Alembic Pre Clone Nasty Occasional Ground Shock...

[Precisionb87]

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

[staindbass]

there must be a ground wire going from the guitar electronics to the bridge/strings . thats making a direct connection. ive gotten some good 120v shocks on the lips ! i removed the ground to the bridge, problem solved. i have an instruction sheet from emg pickups and they recommend the same thing. no connection=no shock.

[billfitzmaurice]

Originally Posted by Precisionb87

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!

Don't lift the ground. When there are ground loop issues causing noise you always lift the signal ground. You never lift the power ground. People have died from doing that.

I hope you do not have a 2 wire AC cord as shown in the schematic. Even in 1999 that was not code compliant.

[beans-on-toast]

I assume that you have this already, but ....

Here is a drawing showing how the wiring should be on the primary of your power transformer. The green wire should be longer than the black and white so that it is the last to break of the power cord is pulled out. The green wire should be attached to chassis with a ring terminal and at a location where there are no other grounds connected.

If you wire your pre-amp like this, and if the PA has a three-conductor power cord, and if the house wiring is up to spec ( ), you shouldn't be getting any shocks.

If the PA has a power polarity switch (sometimes two or three position), changing the position can help.


Power input wiring:

[Precisionb87]

Thanks for the incite so far. Yes, I am using a 3-prong power cord, the ground is tied to the chassis.
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?
@billfitzmaurice:
I would never lift the chassis ground simply for a noise issue. I would never lift the chassis ground for any reason, but in some cases I have no choice. A particular club I play simply has a poorly grounded power strip on the stage, so there is really nothing else I can do.

[beans-on-toast]

I like to use an outlet tester before plugging in any equipment. Well worth the few bucks that it costs.

By testing each outlet, you can ensure that the wiring is correct and that all outlets are wired the same. If there is a problem with the wiring on one of the outlets, it could result in a shock if you touch another piece of equipment.

If you lift the chassis power ground, you may or may not get a shock if you touch another piece of equipment. It isn't a good idea to lift the power ground.

The purpose of the chassis power ground is to provide a path back to the circuit breaker panel. If a ground fault occurs, the circuit breaker at the panel will trip. A ground fault would occur if the line voltage (120 VAC) shorts onto the chassis. Without this, if the line voltage is on the chassis, and you are touching your stings which are normally connected to your amp's ground, and you touch another piece of equipment that is grounded, that voltage could pass through you. Not a good thing.

[Jerrold Tiers]

Originally Posted by Precisionb87

Thanks for the incite so far. Yes, I am using a 3-prong power cord, the ground is tied to the chassis.
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?

There CAN be a voltage, so there WILL be a voltage.... how much is variable, and in any case unless there is a defect, it will be a low current that is defined as "not hazardous"..... but nobody said that it would not give you a good jolt.

Any new amp has to pass a "touch current" test, which is basically an open ground test. It ensures you won't get fried (without a real "fault" or "failure"), but does not ensure you won't feel anything.

Old amps may not be as nice that way..... and they might give you a thrill if not grounded. I prefer grounding.

The only reason you would want to lift ground is for something like a PA connection, and for that you can use a direct box, which will have isolation as an option.

[T-Bird]

Hi.

Originally Posted by Precisionb87

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!

What the heck am I doing wrong here?

You're allowing it to continue, that's what you're doing wrong.

There are El. safety regulations in effect where You live?

Originally Posted by Precisionb87

I would never lift the chassis ground simply for a noise issue. I would never lift the chassis ground for any reason, but in some cases I have no choice. A particular club I play simply has a poorly grounded power strip on the stage, so there is really nothing else I can do.

Yes, You have a choice and yes, there is plenty You can do.

It's Your health you're risking, not to mention anyone elses who has to work on that stage.



An isolation mains transformer is an option that will keep You safe(r), but won't adress the problem.

Forcing the said location owner to redo the wiring is the best, but probably most unlikely solution.

For starters, start using a foam wind cover over the mic, that helps eliminating the shocks.

A "lifesaver" mod to the bridges ground wire will prevent You from dying as well.


Lifting the mains ground is like tossing a few tacks onto Your livingroom floor at night and walking barefoot around. Chances are that You'll miss stepping into one, but there's also a possibility that you won't miss.


Regards
Sam