AC Line Noise. Please Help.

[R.G.]

Bad house wiring can do a lot of funny things. One of the most slippery is an open or high impedance neutral.

Incoming 240 is split into 120-0-120, the "0" being the white/neutral wire. If that's high resistance from the house to the pole, then the voltage on either side depends on what is turned on/connected to either side, because the neutral can't carry the current back. One load by itself can get 240. Electricians try to balance the loads in the breaker panels so usually there's more than one load, and they balance kind of, but it's easy to get one outlet with 80V and one with 160. And when the dryer, oven, or water heater comes on, things go crazy.

It makes noise worse because any noise generated in one side of the 240 has to go through the other side to get back to the pole. You may well have had something like this.

[Morocotopo]

R.G, it would be nice, at least to me,  to know more about house/mains wiring, because most of us, I believe,  know about electronics but very little about the part on the other side of the power plug!!

Any reading material you would care to point me to?

Thanks

[PRR]

RG> Incoming 240 is split into 120-0-120

Morocotopo  - Argentina> R.G, it would be nice, at least to me,  to know more about house/mains wiring

R.G.'s "120-0-120" is true in the US and Canada and a few other places.

The amount of power a modern home demands, it is not economic to supply that much power at a low voltage like 120V. However the US was wired before good rubber (or plastic) insulation, so it could be dangerous to go much over 100V to ground; anyway we have so much 110V-120V stuff that we "could not" switch to 240V.

However it is practical and fairly economic to bring 120V-0V-120V to the house on three wires. If the 120V loads are wired half on each hot line, there is no current in the 0V line; if they are not balanced the unbalance simply appears in the 0V and as less heat/drop in the lightly loaded 120V side. (Big loads such as water-heaters and dryers are bridged across the two 120V lines to get 240V.)

There are several problems. If you stick fingers in _two_ light sockets it is possible to get a 240V shock. The problem R.G. cites is that the "120V" depends critically on the "0V" wire having good connection all the way to the transformer. If it gets tarnished under the screws, the full 240V appears across two loads and divides however it wants. I have seen a coffee-maker and a sound mixer split 70V/170V.... the coffee didn't get hot but the mixer eXPLODed.

-------------------------------------------
You are in Argentina. I do not know, but will guess, that your outlets are 240 Volt, not 120 Volt. This means you waited for good insulation; much 240V electrical work is based on post-1950 products with modern plastics.

AFAIK, most homes in 240V lands are wired one 240V-0V circuit.

And 3-phase power is common in the street.

So your street transformer has three 240V output terminals and a 0V common terminal. One or more houses are wired to each 240V terminal. Houses are connected to the three hot terminals to give roughly equal loading.

In almost all modern systems the "0V" wires are dirt-grounded at each pole and at each house.

Please consult local books and experts. The way you do it is NOT how we do it in Texas or Maine.

[Morocotopo]

 Paul, thanks for the explanation. First of all, I don´t even dream of making any electrical work, I know my limits: a working TS9, a non exploding tube amp...
But I´d like to learn more about mains wiring, just for the knowledge itself.

Over here in Argentina we have 220V (usually lower) mains voltage. The three wire standard in houses is about 15-20 years old, before that all wiring was two wire (no safety ground wire). Two prong sockets, wall outlets, etc. I believe the power systems historically were of English origin, because when the country got sort of industrialized (trains, cow meat processing factories, wheat and cotton cropping and processing, etc.), it was the british who did it, to take advantage of our resources.

I believe 3-phase wiring is somewhat common (not in houses but maybe in business buildings, factories, etc.), I´ve heard people talking about it, and about 3-phase electric motors ("trifásicos" in spanish).
So over there in the US, you have three wires going into each house? The 0V one is not the ground safety wire, right? It´s a separate one? And there has to be balance between the two 120V sides? Complicated...

I´m gonna search for some info on the subject.

[R.G.]

In the USA, we have had several systems come and go.

The modern practice for wall outlets is to take in 3 phase (usually!) from high voltage transmission lines and step it down to three separate secondaries of 240Vac each.

Each winding is center tapped. The center tap of all three separate 240Vac secondaries is connected to a copper wire which runs down the pole into the ground.

Generally, one house ( or rough equivalent) is powered by one of the 240Vac secondaries.

Each house gets three wires: 120, 0, and 120. The 0V is grounded back at the power pole, remember.

Inside the house, the circuit breaker panels are wired with roughly equal numbers of amperes (roughly guessed at by the electrician) from each 120Vac side. The idea is that the currents roughly "cancel" so that not so much current flows back on the 0V wire. The 0V (called "neutral" in electrician-speak) carries the difference between the current used on each 120Vac side.

The house also has a ground wire. This is a wire run to a metal pole driven into the earth. It is connected to the safety ground buss inside the breaker panel. I would have to go get my electrician's handbook to remember if the safety ground is also connected to neutral inside the breaker panel.

The wires from the breaker panel go to the outlets. Each outlet gets three wires: Hot/line/102Vac, cold/neutral/0V and safety ground.

