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Rectifier

What do they do?

Rectifier tubes help the power supply circuit of your amp turn the AC voltage from your wall outlet into a DC voltage. Vacuum tubes need DC voltages on their electrodes in order to amplify an input signal.

How can sound be affected by them?

Sound can be indirectly affected by the rectifier tube mainly in the form of compression. The amount of voltage drop (sag) across rectifier tubes can vary from tube to tube.

When you strike a guitar string hard, the power tubes draw more current from the power supply circuit. When more current is drawn, you get a larger voltage drop across the rectifier tube which makes the voltage on the power tube plates lower. A lower voltage on the power tube plates gives you a lower power output. As the string vibration dies down, less current is drawn from the power supply and you get a smaller drop across the rectifier tube, which makes the voltage on the power tube plates higher. A higher voltage on the power tube plates gives you a higher power output.

Can they be easily changed?

Yes. Just let the tubes cool down, pull the old one out and put the new one in.