Ground loop?
washburned said:
Hi folks. I've had an ongoing problem with my Koch Studiotone since I got it about a year ago. Whenever I try to use the outlet on the back which is supposed to go to the "clean" input of another guitar amp, I get a huge hum. I've tried it into the normal channel of a Crate V30, the Low input (and effects return) of my OR80 and the clean channel of a Fender SS amp with every cable in my bag, including some new "George L"s. Any Koch users out there experience similar problems? I contacted the distributer and they sent me a new pre amp tube, but the hum didn't go away. Other than this, the Studiotone is an incredible amp.
Any suggestions?
Dunno why they thought a new preamp tube would help. If the tube were faulty I would think you'd have the hum without plugging into another amp!
I can think of two possible problems. One is that you're getting a ground loop, where both amps are grounded through their power cords but also "looped" by the grounded shield on the connecting signal cord connecting them.
When 3-wire wiring first came on the scene we saw "adapters" being sold that would accept a 3-wire appliance cord and then plug into an old 2-wire wall receptacle. The adapter had a flying lead with a "fork" terminal that you were supposed to connect to the screw that held the plastic plate that covered the receptacle. The thinking was that even though there was a 2-wire receptacle there was a ground wire in the house wiring that grounded all the metal receptacle boxes. Putting the lead on the screw would connect the 3-wire ground prong to ground.
This at least let you use the new appliance in your old home. Dunno about the ground to the screw, though. Most times if your wiring was that old there wasn't any ground wire in the house wiring anyway.
Anyhow, if you're ever digging through your granddad's garage or basement and you find one of these adapters you should grab it! It is a great way to test these ground loop situations. You just don't use the flying lead and thus have only the 2-wire connection. This will break any such ground loop and stop the hum.
Another trick is to take one of those cheap plastic extension cords that are only "2-wire". They usually offer 3 "plugins" at one end, two on one side and one on the other. You can often plug a 3-wire cord into the top 2-wire receptacle with the ground prong sliding free across the top of the cord's head. Sometimes there's a bump in the plastic/rubber that prevents this but an Xacto blade fixes that really quickly!
Number two is that the signal out circuit in your amp is either bunned or not outputting a compatible type of signal for the way you think it should be connected. If it is a "line out" that is designed to feed a power amp without any preamp circuitry in the way then feeding it into a guitar jack is likely to be a problem. Line signal voltages are much lower in impedance and far stronger than a guitar signal.
Is this a new amp with warranty? I've never worked on one of these so I don't know any specifics about it. Surely they have a factory techie somewhere who can advise you. You may have to get past the sales guys at the store that sold it. Sometimes they think they know more than they actually do and give you a quick but wrong answer instead of actually contacting the factory techical department.
If I had a schematic I could tell you something more useful but this is the best I can do...
