Thanks for your response.
Sorry about the confusion with the forum search field. I did search and did find others with similar cases only I couldn't find the solution.
Going back to the facts...
I have had it working before with the same amp and most of the gear. Only difference is I have removed an effects processor to add a pedal shelf instead... However, right now just for the sake of fixing the problem, I have torned it all down to do a really basic setup. I even have the GCX off the rack to make sure I'm not getting any induced groundloop.
Everything is connected to a furman power conditioner, could that generate the loop?
The three jacks are definitely connected to the amps "channel 1, channel 2, channel 3. For the moment I'm doing a very simple setup to find the problem. This is the signal chain.
Guitar to (loop 1 - IN)
Tube Screamer set up on loop 1
(loop 1 - OUT) to the amp input in the front.
So far everything seems normal, I can toggle loop 1 on and off and all sounds good.
As soon as I connect a cable from one of the loops OUT/NO to one of the channel jacks (not the speaker outputs) I get this huge hum. And yes... the burn smell is not good.. I wonder where the power would come form?
The amp has recently been checked and re valved so I'd be surprised to find out it had any problem...
On the back of the amp, there is a knob to select channel, on what setting should this knob be? 1 2 3 or FW (footswitch)?
I have it setup on FW at the moment.
I'm gonna do some more testing tonight and write down the results.
Hopefully you guys know what it could be.
There are two things about your situation that concern me:
1. You've successfully done this before with a "...slightly different set up..." and,
2. Not only are you now getting "...a huge HUM..." you say you also thought you smelled something burning.
To me this is an indication that you may have crossed a connection somewhere, perhaps inadvertently connecting to a speaker jack on the Mesa instead of a switching jack. I know that sounds highly unlikely, but crossing connections is something I've done more than once over the years, so I know it is a distinct possibility. Even though you've probably already done this, I strongly suggest you review your connections again.
I also found 34 threads referencing the Mesa Triple Rectifier when I used the Search feature, and many of them, if not most, are about channel switching issues. To save you some time, though, here's a link to a Mesa Triple Rec thread that might help you out. If not, try searching again and check out some of the other threads.