If copper were a perfect conductor, the neutral/0V connector in a wall outlet would measure 0.00V to the safety ground. In reality, there is some voltage drop through the wires. This can be several volts, even with the #14 to #12 sized copper wire normally used to carry 15A to a wall outlet. This happens even if there is nothing plugged into the outlet, because more than one outlet is connected to the same 15A circuit breaker, so other loads can cause the "0V" to rise to several volts AC when actually measured.

This difference in "0V" is one cause of hum in amplifiers plugged into different outlets, and can sometimes cause a slight tingling shock.

At least as nearly as I can recall without getting out the books. 

[ayayay!]

Bad house wiring can do a lot of funny things. One of the most slippery is an open or high impedance neutral.

Incoming 240 is split into 120-0-120, the "0" being the white/neutral wire. If that's high resistance from the house to the pole, then the voltage on either side depends on what is turned on/connected to either side, because the neutral can't carry the current back. One load by itself can get 240. Electricians try to balance the loads in the breaker panels so usually there's more than one load, and they balance kind of, but it's easy to get one outlet with 80V and one with 160. And when the dryer, oven, or water heater comes on, things go crazy.

It makes noise worse because any noise generated in one side of the 240 has to go through the other side to get back to the pole. You may well have had something like this.

I re-read this a couple times and was thinking about this, R.G.  An open almost seems reasonable.  Her lights kept blinking, but our really didn't.  It would only take a tiny break on one of those 120V supplies, between her pole and her house, to make this happen consistently.  Her house was struck by lightning about 5 years back, and she said things "never were the same" after that.  Now she says everything is perfectly normal.  I will ask her what they replaced, but I doubt she'll know. 

As a side note, I was actually in her basement with her family when the lightning hit 5 years ago.  We have no basement, so we headed to hers during a tornado warning.  My family and I were huddled with her and her daughter.  We heard a robust, snappy "pop" that made the doggie door blow clean off.  We barely even saw it, but knew we just missed being struck by mere feet.  It was one of those, "Hey, was that...?  I think that was..." moments.  Cooked her downstairs fridge and blew out half the fuses (yes, I said fuses, not breakers) in her panel.  Wow, that was a trip.  

[PRR]

> In the USA... take in 3 phase (usually!) ... and step it down to three separate secondaries of 240Vac each. Each winding is center tapped.

Not on a single transformer, not usually.

Any long large run, from sub-station to neighborhood, is 3-phase. That's so much more economic.

In dense neighborhoods, such as New Jersey, a single 6,000V (or 13,000) line runs down the street, top of pole. Single-phase. Each four (1-6) houses there is a transformer. 6KV in, 240CT out.

In my rural neighborhood, they run 3-phase on the major roads (though a lot of minor roads are 1-phase). This may be lower voltage (because 3-phase is so bulky), 1KV or so. At houses 206, 207 and 208, phase A is tapped 1KV:240CT, at houses 209 and 210 phase B is tapped, at George's lane phase C is taken down the road to transformers near George's several houses.

Residential pole transformers are mostly single-phase.

Go look at your pole-pig. Unless your situation is unusual, there's just one big ceramic insulator where the 6KV comes in.

3-phase transformers are made, and are probably economic, but an awful lot of installations use three or even two single-phase transformers to couple 3-phase power. (Two cores give less than 2/3rd the power capacity, but at 68% of the investment. Since the load on most systems expands, they start with 2 and then when they run hot due to added loads they buy a third. The main step-downs around here is a 3-pot platform with 2 pots installed; when the economy turns-around and I sell my 5 acres for 1/4 acre houses they'll install those 3rd pots.)

There is not a decent way to get single-phase power from a single-core 3-phase transformer.

One indecent way takes two leads at 120 degrees (instead of 180 deg). If scaled for 120V at each leg to neutral, the leg-to-leg voltage is 208, not 240. Most residential large loads are rated for 240V. Some will work at 208. The water heater heats slower (and 208 elements are a cheap way to get full heat). Ovens run on thermostat and will reach 350 deg F slower, but hold 350 F perfectly. A range will boil a pot of water in 13 minutes instead of 10 minutes. However I have seen a piece of photo-gear just go "Urk!" on 208V.

Where there is need for large power or a large investment in large motors, 3-phase is the way to go. In particular, 3-phase motors self-start without the unreliable gimmicks needed to get them going on 1-phase. 3-phase rectification to DC needs little to no filtering and much less stress on the lines.

> remember if the safety ground is also connected to neutral inside the breaker panel.

YES (in the service entrance). However just because it is Required does not mean it got done. Missing damaged and burnt grounds are common.

[DavenPaget]

Paul, thanks for the explanation. First of all, I don´t even dream of making any electrical work, I know my limits: a working TS9, a non exploding tube amp...
But I´d like to learn more about mains wiring, just for the knowledge itself.

Over here in Argentina we have 220V (usually lower) mains voltage. The three wire standard in houses is about 15-20 years old, before that all wiring was two wire (no safety ground wire). Two prong sockets, wall outlets, etc. I believe the power systems historically were of English origin, because when the country got sort of industrialized (trains, cow meat processing factories, wheat and cotton cropping and processing, etc.), it was the british who did it, to take advantage of our resources.

