Help! New JP6BFR with huge hum using 2 cables

[Stageguy]

Hey guys,
I recently purchased a new JP6BFR with ebony fretboard and 5-way switch for the humbuckers (not the 3-way on the schematics), which makes me think it is a 2011 model that sat on the shelf a while. I am trying to use the pickups with 2 cables and the instant I plug in the second cable the amp with the cable out of the magnetics output starts humming loud. So here is an explanation of exactly what I have and use and what I want to do so maybe someone can help with the issue. I have a feeling it is user error, not an equipment problem.

I have replaced the battery and verified that both cables are mono cables so that should not be an issue. I have switched out cables to see if maybe one was bad and that was not the issue.

Magnetics output: Mono cable direct to Marshal YJM100 using Marshall 4x12 late model cab
Piezo output: Mono cable direct to Egnater Renegade using old 1981 Marshall tweed 4x12 cab

I dont think the cabs should matter but you never know... I do have a pedal board but bypassed it for test purposes.

When using only one of the outputs at a time they both sound great. But when I plug both cables in at the same time the hum is only in the YJM coming out of the magnetics output, not in the Piezo output. Also, when I use only the magnetics output and switch the 3-way selector to Piezo there is no sound, so that should be correct since I am using a mono cable.

Any help would be appreciated. I have been playing for years but this is my first Ernie Ball... and I have to say I am loving this guitar, but obviously need help with the complex electronics.

Thanks,

 

[beej]

Sounds like you have a ground loop. You're plugged into two amps at once, and your signal has two paths to ground. The small voltage difference between them is causing the constant hum you hear.

The solution is to isolate the ground from one of the signals. There are a number of things you can use- DI box, buffer, etc. So long as they have a transformer isolated out.

 

[Stageguy]

Thanks Beej, I have a friend that does audio installs coming over to take a look at it as well. It definitely sounds like a ground loop, but louder.

Would a stereo cable to a spllitter solve the problem?
How about a stereo Y-cable out of the guitar to 2 mono cables?

Or is the transformer the only thing that is going to fix it?

 

[beej]

Unfortunately you need the transformer isolation to break the path to ground to one of the amps.

The Y cables are going to do the same thing as the guitar currently does- allowing the ground to be shared by two amps.

 

[auxren]

Or just ground lift one of the amps

 

[beej]

Hopefully not on the amp's power cable

 

[DrKev]

Just to push the point home, please DO NOT lift the ground on your amplifier. The cause is clear, deal with it correctly and don't do the half-assed job that might one day put your life at risk.

 

[Stageguy]

Thanks everyone, we got it figured out. And in the end I think it sounds incredible and probably better just using the mixture of the pickups with a single cable and a single amp (the YJM). The tonalily range with the guitar is like nothing I have ever worked with before and I love it. And I can tell you the YJM does its part of the job very well too, if you ever get a chance to mess around with one I think you will find that it is an amazing head.