A rectifier is a diode that converts AC to DC. Since every portion of a typical linear power supply in a vacuum tube amplifier is coupled (in part) to the output, changes in any component in a power supply can often radically effect the resultant sound - rectifier tubes, power transformers, resistors, capacitors, and inductors all can and do effect the sound.
The getter is not a tube, but rather a portion thereof. The getter is a metal element within the envelope coated with a substance which reacts with excess gasses trapped in the tube as the vacuum seal is applied. The getter both aids and maintains the vacuum within the tube, however it has no electrical function per se.
To explain with any more breadth requires at least an elementary knowledge of the topic. In other words, it is difficult to explain what a diode is without referencing other basic electronic terms and concepts. If you are interested in further exploration, several excellent online tutorials exist regarding basic electronics.
The getter is not a tube, but rather a portion thereof. The getter is a metal element within the envelope coated with a substance which reacts with excess gasses trapped in the tube as the vacuum seal is applied. The getter both aids and maintains the vacuum within the tube, however it has no electrical function per se.
To explain with any more breadth requires at least an elementary knowledge of the topic. In other words, it is difficult to explain what a diode is without referencing other basic electronic terms and concepts. If you are interested in further exploration, several excellent online tutorials exist regarding basic electronics.