groundlift question

[eddiewagner]

my new old amp has a groundlift switch. i have been told, that the wrong use of this device will give you a quick rock ´n roll death, without the fun part first. what´s the deal with that switch ? i got no idea....
eddie

[BrassRat]

What the switch should do is defeat the chassis ground.

It should be used if (and only if) you have done something that creates a ground loop in your rig.

The quickest way to get this is to split your guitar signal into two different amps. The ground from the electrical power line will then ground both amps, which will be joined back again on the outer conductor of your splitter cable. This creates a big ground loop, which will pickup hum from the AC power fields by inductive coupling.

If you "lift" (i.e. disconnect) the chassis ground from one of the amps, you break the ground loop, but you maintain that chassis' ground by the path back through your signal cables (as long as they are still connected).

If you lift the ground on a single amp rig, you're effectively just running through a two-conductor plug - like snipping off the ground pin. In this case, it is possible through some combinations of component failures to get the hot voltage onto chassis. At that point, the path of least resistance for the power to ground could be through your signal cable, your strings, and your heart (insert rock-and-roll obit here). Unlikely, but possible.

Short answer - don't defeat the ground unless you've got AC hum issues and reason to believe that you amp is creating a ground loop.

[EMan]

This sounds bad. This amp, and all amps, need a three wire AC cord with safety ground on the chassis at all times!.

[stxrus]

get the amp to a reputable/reliable tech and get a proper grounded cable/plug installed. ASAP

steve

[BrassRat]

Eddie -

What is the make/model of the amp?

Does it have a balanced XLR pre-amp output?

If so, the switch might defeat the passing of the local ground to a mixing board or other downstream electronics (in which case, it isn't messing with your chassis ground).

The whole electrocution question made me think on the power side, but there are common signal-side ground lift applications. The only ones I know use balanced signals (two wires for signal, plus a ground, like typical mic cables). In an UNbalanced signal, like a guitar cable, the signal and power grounds are common on the braided shield wire.

[eddiewagner]

Originally Posted by BrassRat

What is the make/model of the amp?

it´s a 70ies fender pro reverb. i have never seen that switch before....

[BrassRat]

In that case, what you've got is the usual ground polarity switch, not really a ground "lift". That switch changes which side of the power tranny input is the hot side and which is neutral.

If you read geek, Ampwares has the schematic here (this is for the '76-82 circuit). You can see the AC and ground switches labeled in the power section at the bottom center.

The ground switch's original purpose was also to minimize hum by swapping the hot and neutral power wires to get the quietest idle (if you had other gear plugged into the circuit "backwards", this allows you to compensate by making the amp "backwards", too).

There is no need for this switch on a modern circuit with a three-prong grounded cable. If your unit doesn't have one yet, it is highly recommended to have one put in - as the fellas have already pointed out. The reason for this is the infamous "death cap" that you see between the center pin of the Ground switch and the triangle indicating the actual ground. If this cap blows and results in an open circuit, it is (again) possible to put live voltage onto your chassis which could give you a nasty jolt, or worse.

The only weird thing is that the AA1069 schematic seems to indicate a grounded cord should be attached - I'm no expert, but I haven't seen a factory model with both the "ground switch & death cap" combo, and a grounded cord, but maybe they started transitioning to this in the late 70's or early 80's?

The key safety issue is to be sure that your power main has a good ground (beware seedy clubs and old residential wiring) and your three-prong cable is permanently grounded to your amp chassis. If you've got that going for you, you've nailed the major risk factors.

[jhundt]

Eddie - every Fender amp had that switch back in the old days. My '83 Princeton has it, I don't know when they stopped.

It used to be very important. Our wall plugs in the USA had 2 pins, one was HOT (120 VAC) and one was neutral (0 VAC). You wanted every amp to have the same neutral "ground" - at 0 VAC. So Fender put a switch on the amp.. if your plug was inserted "upside down" your ground was different from other amps. By flicking the "ground" switch, you effectively turned your plug around, without having to go back and find the plug and reverse it.

I grew up with this, it was just normal for us. Now, everyone is scared of electric shock and electrocution and all of the legal liability problems!

And for good reason - in the USA (120 VAC) you could get a nasty shock... in fact, Keith Richards was knocked-out in Sacramento CA in 1966 - rumors quickly spread that he was dead... lucky for us, he lived!

In Europe, with 240 VAC at the wall, the problem is even more serious.

after the 60's the USA began to use plugs with a GROUND pin, so you could only put the plug in one way. Even so, Fender kept the ground-reverse switch for many years.

Here's how we used to check for safe grounding back in the 70's:

turn your amp on

be certain that the PA is on, too

check that all other amps on-stage are on

plug your guitar cable into your amp (don't stick it into the guitar yet)

hold the guitar cable in your hand like you would hold a pencil, let the side of your index (1st) finger rest on the tip of the plug

while holding the guitar cable, touch the tip of your finger to the mic cable housing, and to the mic, and to the wind-screen of the mic. You WILL FEEL the shock if your ground is incorrect. It will shock you, but not kill you.

if you felt a shock, go back and switch the "ground" switch.

try it again, now your "ground" should be OK.

every instument should be checked this way. And, to be sure, you should check each instrument's cable to other instrument's chassis/ground.

It seems like a lot of work, but it only takes a few minutes. Nowadays we are supposed to believe that European power outlets are all 100% safe. I will NEVER believe that, especially here in Holland it is too easy to have "reversed polarity" with my USA amps.

So you guys in Europe never checked grounds when you played...?

[fakeocaster]

Even if you use a grounded supply its worth checking that the earthwire connection on the mains plug is secure.And despite what some people say you should never break the chassis earth connection to prevent earth hum.There are safer ways to do it

[woodman]

definitely install a three-prong power cord, but save the old one in case whoever you sell it to is a collectible nut.

[Axis29]

I had a Mesa that had both a three prong cord and the ground switch. there were a couple of times when I would use it to quiet a little bit of hum, but for the most part it didn't seem to make a difference.

When it comes to playing in places I don't know, I bring one of these:

http://www.idealindustries.com/produ..._e-z_check.jsp

I feel much happier when I know for sure... and for the ten bucks one of these costs, it just seems like a mildly smart thing to carry in my gig bag.

[eddiewagner]

thanks guys, this is all good info. i guess i will give the amptech a phonecall about that switch, maybe he can do something about the safety-situation, since he is working on the amp right now.

[eddiewagner]

false alarm! the european amps have the switch, but it is not connected at all. the tech just told me.