The triode (tube) often used in single-ended tube amps is what is known as *directly-heated*. That is, the filament (the bit that glows) is also the cathode connection to the tube and therefore part of the audio circuit. Examples include the 300B and 2A3. Since the filament is typically driven by (low-voltage) AC power, there is the potential for AC hum to get into the audio signal. The least hum is introduced when the audio connection is through a point that is exactly half-way in voltage to the AC signal going to either end of the filament---in other words, zero AC volts. The hum-pot allows adjustment to get to this condition and thus get minimum hum.
