Alembic Pre Clone Nasty Occasional Ground Shock...

Precisionb87 · #1 07-26-2011, 08:34 PM

I built a Alembic FX-1 preamp clone which has sounded and worked great for several years now. But whenever the ground via power cord is lifted (against my will, believe me...) a potential is created between my strings and the PA vocal mic ground.
One time I measured it with my DMM and got a reading of 70vac!

Needless to say, I and sick of getting shocked on the lips, or anywhere for that matter. What the heck am I doing wrong here? Here is the schematic I followed (with the exception of the 12ax7 pinout correction) Any thoughts or quick fixes? Thanks.
Schematic:
Google Images

SGD Lutherie · #2 07-27-2011, 02:36 PM

This can happen with amy amp and PA when they have the wrong polarity or one is missing a ground.

I also ready about 70 volts once between my amp and a mic. I started carrying a neon bulb output tester. I would touch it between the strings and the mic, and if it lit up, I needed to address the ground situation.

They make isolation boxes that go between your bass and the amp to stop shocks. They use an isolation transformer.

wideload · #3 07-27-2011, 02:42 PM

I understand you have a problem that needs to be addressed, and I can't help with that other than to say "ouch!!!". BUT... wouldn't a wireless from your bass to your preamp eliminate the "ouch" until you find the problem? It also would be an extra buffer in case another problem outside your control pops up.

SGD Lutherie · #4 07-27-2011, 02:43 PM

A wireless would work, if you like the tone of a wireless.

wideload · #5 07-27-2011, 02:49 PM

Right you are. I just so dislike being shocked that wireless is an automatic outdoors or if I perceive dicey electricity!

Precisionb87 · #6 07-27-2011, 06:45 PM

So what you guys are telling me is with any vintage style amp, (signal ground directly tied to chassis ground) whenever the ground pin is lifted, a dangerous potential is created between guitar ground and PA ground?

SGD Lutherie · #7 07-27-2011, 06:54 PM

Quote:

Originally Posted by Precisionb87

So what you guys are telling me is with any vintage style amp, (signal ground directly tied to chassis ground) whenever the ground pin is lifted, a dangerous potential is created between guitar ground and PA ground?

Yes. Even with not so vintage style amps. And usually it's the PAs fault.

Don't lift the ground pin. If you only have a 2 pin receptacle, use an adaptor and screw the ground lug or wire to the outlet box or some other metal like a conduit or water pipe. Not that you will find water pipes on stage.

If you can get a grounded AC powered effects pedal in between your amp and the bass that would also help.

Bassamatic · #8 07-27-2011, 08:20 PM

What is happening is that somewhere in the wiring of the venue, the PA amp and your bass amp have their hot and common switched. The ground is for safety, and in general should not be lifted, unless you know that all the AC is OK and you have an audio ground loop.

Get an AC outlet polarity tester for about $10 and check all the outlets. You should do that as a matter of course in any venue.

Use a foam windscreen on the mic for insurance.

Be careful!

sturoc · #9 11-13-2012, 09:59 AM

Have to say am going thru this scenario right now with a vintage bass head and and narrowed it down to all the P.A. and recording gear in the room we rehearse in.
The power situ on that side of things looks like a rats nest so I got my work cut out for me there. btw: I did not set this system up (i know better than to use a myriad of powerstrips etc).
BUT one thing is for sure that the remedies given such as use a Wind /Pop screen for the mic or go wireless do not solve the problem , they only jury rig it. The potential still exists.

Be careful and do the safe, right thing, eliminate the fault properly.
Cause it's yours or a bandmate's life that could be at risk.