How much hum is normal from a new tube amp

Any loose bolts on the power transformer?

That's the low hanging fruit.  If you press on the transformer, does the hum stop?

Is hum from the transformer or from the speakers?  Speaker hum is generally from AC heaters and low level is often considered acceptable.

Hum from transformers is a different story.  power transformer, not output transformer.  There are many threads chasing transfoermer hum.  many chase  "dc offset" but I find that highly unlikely--maybe I've just been lucky.

It is generally caused by either poor design or an unbalanced load.  Some designers put fuses in the outpot of the voltage rectifier tube.  there are 2 legs to this tube and if one leg blows a fuse the amp will still play fine but the transformer will hum.  I've found this once in an amp I owned and helped others find it twice.

Jerry

@blue-magoo 

None!  

In time you might get a hum from a transformer, but you should never get a hum from the amplifier itself.

Tube amps should have zero hum. Return it.

Know that I think of it I had a hum that was intermitant and it turned out to be a loose wire on a Cap. BE CAREFUL investigating that one.