Vox VT30 Hum

My Vox VT30 has suddenly started humming and not in a good way.
I have been reading about others having this issue but have not found the fix yet.
Here's my scenario:
1. Amp has a hum like a 60hz ground loop after turning it on and with the volume control in the back of the amp all the way off.
2. Lifted the ground and no change.
3. Tried different AC outlets and no change.
4. Reset the amp holding Channel 1 and 4 buttons down, turning it on and then pressing Bypass.  This resets the digital brain and there was no change.
5. Sent an email to Sweetwater Support to see if they know anything about this.
6.  Ordered a new tube today which I will get right after Christmas.
 
When I first got the amp, it was really quiet.  I have barely used it in this last year.  It cost $200 and I don't think I want to spend $100 getting it fixed so I posted this thread so see if anyone has any clue as to what might be going on.
I would love to get this amp working again.
TIA
Rimshot

Yes but I am out of warranty now...

Hi John.  I will check that out.  Thanks much for the tip.
In your experience, could this issue be related to a bad tube?  I am concerned that there is hum even when the master volume control is turned all the way off.  
 
Thanks.
 

ampfixer
It depends where the master volume is located in the circuit. I thought it was a Solid State amp.

I've had little luck finding a schematic, but it looks like it has a solid state front end and tube output.
 
Rimshot, if you don't know what you're doing, I wouldn't go sticking my hands into your amp, on or off.  There are voltages around 400v, sometimes stored in large capacitors that aren't automatically leaked when you turn off the system.
 
Changing a tube shouldn't be a problem if you can get at it without touching anything else.  Not sure you'd be getting any sound if there was a problem with an output tube, which I think is all you have.  How many tubes to you see?  Just two big ones, two big ones and a small one, or some other combination ?
 
 

I see just one tube.  It is a 
Electro-Harmonix 12AX7EH Preamp TubeThe volume control is located on the back of the amp.  
Here is some help on the subject:
http://www.valvetronix.net/forums/vt30-problem-loud-hum-t4484.html
 

 
http://www.voxshowroom.com/uk/amp/vt30_hood.html

 
There are several flow charts showing the Vox Valve Reactor™ topology but I haven't found one specific to the VT30.
 
In the VT30 the "output transformer" has been replaced with Coupling Capacitor Technology™.
 
 

 
So much for high voltages and large power supply caps.
 
Here's something that may apply :

 
From :  http://www.seymourduncan.com/forum/showthread.php?201710-Vox-VT30-LED-under-the-tube/page2

My money is on a bad capacitor in the power amp. Likely a 2 cent component. Have a really close look at the electrolytic caps. The little cans that are soldered onto the board. One is likely bad or has a bad connection to the board. 
 
Merry Christmas

ampfixer
My money is on a bad capacitor in the power amp. Likely a 2 cent component. Have a really close look at the electrolytic caps. The little cans that are soldered onto the board. One is likely bad or has a bad connection to the board. 
 
Merry Christmas

Will check that.  Thanks.

Rimshot
Will check that.  Thanks.

Unplug the amp first
 

Rimshot

fireberd
Is that 12AX7 running low plate voltage ("starved plate") operation?

Hi fireberd, I am sorry but I don't know the answer to your question.  Where can I find out?

I'll offer a creative way to do this safely for a non techie person:
 
Look at the largest capacitors  (the blue canisters with a white stripe) and read their voltage rating.
 
The 12ax7 is nominally run at 200vDC. A cap on the "B+" voltage supply will be rated just above what this amps run the tube at. It could be a 250v cap on a 200vDC supply or it could be just about anything lower now that using tubes, in solid state amps, as a bullet list feature has become so routine.