Here's a great resource if you want to understand tubes: the Navy's free electricity and electronics training materials - http://www.phy.davidson.edu/instrumentation/NEETS.htm They have a whole chapter devoted to teaching you the basics of vacuum tubes from the ground up: http://www.hnsa.org/doc/neets/mod06.pdf
One of the defining characteristics of a vacuum tube is that current can only flow one way through the tube - from the filament/cathode to the plate/anode. Rectifiers are thus the simplest sort of vacuum tubes. Apply AC to heat up a filament, in a vacuum, and electrons float across the gap onto a positively charged plate, which outputs current flowing in only one direction (DC). To be clear, no signal passes through this sort of a tube. It is merely part of the power supply of the amp, so whatever sonic changes are perceived as between rectifiers are only the indirect results of changing the power supply circuit.
Signal tubes are far more interesting because you are using the same principals to amplify the voltage of the actual audio signal, which is introduced into the tube through something call the "grid." The amplified signal is then (typically) picked up off the anode. Rectifiers don't have a grid. In any event, I highly recommend the neets modules if this is of interest!