all tubes lit no sound

My st 120 has all tubes lit but no sound . I am taking it in wednesday and am wanting to know if it could be something very expensive or could it also be something not too costly ?

I would check your B+, see if there is a problem with that.
The filament supply is separate from your B+, so the tubes would be lit even if the B+ is not present.
I am pretty sure I'm right with this, perhaps other members can chime in here as well.

maybe just one missing wire somewhere!

is this a new build ST120?, did you build it if it is?
As tubes4hifi said, check the wiring, but like my first reply, if you have a meter, check your B+

Got some spare driver tubes you can plug in? No signal, no sound ... try swapping the center and one of the side tubes and see if you can get one channel to work. That happens, replace the center.

You could also try re-tensioning the sockets. There's three tabs for each pin that can loosen up and go intermittent. Those can be pushed in a bit - carefully - a jeweler's screwdriver works nice for that.

As always, power down when playing with tubes, and let the amp "rest" a couple minutes before powering it back up so the caps can discharge properly. Fast cycling the power can cause all sorts of grief.

Just from personal experience, check/re-solder your bias adjustment pots on the driver board. I had a lame solder connection on one of the three pins and no sound came out. (except some expletives from me).

Get yourself a spare 5AR4. If you have a Guitar Center nearby you should be able to pick up a Sovtek 5AR4/GZ34 for less than $15 there.

Hi It turned out to be a problem with the soft start. I have had this amp for 16 months and only used it for about a week and wow it sounds great my friend said it didn't sound like a tube amp so much as it sounded like a band playing right over there .! Nice Amp Bob