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Guitar amp ground lift myth?
Guitar amp ground lift myth?
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Old 20th September 2019, 04:51 PM   #11
voltwide is offline voltwide  Ireland
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I urge you to repeat this not too often
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Old 20th September 2019, 05:05 PM   #12
Max Headroom is offline Max Headroom  Australia
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On the contrary, the occasional 'bite' is good lol.
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Old 20th September 2019, 07:36 PM   #13
Gnobuddy is offline Gnobuddy  Canada
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Originally Posted by Max Headroom View Post
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"?



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.


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Old 21st September 2019, 02:50 AM   #14
PRR is online now PRR  United States
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Originally Posted by 6V6dude View Post
....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....
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Old 21st September 2019, 03:05 AM   #15
john_tracy is offline john_tracy  United States
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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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Last edited by john_tracy; 21st September 2019 at 03:08 AM.
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Old 21st September 2019, 03:33 AM   #16
6V6dude is offline 6V6dude  Australia
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Originally Posted by 6L6 View Post
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.
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Old 21st September 2019, 04:20 AM   #17
john_tracy is offline john_tracy  United States
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"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?
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Old 21st September 2019, 05:02 AM   #18
leadbelly is offline leadbelly  Canada
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Quote:
Originally Posted by john_tracy View Post
"It is better to remain silent and thought a fool, then speak and remove all doubt." A. Lincoln
Quote:
Originally Posted by john_tracy View Post
Your guitar I assume is plugged into the preamp whose circuit ground is connected directly to the safety ground.
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Old 21st September 2019, 06:17 AM   #19
john_tracy is offline john_tracy  United States
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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.
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Old 21st September 2019, 08:11 AM   #20
Max Headroom is offline Max Headroom  Australia
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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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