Please don't listen to Bobbe on this one (just this one!)

[Mark van Allen]

I love reading the Tech Tips newsletters my pal Bobbe Seymour sends out, wonderful stories and information. I have to take exception to something in the one I got today, referring to two-amp setups:

One more important thing here. You must have a ground lift on the end of the amplifier AC power cords so that neither amp is actually grounded. This can be done by cutting off the third prong on the AC plug or by putting on a two prong adapter with the little green wire that sticks out on the side.

This is the way 95 percent of folks should be doing their two amp setup.

Please, please don't do this! There are several ways a malfunctioning amp or electrical system (and nightclubs are notorious for that!) can deliver a fatal shock when ungrounded. There's a good reason EVERY amp manual includes a warning to NEVER remove the grounding plug from an amp cord.

There are several electronic alternatives including connecting the amps with a (guitar) cord that is only grounded at one end.

I've been very badly shocked by improperly grounded gear. And I'm lucky.

[chris ivey]

then what would the proper way be...and how are they patched together? i know lots of pickers use two amps. i always get hum if i don't lift one amp ground?

[John Ummel]

Not quite sure what is the exact scenario Bobbe is referring to where using two amps has a hum problem. I used two amps driven out of a stereo chorus for a long time and no problem with hum. A couple of things though, when you remove that ground, what you have left is "Hot" (the black wire) & Nuetral" (the white wire) if you swapped those on ONE of the amps you could produce a dangerous condition.(and removing the ground terminal on your AC plug could allow to turn it over and plug it in backwards) Also some amps use the chassis for both signal ground AND AC ground, others (like the custom made amp I use)isolate signal ground from AC ground. As a professional electronic technician I've seen there are times when a ground must be lifted in order to prevent a ground LOOP from being created, but again, two amps both sharing a safety ground...I don't see why that's a problem. (There I've Really muddied the issue!)Mark's point is well taken..BE SAFE!

[Mike Mantey]

Also regarding the newsletter, Mullen does make threaded nylon tuning nuts. We go the extra mile so you don't have to (they thread right on). They are designed not to wear out any quicker also.

Anyway back to the subject, be Safe!!

[Ray Minich]

Ground lifting is one of them unapproved, last minute, NEC violating, get yourself in liability trouble, taking a chance, hope it ain't wet, don't touch that, risky things that we don't talk about.

If ya get caught, it ain't gonna be pretty.

It's a lot like fishing with dynamite.

[Bobbe Seymour]

Lift the ground one one amp, not both, was what was meant to be printed, the proof reader said "both".
You HAVE TO lift the ground on one amp or the humm will kill you!

Bobbe

[Bobbe Seymour]

John Ummel is correct, as most of us already know. However John, it's hard to get these reversed when one prong is bigger than the other, thank goodness.

Chris Ivey, yes, use a standard ground lift to prevernt hum. Remember guys, electricity was around 75 years before the third prong (ground)was even invented. Amazing the whole world wasn't killed, huh?
Bobbe

[Bobbe Seymour]

Mike Mantley, I was just talking about major guitar companys!


Wake up Mike and leave me alone, where are my guitars? Shipped them yet?

Saw Russ and Herby last weekend, they said they will get a payment to you right after the NewYears gig!

Happy Thanks Giving to you and the family.
Bobbe

[Ray Minich]

My common mode has been rejected. Instrumentation amplifiers for everybody! My normal mode rejection ain't much better either.

In today's litigous society 'tis dangerous to recommend certain things that we might otherwise do without tellin' anybody.

I have some asbestos rope... great for sealing woodstove doors.

[Pat Comeau]

I've had 110 and 120 volt shocks many time and it does wake you up abit but not a life treat, my expirience with hum is that sometimes it's the wall plug that is reverse and to get rid of the hum you have to break the ground of the plug so you can reverse the connection but like Bobbe said some plugs now days have one side bigger then the other one and only fits one way in the wall plug, when i bought my first laptop i had a big hum problem with my sound coming out of my laptop when it was connected into my main system so i tried everything from changing driver for my soundcard and even format the ol' thing but the hum was still there so i called the company but couldn't get any solution and i was on the point to return the laptop for another one but i was still puzzeled cause when the laptop was running only on the batterie it worked fine but as soon as i would plug the adapter the buzz and hum would come back so from there i thought it was the adapter that was the problem so i called a technician and he said it was my adapter also so i went to the store where i bought the laptop and ask for another adapter and a guy working there told me that there was another guy who had the same problem and that he tried several different brand of laptop and they were all doing the same thing they all had a hum when using the output in a sound system with the adapter pluged in, so i said to myself there's no use trying another laptop and i was at the point of giving up and going back with a desktop computer so i came back home and tried one last thing ... i had a 2 prong adapter with the green wire sticking out like Bobbe said... i pluged my laptop adapter in it and pluged it in the wall plug and "VOILA" the buzz and hum was gone and i was a happy camper.

