Guitar amp ground lift myth?

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I have a stand alone tube guitar preamp connected to guitar amp which is in another room. This creates ground loop and hum. How about ground lift on the preamp to eliminate the hum by just using the lead for ground. By chance I came across article on some website claiming one can get electrocuted this way which I believe is impossible. What are your thoughts on this? Excerpt from the article: "Do not do this. If you're using a ground-lift adapter on one amp that is connected to the other grounded amp through cable, when you touch both guitar plugs at the same time, you will get a jolt of electricity through your body."
 
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6L6

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You've got the answer already - the devices are grounded through the instrument lead.

So if there's a fault, those currents will be attempting to find ground through the whole circuit, including the guitar itself. Painful. Also, there is a chance, depending on the various devices being used that the whole circuit becomes elevated since it has no ground reference, and if you touch anything on ground, it's going to cause a zap.

The worst case is if you use a cheater and have the live and neutral reversed - that will usually make for a painful experience when you touch anything else.

So is your question a myth? Absolutely not -- you've just been lucky so far.
 
I have a stand alone tube guitar preamp connected to guitar amp which is in another room. <snip>

I don't think anyone here will sanction using a ground lifter as a permanent solution. For testing OK. The protective ground is to be certain no lethal Voltages (and currents) are on the ground. I don't know if it's still done but it was common on tube gear with a two wire cord (non polarized plug) to use use a 1nF cap from one terminal to the chassis. The instructions were to reverse the plug to reduce hum. That connection to the power line is a big part of the problem and while it didn't kill me, it DID get my attention. Look into isolation transformers for either the power line and / or the signal cable. That will break the ground loop without being a safety hazard.

 
I'm with Bill and Tesla. If you've got hum problems, use an isolation transformer in the line level lead between the pre-amp and amp. Preferably a pair (one at either end with a balanced, 600ohm line between). Jaycar and Altronics both sell these (10k:10k and 10K:600CT) for not much money.

If that doesn't fix your problem you need an electrician. One with a brain. As your building wiring is broken (probably in a weird way) or your amp/preamp is faulty.
 
That one was fake, by own admission.

Try this one for real :(

YouTube

the electrocuted Guitar player suffered MASSIVE heart damage, now a good part of it is non functional scar tissue, can´t play any strenuous sport, his life is ruined.

he´s alive only because his Father in Law who was on the first row had recently finished a CPR course and assisted him immediately, plus by sheer chance there was an ambulance (and paramedics) in the premises so they were by him within 4 or 5 minutes.
 
On the contrary, the occasional 'bite' is good lol.
Please IGNORE THIS POST. It is wrong, and dangerous, and could cost you your life. Hopefully the moderators will step in and delete it.



How would we feel about a gun enthusiast who has repeatedly shot himself by accident, and who goes online to say "the occasional bullet wound is good"? :mad:



Anyone who gets shocked repeatedly while working with electricity is doing it wrong. Please quit immediately, and take up knitting, or bowling, or something else that won't kill you because you lack the discipline to follow good safety practices.


-Gnobuddy
 

PRR

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....one can get electrocuted this way which I believe is impossible.....

The "impossible" happens regularly.

If I understand your setup, if there is heavy leakage, AND you unplug that cable with one hand on plug and one hand on amp-metal, you have line-voltage through your heart.

So: not while you are using, but when you break-down. Yes, you could plan to pull the power plugs before that wire, but it's late, your lover is clingy, your head is full of beer....
 
Connect the safety ground (green wire) to both chassis. Ground the preamp circuit ground to its chassis. In the amp, AC ground the circuit to the chassis with a film cap and add a CL-60 thermistor in parallel to the cap. The resistance of the thermistor should take care of the ground loop and still allow a fault to trip the circuit breaker though the safety ground. That' assuming the amp doesn't use the chassis as a circuit ground. If it does, changing to a "star" grounding scheme would be a hassle; but, the amp would most probably be quieter.
 
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So if there's a fault, those currents will be attempting to find ground through the whole circuit, including the guitar itself.
I can't agree with that. If there is a sudden current leak to chassis, it is immediate. The current is not taking time looking for anything. It just jumps to alternate ground which is in the guitar cable. There can't be any argument that the cable is too long to be effective because power cables are long too.

I think the only way to get electrocuted is to lift ground. That is, isolating the amp from ground completely. Would be good if an actually engineer chimed in on this one. In any case, my preamp is always disconnected from mains when not in use and guitar goes through in bypass mode.
 
"The current is not taking time looking for anything."

The amp chassis is directly connected to the safety ground. Any leak to chassis is grounded directly. You might mean a fault to the circuit ground. In that case 60R (initially, it should drop fast) to the safety ground is not that high of an impedance. At 300V B+ 60R would pass 5A if the PS could supply it. Which it can't (unless you have some godzilla super amp). It will pull the voltage down and probably blow the amp's fuse. Your guitar I assume is plugged into the preamp whose circuit ground is connected directly to the safety ground. The safety ground is a low impedance ground at DC and 60Hz AC, or should be. So how are you going to get shocked?
 
Mr. leadbelly
If you read my original post above, the suggestion was that both the preamp and the amp chassis were to be connected to the safety ground. And that the preamp circuit ground be tied to the chassis ground. That was the assumption. I was only trying to suggest a way to defeat ground loops without removing the safety ground connections to the chassis. If someone wants to muck around with equipment without safety grounds that's their business. It's a free country. They can roll the dice if they wish.
 
6V6, you need to run AC extension lead from amp power source to supply the preamp, ie both items are on the same power circuit. This is routinely done in live sound and eliminates such earth noise problems. Disconnecting the safety earth at the preamp is actually illegal and as we know inherently hazardous. My joking about electric shocks is that such shocks are a reminder that safety earth conventions are for good reason and must be followed.

Dan.
 
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