I don't recall where or from whom, but I heard that switching on an amplifier head without a connected load (a cab, that is) may cause damage—presumably to the tubes, but perhaps something else, I'm not sure.
Is there any truth to this? If not, is there a related problem that might have inspired such caution?
It is true that a tube amp should not be operated without a load, i.e. without speakers (or a dummy load) connected. Solid state amps don't have that problem. The reason is that (almost) all tube amps use an output transformer, which can produce high voltage peaks if its secondary (output) winding is not connected, because the energy from the primary (from the power tubes) cannot 'go anywhere'. So it will be fed back to the primary (where the power tubes are connected). This will cause damage either to the transformer, the power tubes, or both. If you want to know more technical details, check out this excellent paper by Peavy (mainly page 6).
Having said that, don't worry too much if you've operated your amp without a speaker for a short while. It has happened to me too, and everything was fine. So apparently the transformer and the tubes will not immediately be destroyed but it may take a while till things get bad. There are also some tube amps which are protected against being operated without a load. My Koch amp has a built-in dummy load which is connected automatically as soon as the speaker cable is disconnected.
I disagree.
I've just seen a blown up an acoustic 260 head.
The power cord was plugged into the wall, sharing with a window based air conditioner unit.
There was no speaker load connected to it.
The power switch was flipped to "on".
In this state, it was plugged in from the back with the cord, providing instantaneous power to the whole thing, and within 1/2 of a second a click sounded inside, bad smelling smoke was released, IN VISIBLE CLOUDS NO LESS, and the light went out, never to come back on.
Always pretend it can be worse and play it safe.
A modern valve amplifier with a push-pull output stage should have diodes across the valves to clip spikes caused by the transformer inductance when unloaded. Check that it has. If not, you can put them in, diode cathode to valve anode (reverse biassed across the valves). Just make sure the diodes are rated at greater than 2x the DC line voltage. Say 1000V for a 400V line. If you used 1N4007's use the glass passivated type 1N4007GP. The ordinary ones are slow & may overheat at high frequency.
Way back when I was a young whipper snapper I inadvertently turned my first stereo amp, a Marantz 1060 solid state integrated, on with no speakers or dummy load and within a couple of seconds POOF, it let the magic smoke out of the output transistors on both channels, so YES, ASSUME IT WILL DAMAGE THE AMP SO DO NOT DO IT!
