Turn on amp w/o speakers

it is harmful to most tube amps but not harmful to most solid state amps - is my minimal response

i thought it was the other way around to what Philjolet posted

Why would you power it up?

my amp maker tells me to adjust the bias without the speaker leads connected. it is a tube amp

Be very careful with NuForce amps. Powering up without the speakers connected (or some sort of resistive load) will burn up internal components. I know people who have had this happen.

Cayin also advise to bias their A-88T amp by powering the amp up with no load connected. Haven't adjusted bias yet but will trust the manufacturer when I have to.

Phil Jolet has it right, based on those I have asked. That said, it is best to contact the manufacturer and ask them; it is better to be safe than sorry. (I have done this with a few SS amps.)

Tube amps will not be damaged by being powered on with no load. They might be damaged if you apply a music signal to them with no load. Here's why.

Most tube amps have an output transformer. It works because the alternating output tube currents create expanding and contracting magnetic fields in the primary which transfer energy to the secondary. This energy has to go somewhere. Without a load on the secondaey to absorb this energy the contracting fields kick back energy to the output circuit. This kickback can create a very large voltage spike which can arc over inside the output tube or other components.

If you don't play a signal this can't happen so no damage is done. Solid state amps in general don't care. Amps like the NuForce that use switching supplies are an exception and they warn against no load. As suggested, when in doubt it is best to check with the manufacturer.

My ARC manual said to bias without a signal, after a 15 minute warm-up, but nothing about removing the speaker load FWIW.
.