I have a Marshall valve amp and for some reason whenever the FX loop is on, there is this loud hum. The kind of hum you get when you put your guitar really close to the head, but it's much louder. It hums on every channel when the FX loop is on, but doesn't hum with the FX loop off. My first thought was that it was a dodgy lead so I switched my leads around and it only made the hum slightly quieter. The strange thing is, when used the leads that were originally used for the FX loop, to go straight from the guitar to amp (and the FX loop turned off) there was no hum. If it was the lead, shouldn't there have been a hum whenever I used it?

Anyone have any ideas as to what could be the problem? It's never done this before. My thoughts is that most of my leads are dodgy, but I would like to hear your advice!

Thanks in advance
Which Marshall valve amp?

Epiphone Les Paul Standard w/ SD Alnico Pro II's
Fender Aerodyne Telecaster & Stratocaster
Marshall JCM 800 4104 combo

E-Married to Funny_Page
It's a TSL. I'm only running a Digitech delay pedal, it's about $200 AUD. It's only a simply, relatively cheap pedal but it does the trick. I also noticed a swirling type of sound when it's on and I'm on the distorted channels. I think the hum is my leads and the swirl is the pedal. I've never heard anything like it before so I could be very wrong!
I suppose it could also be my power supply, but I've never heard of a power supply giving too many problems. Although, I am no guitar tech
ah why didn't i think of that! I haven't, but I will. I've tried running just a chorus pedal thru the FX loop and the hum was still there, but a slight bit quieter. It could have been some sort of interference from the lights or something so I'll try playing my amp in a different room too.
Try to use quality chords for the effects loop, not cheap ones. Since the send/return chords are on the back, make sure they aren't too close to the power cord. If you need to cross them always use 90 degrees, and never run them parallel to power cords, as that can cause an AC hum.
I'll try that, thanks. I think it is my leads, they're only cheap ones. I'll have to buy good ones some time soon then