is it ok to leave a tube amp on without speakers connected

NO. NO NO NO NO NO. Had a pari of VTLs without my WATTS attached. Scared the color off me (I'm Hershey Brown colored). 

NO!!!!

With tube amps the answer is NO!.you  will fry the output xfmr. SS amps may  be left on though however connecting and unconneti g spkr cables leaves you vulnerable to accidentally shorting the leads.

What does the manual/manufacturer suggest or warn against or not. It's a yes and no answer. My Cary Rocket 88R  does not need to see a load and will not damage the amp by design.  Others will require a load to be safely used and not damage the output trannies.  Never follow the advice of anyone other than the manufacturer of YOUR product .

Like others report: My research indicates the voltage builds up and damages the output transformer. I wanted to disconnect one speaker so I could do the Soundoctor.com trick to integrate my sub. Was told never run tube amp without load (by Upscale guys) but you can use a 10 ohm 10 watt resistor in place of the speaker https://amzn.com/B0087ZD6FQ. I think they do that when they are working on an amp and connecting a speaker is inconvenient. Decided for my purpose I could just disconnect the input from one amp channel.

Maybe this one worth reading

Dynaco's ST70 manual does not recommend leaving the speakers off.

The trick is make sure there is no signal input.

For modern amps best to check with the manufacturer.

So it's a good test. No input signal, no speaker load, if it gets damaged - it's a junk amp.

My Audio Research VS60 can be left on with no load.  Matter of fact, I was told to bias the amp in that way with no input signal.

It's your gear and your risk. Like Ralph says, it could be okay and probably won't damage it. For example, Dynaco's ST70 manual does not recommend leaving the speakers off. Probably best not to risk it. 

The answer is yes IF the amp is stable. If it is not, then the amp can be damaged. If left without a load, the amp should not have a signal applied to it as without a load the transformer can arc. But I've seen many tube amps you could leave on all day with no worries - Dyna ST-70, HK Citation 2, Marantz 8b and many others.

Our amps don't care much what kind of signal is at the input. They are pretty hard to damage and we run them without a load all the time. But they don't have output transformers that can arc.

At any rate, stable design is an important feature in any amplifier; if a tube amp can be damaged without a load while at a zero input signal condition, its a pretty good bet it will be less reliable in general use. Imagine what might happen if a speaker cable accidentally is disconnected- the amp had better be able to survive that sort of thing, and most of the vintage amps from the old days can.

 

No

The answer is clearly yes you can, but not without certainly experiencing the aforementioned damage to your tube amp…but yeah…you can.

Most likely the output tubes will be damaged once you remove the speaker load as the inductance in the primary is converted into a very high voltage and the power tube shorts that voltage to ground. But then again, the transformer may damage as that energy arcs across the windings since it has nowhere to go.

Depends on certain amps.  I think my previous Cary 211fe didn't need a resistive load connected meaning you could disconnect the speakers.  But I wouldn't bother or mess with it.

As previously said, you can burn the output transformers. 
I also have two amplifiers and two sets of speakers. I made a switch box that allows me to choose which amplifier am I listening to, connected to which speakers. There are some models on the market (selectores and switches), but I decided to build my own and I'm very happy with it. 

Thanks for the responses.  Makes perfect sense.

No, and agree you should never connect and disconnect speakers with the amps on in any case.

Octave Audio tube amps can be run without a load but, you really shouldn't be connecting/unconnecting speakers under power any way.

You will damage the output transformer with no load on the amp

No. 

No, I don't know the specific reason, but was told that the speakers provide a load on the amps, and especially for tube amps, it's not okay.