Triode tubes are inherently low noise devices, and extremely linear when used properly. This means that the oscillation frequency wave it produces is very pure and clean. The noise in the sidebands, from 10Hz to 100kHz on either side of the oscillation frequency, is extremely low. This is the important spectrum for audio. Everything below and beyond that can be ignored.
Each square wave seen below is measured on a Philips PM3295A analog 400MHz oscilloscope using LeCroy 10:1 probes. These probes attenuate the signal by a factor of 10, in order to not load the device under test with any load, resistive or capacitive. So the vertical resolution on the scope image has to be multiplied by 10. For instance when you see ~0.1V, then it is in reality measuring ~1V(AC).