If you've recently replaced your tubes, then knowing how to adjust guitar amp bias is a must! Without proper biasing you run the risk of working your tubes at uneven rates, which will ultimately burn out your tubes and screw with your tone. Here's a straightforward approach to adjusting guitar amp bias. Keep in mind that amps pack enough power to kill you, so if you don't feel completely comfortable with this process do not attempt to adjust the bias on your amp!

1. Buy an amp-head bias tester probe. Getting one of these could save your life! Bias testers let you plug each individual tube into a probe before connecting the probe to your amp. A probe will allow you to measure tube amperage in millivolts, which you'll want to know in order to properly adjust guitar amp bias by setting each tube to the appropriate power level.
2. Determine a tube's output. Here's where it gets scientific! To figure out the plate dissipation of an output tube, the equation (P = E * I) is needed. E stands for plate voltage, while P equals the wattage you want to run your tube at. Say your amp runs at 400 volts and your tube at 20 watts. The equation would now look like (20 = 400 * I). Simply solve for (I) to figure out the correct millivolts to adjust your guitar amp's bias to-which in this case would be 0.050.
3. Adjust your amp's trim pots. When you crack open your amp, you should see a couple trim pots specifically meant for adjusting the guitar amp's bias. Using the equation above, you'll want to adjust the bias to 0.05 millivolts for each tube. Simply turn the nob until your probe reads 0.05 millivolts. Plug the tubes in the proper way and you're done. Congratulations, you've just adjusted guitar amp bias!

Though adjusting guitar amp bias sounds straightforward, again do not attempt this if you're not familiar with the process. Mucking around in your amp without care could kill you, and that's nothing the amp manufacturer wants to see! Play it safe and seek a trained professional if you're at all unsure.