Quality answer to dangers of Tube amps running under no load
Messing with my VAC tube mono 160's, wound up surfin' tube related stuff and came across a guitar techs answer to a blown Marshall. Thought it was cool, figured I'd share:
"The answer lies in impedance and its sibling inductance, which affect AC signals like those that pass through your amp in use. The power tubes generate AC current, which works against the impedance offered by your output transformer and speaker together to create voltage and thereby power. Basically, current (I) times impedance (Z) equals voltage (V).
Now, transformers don't really have an impedance on their own. They multiply (in a fixed ratio) the speaker ohms a few thousand times to a level that's ideal for your tubes, which is a few thousand ohms. 8 ohm speaker = 8000 ohms to the tubes, for example. The tubes work against that impedance to make AC voltage, which can normally peak at around 800V or more.
If you have the speaker unplugged, the load is now nearly infinite ohms. That gets multiplied by the transformer to an even higher number of ohms (hundreds of thousands or even millions) that the tubes are trying to work against. The result of this is that instead of making hundreds of volts, the tubes now try to make thousands of even tens of thousands of volts. That much voltage can destroy the insulation in your output transformer, and can cause tubes to arc and self destruct. Some tube amps, but not all, use shorting jacks on their speaker jacks to help protect against this but plugging in a cable with nothing on the other end can bypass this feature and still cause damage".
