How much hum is normal from a new tube amp


I just bought a great sounding Lab12 Suara KT120 tube amp. The issue I am having is a hum that is coming from the transformers. I can hear it with no music playing faintly from my listening position 12 ft. away. The question is, should I return it, or is this normal acceptable. 

128x128blue-magoo

@elliottbnewcombjr  Yes I meant to say KT150, that's what I have.

I have rebiased all the tubes.The hum is present with the preamp off. I used the DB meter from my phone and tracked the loudest reading and it is coming from the back of the amp. I have also changed the power cable, and I just tried plugging it into a different socket. I checked the speaker cables. I have used all this equipment setup with different amps and they are dead silent. 

@blue-magoo You could try a DC blocker. Ensure it's on a flat surface. Any loose bolts on the power transformer?

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