You've probably seen a few of my posts trying to figure where my static/feedback sound is coming from. I've got a LP Traditional Zebra with a new Hughes and Kettner Tubemeister head. Today I went out and purchased a ISP Decimator pedal, the guy at GC tried to talk to me into the ISP Decimator G String but it was double the price. This is my only pedal. I started by hooking it straight into the amp. This made the horrible sound stop until i start playing. Then the static and hiss returns. I've tried switching cord and guitars and nothing changes. I tried hooking the Decimator into the effects loop and it made the sound really bad. It had the sound when I was playing or when it was just sitting. So now I am at my wits end. Do I take the pedal back and try the G String version? Is something wrong with my amp?Does GC's return policy go from the date of purchase or when you receive it when you order it online?

Any other suggestions?
here is a visual guide if it helps.

the thing I would do in addition is spray some contact on the tube pins while you are at.