and also i never had any problems with reversing a plug. the current is the same on both, but i wouldn't go without a ground in wet conditions.

just my 2 cents.

Pat.

[Tracy Sheehan]

Back shortly after electricity was discovered i was getting into ham radio.Back then you had to take a code test and test on electronics.MY teacher was a friend who was an electronics tech for the FAA.After i got my ticket before the FCC he was teaching me how to work on rigs which were all tube type back then.
He taught me about grounding and said more people had been killed by the lowley 120 volt A/C current than any other.
I also liked his humor which has nothing to do with this,he showed me how to ground the caps on the finals and keep them grounded with the rig turned off and unpluged as the caps could and would build up voltage again.Said if i touched one it would not kill me,but i woulden't do it again.He was so right.LOL

[Jonathan Cullifer]

The easiest way to eliminate ground hum is to plug both amps into the same power strip.

I have been shocked at places with faulty wiring and won't play anywhere that doesn't have a functional third prong. I always carry an outlet tester with me.

[Brad Sarno]

The idea here is to eliminate the ground loop thru multiple power cords. Once wired up with the audio cables, technically, and like Bobbe corrected himself to say, you can lift the ground of one amp to eliminate the loop, and then the audio cables create the safety ground path to the ground-lifted amp. Legally and officially this is not recommended, so I'd never officially recommend that, but I'll admit I do it if I need to. But I'll only do it if the ground-lifted amp is connected to the grounded amp via audio cords that provide the ground path. I don't like an ungrounded amp just floating out there in space.

This new Hum-X device is said to really solve this problem and allows for both amps to be grounded. The thing apparently is able to actively cancel the ground loop hum while still being grounded. Sounds like the best solution.

What Jonathan said about being sure to plug both amps into the same strip is a good idea for sure, but may not always eliminate the hum. One of the first things to try is to make sure both power cords are near eachother, or even better, twisted together on the way to the power strip. This reduces the area inside the "loop", thus reducing the hum. Once you understand the nature of the "loop" and know that the ground "loop" shared by the guitar cords and power cords is the antenna that picks up the hum, you can often use common sense to diagnose the issue.

But the most critical rule is that there should ALWAYS be some connection to ground from both amps should anything fail or short or whatever in either amp. That 3rd pin is there for your safety.

And another tip I like to share that has saved lives. If you're a guitar player or steel player with a vocal mic, and you're at soundcheck and want to make sure your vocal mic won't shock you while your lips touch the mic and your hands are on the guitar... DO NOT grab the mic in the palm of your hand. It's possible that if there is a shock to be had, it could make your hand grip the mic even tighter getting you stuck in a real bad place. The trick is to touch the mic with the back of your hand so that if there's a shock, your body automatically recoils from it instantly. This really has saved lives.

Always try to make sure that your guitar rig is powered by the same power circuit that the PA system is, and ALWAYS power your personal rig from a single point of power. I've seen guys who will have a complex rig and they just look for the nearest outlet on stage, one from the next guy, one from the wall behind the drummer, one from the quad box in front, and they get buzz out the yin-yang. Like Jonathan said, always power everything in your rig from the same strip.

Brad

[Bobbe Seymour]

Amen Brad!
This is why you have so much respect in the world of steel amplification. I agree 100%.

And thank you Mark for pointing out the flaw in my news letter, be sure to have at least ONE amp grounded or do what Brad or John Ummel have suggested.
You guys are great!

Bobbster

[basilh]

Whatsoever draws the most current has the least resistive path to earth (ground), so, Earth (Ground) that and let the rest be earthed (Grounded)via the screen..Would be the way I would look at it. Common practise in studios and places where earth (Ground) loops are frowned upon.
It's usually the power amp that has the most efficient earth (Ground)

BTW Where on earth did the terminology "Ground" come from ?

