biasing is done using DC. The point of biasing is getting your signal (audio) to 'sit' on a dc level, giving the transistor maximum swing for both half cycles (class B).
if you over or under bias a transistor, you will only distort your output signal by clipping the waveform, so experiment.
by tubes, i hope you dont meen Traveling Wave Tubes! if you mean valves then there is always a strong positive voltage on the annode, a positive voltage on the screen (if used) and a negative voltage on the cathode and control grid.
Quick lesson on valves=
the cathode is heated by a element. This warms the Cathode to free up the electrons that are sat on it. The positive potential on the screen starts to attract the electrons toard the Annode, which itself attracts the electrons. The control grid, which has your signal on it changes how many electrons are atracted to the annode which will be proportional to the signal, thus amplifing (and inverting) it.