amp with no load - speaker swapping while on

[xtian]

We know that to run a tube amp with no speaker or other load attached is a Very Bad Idea. Conceptually I have no trouble imagining why--I just imagine pedaling a bicycle up a very steep hill as hard as I can, and then imagining what would happen if there was suddenly no resistance to the pedals. Ouch.

So I'm wondering about an amp that's on, but not doing any work (that is, silent). Will it be stressed with no load?

I guess the more practical question is, can you hot-swap speakers while the amp is on? Could you/should you?

[TheGimp]

I would venture that it depends on the amplifier stability.

An unconditionally stable amp should have no problems.

If the amp is not unconditionally stable, it could break into oscillation when the load impedance changes to a very high state. This would be the equivalent of being driven with no load.

If you are going to swtich loads with it on, use dual jacks and always plug the second speaker in before removing the first. Then the amp will see a much lower impedance load temporarly, but never a very high impedance (open) load.

[xtian]

Good advice.

And what about in Standby? Safe to swap?

[dartanion]

Put the amp in standy. This means that only the filaments are seeing power, so no worries about swapping speakers/cabs in standby.

[Bob-I]

I've done plenty of speaker swapping while the amp is on, not that I'd recommend it. As long as there is no input signal and you do it quickly it's fine.

What I'm saying is if you make a mistake and forget to put the amp on standby you'll prolly be ok.

[C Moore]

What is the situation when you cannot turn the amp off to do a speaker swap?
Thank You

[d95err]

Put a resistor across the OT secondaries to make sure the amp always sees a load. 200 - 300 ohms should keep the amp safe and won't affect tone. This also protects the amp from glitchy speaker jacks, playing without a speaker (common OT-breaking scenarios).

[Scumback Speakers]

I always put the amp on standby, then I can switch ohm settings, cabs/cables/speakers without an issue.

Leaving the amp on (standy off, powered on to play) during a speaker/cab/ohm change is just burning up your tube life, and in some cases more.

[xtian]

What is the situation when you cannot turn the amp off to do a speaker swap?

I don't do it. Just curious. And why sit around wondering when you can just ask, and a lot of really smart people will answer? I love this board.

[jojokeo]

I can see why you wouldn't want to unplug and re-plug speakers in while the amp is on & playing but if you have a simple A/B switching box (w/ two different 8 ohm speakers for example), what's the harm in going back and forth btwn speakers to hear the subtle differences btwn them? I do this all of the time w/out issues to date.

>>An unconditionally stable amp should have no problems<<

What is this definition to have an "unconditionally stable" amp?