[ajm]

"I've had 110 and 120 volt shocks many time and it does wake you up abit but not a life treat...."

If you really meant to say "threat" and not "treat", and you think that 120v is not life threatening, well, I'm not sure how to answer that one.

Also, in the first post, it says to either use a three to two prong adapter (maybe OK) or to CUT OFF the third prong on the power cord????? An adapter is reversible (just remove it). Cutting off the pin on the cable is semi-permanent. In spite of all of the documentation out there saying to never do this, there are some who still think it's OK. I personally would never think of typing this, let alone doing it.

[Lynn Oliver]

I guess you guys are going to do what you want, regardless. But this is a public forum and I feel obligated to comment.

120V can easily kill you. To think otherwise because you have survived a shock or two is simply wrong. There are a lot of variables that determine the actual path of the current, but this is not something to take lightly.

There are at least two unsafe situations when using guitar amps without proper grounding. One is if there is a short to the chassis, which happens from time to time; the second is if the neutral and hot legs are switched, and even if you have a polarized plug the outlet could be wired incorrectly. In either case you want the lowest impedance path to ground to be something other than your body, and relying on the shield in a guitar cable is risky. Think about how much abuse some guitar cables receive.

If you keep a circuit tester, GFI, and isolation transformer in your toolkit there is never an excuse to use an unsafe setup.

[Lynn Oliver]

Remember guys, electricity was around 75 years before the third prong (ground)was even invented. Amazing the whole world wasn't killed, huh? Bobbe

The point is, some were killed. Too many as far as I'm concerned.

[Dave Mudgett]

I guess I just have to say something. I'm sure Jim Sliff will join in at some point.

There are other dangers besides grounds that are reversed with respect to either the the "true" earth ground or another player or PA reversed from being plugged into a different circuit. Of course, these are potentially dangerous - 120 VAC is potentially highly lethal - and the approaches Brad suggests can be used to reduce this danger somewhat. I always use a circuit tester and check everything I can, always test the PA for reversed grounds, and am extremely gun-shy of electronically-ignorant bandmates and low-grade PA people. But I still see problems that leave me shaking my head occasionally. I took to using a wireless on 6-string for a long time, but that's kind of a drag on steel.

In either case you want the lowest impedance path to ground to be something other than your body, and relying on the shield in a guitar cable is risky. Think about how much abuse some guitar cables receive.

A very good point, IMO.

Yet another fly in the ointment is the possibility - admittedly remote - of some type of electrical fault inside an amp - especially a high-voltage tube amp - putting something like a high B+ voltage onto the ground leg of the circuit. This is admittedly very rare, but it is in principle possible to have a component short or some other type of wiring fault cause this. Then you're not talking about 120VAC, but on something like a Twin Reverb or heaven forbid certain Music Man or Ampeg amps, more like 500 or even 700 VDC. I have only seen this happen once, but it is not impossible. I'm a gonna tell you, this kind of a jolt makes a reversed-ground jolt seem like a walk in the park. I've had a couple of high-voltage jolts in the lab - the worst was a very high-voltage electophoresis experiment. Luckily, I had put a very sensitive current limiter on it, but it still dazed me.

There have been some pretty spectacular rock and roll bad-ground electrocutions - guitarist Les Harvey with Stone the Crows in 1972, Shadows bass player John Rostill in his studio, and probably the most well-known - former Yardbirds lead singer Keith Relf with a regrouping of Renaissance in 1976. There are no doubt many other less well known examples. I've seen people get some pretty bad shocks, and have been jolted a few times myself on gigs due to someone else screwing around with my grounds.

No doubt, I've reversed grounds and lived to tell about it. But it seems to me that with the HumX type of solution, there isn't any need for this anymore.

Here's an article on this: http://www.guitarnuts.com/technical/ele ... /index.php

Just my view.

[Dave Mudgett]

BTW Where on earth did the terminology "Ground" come from ?

This wikipedia article more or less explains it - http://en.wikipedia.org/wiki/Ground_%28 ... rsus_earth

Voltages are always relative to each other, and a circuit ground is simply a (hopefully) constant reference voltage that the other components can be measured with respect to. On the other hand, earth ground is the voltage of anything directly connected electrically to the earth itself. Of course, no voltage is absolute, but the idea is that the earth potential is (hopefully) constant over a local area and that dangerous voltages can be safely dissipated into the ground via a high-capacity link to a grounding rod.

