Hartke Bass Combo Grounding Issue

[StradivariusBone]

I run sound for my church part time and also play bass with them on occasion. We did a restaurant gig a week or two ago and when plugging everything in I felt something that I was pretty certain was AC current coming from the rear of the Hartke 2500 head as I plugged in a tuner on the effect loop. It went away after about 10 seconds, but a bit later I felt it again on my strings. At the time I wasn't sure it it was current because I couldn't replicate it, so at the time I attributed it to nerve pain or something.

Sunday I'm running sound in our church, same amp is patched into the system, but is turned off. They're doing an acoustic set and the amp is hooked up to the direct out in the back and nothing is patched into it. On the board, the bass amp is showing signal and checking it through PFL I can hear a clear signal of our vocal monitor mix.

Again, I stress that the amp was not turned on. Nothing is plugged into the amp except the DI line to the board and the power to the wall. The signal goes away when I unplug the DI line or when I unplug the power cord to the amp. I eventually discover that the brass plate surround the receptacle where the monitors plug into on stage is touching the side of the L shaped TS plug. When I stopped it from grounding out, the signal stopped coming from the bass amp.

So my theory is that the monitor feed was shorting to the conduit and entering the building neutral/ground, somehow completing a circuit through the building electric. I'm not sure how it ended up passing through the Hartke, but that's where I'm wondering if it's possible for a grounding problem internally to allow a signal path like that, even when it's off. We have had problems with the tube pre amp that was not resolved by replacing the tube. The amp shares the same building ground as the rest of the system. The more troubling issue is that the current I thought I felt now seems more likely and a bit more dangerous. We've taken the amp out of service for now. Anyone have insight?

 

[FMEng]

Has the ground pin been cut on the AC plug of the amplifier, or was it plugged into a "lifter?" Was it plugged into an extension cord or a plug strip with the same condition? If so, that's a dangerous thing to do. Restore all AC grounds immediately.

AC grounds get lifted to solve audio hums because people don't know how to properly prevent them. In doing so, it creates a safety hazard. Your Hartke amp has an electronically balanced direct out. That offers no isolation, so it can cause hum under many circumstances, hence the need to lift the AC ground. Just because it has an XLR connector doesn't mean it works right. Stop using the direct out on the amp and get yourself a transformer isolated direct box and you'll have no hum issues when the safety ground is fully intact.

Was was happening in the church scenario was large amounts of current from the monitor speaker found a path onto the shield of the audio line plugged into the bass amp. It's not surprising there was some coupling of audio.

With a new power cord on the amp, I would measure for continuity between the AC plug ground and the chassis or the sleeve of the guitar input. If you do not have essentially zero ohms, then the amp has an internal problem. The tingle you felt was leakage current from the AC transformer that would normally get shunted, but it can't through an open ground.

If the amp and its cord was properly grounded through all of this, then I would suspect that there is a problem with the wiring in the venues (or the extension cords). It would be worthwhile to suggest that a licensed electrician inspect the circuits. Churches often have work done by well meaning but inept volunteers.

 

[StradivariusBone]

The ground was intact. I don't pull ground plugs off equipment and I repair or discard cables that have been modified. The amp is run through a surge protector that has a functioning ground. There is a ground lift switch on the amp itself, but as far as I understand that just lifts the cable shield (pin 1) for the line out. I don't recall if it was enabled or not. I don't lift it when I use it, so I'm erring on the side of it being off and having a closed ground.

We don't have hum issues with this amp, the noise issue was a scratchy, static sound when the tube pre amp was turned up. Replacing the tube did nothing, but if the pre amp is off it's fine. I get some hum on this amp using my bass, but it's a single coil J-bass so that is to be expected.

I'll run the test meter on the amp when I get a chance. I'm more towards a problem with the amp and the venue. The floor box where the monitor plugs in is conduit and the conduit is grounded to the same point as the rest of the building electric. So it makes sense to me that a short from the power amp could send amplified signal along the building ground, provided it made a circuit on the neutral. The thing I don't get is how would the bass amp produce that signal from the wall outlet to the direct out while turned off? None of the other amps on the stage were exhibiting that problem.

 

[FMEng]

Conduits can get broken or loose, either accidentally or intentionally. Just because an outlet is wired in conduit doesn't mean it has a low impedance ground. If the outlet didn't have a good ground, then the path for all of that current was through your amp's power cord, and back to the audio console through the audio line shield (pin 1). That much current on the shield is going to sneak into the audio circuits inside the console. For this to happen, the active circuitry in the amp wasn't required at all. It is likely that the other amps did not have a complete circuit from AC ground to direct out pin 1.

 

[derekleffew]

See also this thread: http://www.controlbooth.com/threads/musician-severely-shocked-by-microphone.31756/ .

 

[StradivariusBone]

The reason I'm guessing it to be grounded is that when it was shorted to the conduit, the monitor speakers attached to that run dropped significantly in volume. Aside from that, if it's grounding out and everything is powered down, you'll hear a very faint 60hz hum through those monitors. Now I haven't metered it, but I'm guessing it's attached to something somewhere since it's behaving like that. I see what you're saying though, that the signal could have carried through the chassis itself and then across pin 1. I'd be interested to see if lifting the ground would cause it to stop (not as a solution, but testing a theory). Either way, just so we're all clear the amp is out of service and is going to be serviced before anyone uses it again.

That's scary stuff, Derek, especially considering what I just experienced. I was thinking while it was zapping me that I must be creating a path to ground somewhere, but couldn't figure out how. I guess I'm glad I don't play barefoot!

 

[venuetech]

so be sure to carry your non contact voltage tester in your bass case.

 

[TimmyP1955]

There's a difference in potential between the ground of AC that's feeding the amp and the ground of the AC that's feeding the console.