9

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?

  • 1
    It's been 'common knowledge' since I first started playing in the 70's. I've aways avoided doing it, but actually have no real idea if it's actually true or not - it 'feels' like it ought to be true... but... hopefully someone with proper electronics knowledge will confirm or refute. [this post really just so I get notifications on answers] – Tetsujin Sep 7 '14 at 19:19
11

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.

  • Thank you for that clarification of my long-held 'voodoo' belief, Matt. Most appreciated. – Tetsujin Sep 7 '14 at 20:14
  • 1
    The backline at the festival we played last night did get killed by doing exactly this - the techs clicked it off standby without telling me, and without having a cab attached. The nice red light on the front died instantly, and no noise came out. Needed to get a runner to find a new Marshall 120W head :-( – Doktor Mayhem Sep 7 '14 at 22:50
  • Transistor amplifiers drive more power into low-impedance loads than high-impedance loads, while tubes do the opposite (a short is zero impedance; an open is infinite impedance). Curiously, I seldom see any mention of this phenomenon in discussions of "tube sound", even though the impedance of a typical speaker will vary with frequency, and the way tubes interact with that varying impedance is very different from the way transistors interact. – supercat Oct 21 '14 at 3:27
  • i killed some valves doing this trick , ileft the amp to warm up for about 15 minutes and heard a bang , and then realized i had disconnected the speaker previously – aron May 9 '17 at 3:30
1

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.

0

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!

  • 1
    All modern professional solid-state power amps can handle running without load just fine though. – leftaroundabout Jan 28 at 22:09

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service, privacy policy and cookie policy

Not the answer you're looking for? Browse other questions tagged or ask your own question.