Amp Friend is a no-frills, one-trick-pony calculator for tube amp repair technicians to quickly and easily calculate the power being output into a particular load, or calculate the expected voltage for a specified wattage.

Supports RMS or peak-to-peak voltages, and power (watts) as input.

Simply measure the output of your amplifier in volts, either RMS (with most multimetres), or peak-to-peak (with most oscilloscopes), and type the numbers into Amp Friend. It will then show you the wattage the amp is producing into each load.

You can also type in the wattage desired, and it will show you the voltage you should expect.