2. amps-preamps
  3. 194849-98963

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".

cavscout19d

Showing 1 response by gs5556

gs5556
1,245 posts
 

I don't really agree with that explanation, it applies more to an ideal transformer. If there is no load on the secondary there is no magnetic field from the secondary current to reflect so the primary becomes one big inductor. An inductor's impedance is frequency dependent, so when the power tube's current hits the higher impedance the anode to anode voltage surges. The higher surge voltage does the damage.

A more immediate threat is if the speaker cable is suddenly disconnected with a current through the secondary. The magnetic field collapses and the stored energy in the primary and secondary has nowhere to go and causes arcing from the induced voltages. Same as a car's ignition coil operation, only the collapsed magnetic field has a relief path across the spark gap. The tube amp OPT has no such safety valve unless you put a few diodes in series with the B+ to ground. But the problem with that is diodes tend to short and that would cause a short through the transformer. 

As a precaution, what I do with spade terminal speaker cables is mount them facing down on the amplifier binding post so if they should become loose the weight of the cable won't  pull it off the post. 

More to discover