In most structures, the internal wiring is "earthed" by connecting to this grounding rod which is sunk deeply in the earth. Electrical codes specify how this should be done - for example, here: http://standards.ieee.org/nesc/NESCIR536.pdf

I always distinguish between a relative circuit ground and earth ground. The ground wiring - in the amp and throughout the structure - is a high-current capacity link to the earth ground. I always want a high voltage on my guitar strings (which are generally connected to the amp's ground circuit via a "string ground") to have a helluvalot easier time getting to earth ground through the ground circuit instead of through my body.

[John Bechtel]

I have, in the past; had certain brands of effects stomp-boxes that created a hum if plugged into the front of an amp and using an AC-Adaptor. Removing the Adaptor solved that problem, but; ate up batteries, however; by plugging the box w/Adaptor into the rear effects-loop also cancelled the hum! I've never been able to get rid of the hum when plugged into 2-Amps, even w/o any effects boxes! Lifting the 3rd. wire on one Amp. does nothing for me! I always plug into one AC-Strip and use a 2-prong plug on one Amp., just in case it might work some time! I'm using 2 N–112's, but; I've used 2 N–1000's 2 N–400's, and 2 of several other models and they all result in the same type Hum! The only time I didn't have a Hum was when I was using a ss-pre-amp. along with a pair of Peavey Power-Amp. sections. Is there really a solution to my Hum-problem? BTW: I live in a Mobil~Home. Could it be simply a Low-Voltage Hum? I can't really say that I noticed any Hum out on the job, but; that's been a long time ago also! Usually it was too noisy to tell on the job anyway!

[basilh]

Thanks Dave for the explanation, here in the UK (And Ireland) we have a system called neutralised Earth.
Where the Earth or Ground is supplied Via the Electricity supplier's connection and consequently because of the long power cable runs to the nearest sub-station, it's NOT a TRUE earth.

Most houses also have an "Earth Rod" connected to the system, but as you can imagine, current leakage in components can put a potential between the neutral and ground. This is lethal if someone connects an amplifier to a socket that's on a different phase of the three phase supply that's in most commercial premises. The difference between the two APPARENT grounds (which are actually neutrals) can be as much as the total 3 phase supply (Nominally 440 volts)

Foolishly some entertainment venues have power outlets just under the FRONT of the stage for floor cleaning equipment, this is on the "Motive Power" phase whilst the normal outlets ON Stage are usually wired to a different phase.

The reason :-

In single-ended single-phase systems where the Earth and neutral are combined (TN-C, and the part of TN-C-S systems which uses a combined neutral and earth core), if there is a contact problem in the PEN conductor, then all parts of the earthing system beyond the break will rise to the potential of the L conductor. In an unbalanced multi-phase system, the potential of the earthing system will move towards that of the most loaded live conductor. Therefore, TN-C connections must not go across plug/socket connections or flexible cables, where there is a higher probability of contact problems than with fixed wiring. There is also a risk if a cable is damaged, which can be mitigated by the use of concentric cable construction and/or multiple earth electrodes. Due to the (small) risks of the lost neutral, use of TN-C-S supplies is banned for caravans and boats in the UK, and it is often recommended to make outdoor wiring TT with a separate earth electrode.

P1-ph = Vp x Ip

P3-ph = _/3 x VL X IL x cosφ

= _/3 x 3 x Vp x Ip

[Alan Brookes]

...BTW Where on earth did the terminology "Ground" come from ?

Of all the American terminologies, "ground" is probably more realistic than most. We "earth" by connecting to the ground. I don't know any other way of earthing.
As Basil implied, electrical supply companies supply a virtual earth which is not really earthed except at their substation. We take it for granted most of the time without ever checking it. Likewise we take for granted the voltage levels, which are often lower than that stated. From time to time I've noticed here that my fluorescent tubes don't come on, and it's because there's a voltage drop.

[Robbie Daniels]

I have used two amps for years and I have found the best way to cure hums, etc. is to use a HumX attachment to one of the amps. It retains the ground plug, but stops any hums etc. and I use a digital echo stomp box with a double outlet to split my signal to the two amps. HumX is a bit pricy for such a small unit, but well worth the money.

[Curt Eversole]

Hey, if you guys would teach the amps the words they wouldn't have to hum so much.
My input on the subject