be other sweet spots, depending on the tone you like and the response you want from your amp.)
If you’re looking to use your amp for dirty sounds as well as clean, repeat the above with the gain knob. With the gain really low, your tone should be pretty clean, and as you turn it up, it will get more and more distorted. However, at a certain point you may notice the tone doesn’t really sound more “distorted,” rather it sounds increasingly compressed and possibly more fizzy. The point at which your amp reaches maximum distortion without being overly compressed or fizzy is a “sweet spot” for gain. (Again, there may be more than one — one for crunch, one for riffing, one for soloing, and so on.)
Some vintage-style amps don’t have a gain knob or master volume, and the only way to get more grit is to turn the amp up to blistering levels. With those amps, you might want to go for a nice “edge of breakup” sweet spot, where the tone just gets gritty when you dig into it but otherwise stays relatively clean, and then use pedals for extra overdrive and distortion.
Knowing the sweet spot(s) of your amp opens up a world of tonal possibilities, and it’s worth taking the time to explore those settings!