Running a valve amp without any speaker connected???

[barkfield]

Can someone (probably SteveD) help me with a question I've never found the answer to? I'm an electronics engineer as well as a guitarist so I understand valve circuitry quite well but I've never worked this one out because I've never had to do design.

Given that the manuals always say "do not switch the amplifier on without speakers connected" and it's generally accepted that you will do damage if you do run your amp without a load (speakers, hot plate, powerbrake, etc), I have a couple of questions.

1) what will get damaged? - valves, transformer, output capacitors, what????

2) at what settings do you have to run to do the damage?

3) how long do you have to run the amp without a load to do damage? e.g. like most people will have done at some time, if you've accidentally switched the amp on for 10 minutes before plugging your guitar in and realising you've not connected the speakers will you do any damage?

[kissfanps]

Steve can probably fill you in on the details, but I believe you can pretty much blow up the output tranny.

[slowpokerhino]

I believe Steve has stated that if there is no signal going through the amp (i.e. you're not playing through it) it will probably not do any damage.

[SteveD]

If the output transformer is unloaded when a signal is injected into it's primary, there can be transient spikes of a few thousand volts generated in the secondary that will quite easily blow a hole in the insulation between the windings.