I believe 3-phase wiring is somewhat common (not in houses but maybe in business buildings, factories, etc.), I´ve heard people talking about it, and about 3-phase electric motors ("trifásicos" in spanish).
So over there in the US, you have three wires going into each house? The 0V one is not the ground safety wire, right? It´s a separate one? And there has to be balance between the two 120V sides? Complicated...

I´m gonna search for some info on the subject.

Yep , it is all british origins , same in singapore too , that 3 pin semi-circular rectangle pinned plug , it carries 220 on the Brown , 0 on the Blue , and of course "safety grounding" on the green
For people in the NA it's 2 whites/hots (120-0-120) and a black (neutral)

[PRR]

> 2 whites/hots (120-0-120) and a black (neutral)

PLEASE try to get it right!!

In the US, the groundED wire (which is Neutral in the drop) MUST BE IDENTIFIED WITH WHITE PAINT (now plastic).

The GroundING conductor may be bare or GREEN.

"Hot" was formerly natural rubber (black); may be any color EXCEPT WHITE or Green.



There are three main hubs for Electrical Systems.

Very early, Edison/GE/Westinghouse wired the US, Canada, and some other areas. Siemens and others in Germany made better systems. The UK was a hodge-podge until 1945 when large parts of the system had to be rebuilt, fast, with little material, and a British Standard laid the plan for that.

Of course each nation may choose to buy the big machines and cans from one of the Major Makers, then promote local industry by favoring a different plug only made by the Presidente's brother. However Global Trade has weakened these local oddities. Most of the "modern" world is using US, DE, or UK devices, and basing local regulations on those systems.


> 3-phase wiring is somewhat common (not in houses but maybe in business buildings, factories, etc.)

Exactly. The amount of power used in a house does not justify the high initial cost of 3-phase service.

The large power in factories, shopping malls, universities does justify 3-phase equipment.

Any long run justifies 3-phase.

Why?

R.G. teaches that in 240V/120V systems "the currents roughly 'cancel' so that not so much current flows back on the 0V wire." There are 3 wires. If loads balance, the CT wire carries no current, waste of copper. If all loads fall on one side of the split, the other leg carries no current. There is no way to get full use of the three wires we paid for.

3-phase with balanced load gives better use of the copper. 3-phase with unbalanced load is not so good, but at-worst it equals 240V/120V 3-wire system. Also note that 3-phase is usually run "above" the level of single users/loads. When you have a whole street or neighborhood or large factory or office, you take care to check that loads are balanced on all 3 phases.

> in the US, you have three wires going into each house? The 0V one is not the ground safety wire, right? It´s a separate one? And there has to be balance between the two 120V sides? Complicated...

The standard "drop" to the house is called "triplex". Short runs, we use red and black insulated stranded wires, plus the bare strands from a third wire wrapped around the red/black pair. The external strands protect the insulated wires. Another type (which I have) is two insulated wires wound around a _steel_ cable. This is much stronger for long runs (I am far from the street), but not well protected (I have a lot of weak trees), and there is more resistance in the steel. There is another type with three insulated wires, popular in underground feeds.

ANY wiring system _must_ be connected to dirt near any place we can get "in contact" with it. Transformers leak a little. Lightning loves pole-top wires, and gets into underground wires. If the system were left "floating", an ill transformer could bleed 6,000V onto the system, and lightning-hit may be a million volts. While we can not hard-ground both/all wires, we can pick one, ground that, and it tends to limit stray voltage on the other wires.

So all down the street there are wires from the top of the pole to the dirt. They bond one of the 6KV wires (often bare here). This is also tied to the 0V wire in the drop to the house. However a heavy fault on the street can pull-up "ground potential" at the street to be different from ground potential at the house. Also a fault in the house can pull-up the house many volts different from ground at the street. Therefore you must have (in most situations) a dirt-rod at your end of the drop-wire. R.G. is right: you really must have the Code Book to know how this is done. And Code is changing: used to be metal water pipe maybe with a dirt-rod. But dirt-rod won't absorb big faults. And water pipes are often plastic today (my copper pipe turned to plastic a foot outside the house). And the water company hates to be the electric dump (one reason they went plastic). Rods embedded in large concrete are very good, if you do it before the concrete sets. There are standards for large plates (corrosion is a problem) and ground-rings (a lot of digging, a lot of fat copper). The "service entrance" needs clarification.... meter? Fusebox? And how about a detached garage? Swimming pool? In the US, you expect to find a large bare ground wire in the fusebox which can be traced to dirt; but other methods are used also. (I'm "grounded" at the meter 50 feet away from the house.)

3 wires from the street. Dirt-rod at both ends.

INSIDE the house, all metal objects (water pipes, stoves, guitar amp chassis, even aluminum siding) should be connected back to the ground in the FUSEBOX. No shortcuts out the window to dirt. No tying white to green (VERY dangerous, and hard to detect).

Which means that a combo 120V/240V load (stoves may use 120V for medium and 240V for high) really must have _4_ wires. Hot, hot, neutral, ground. Until a decade ago, this was an Exception: a stove was presumed grounded via the Neutral wire. In fact some people got hurt this way. Now all new stove and dryer outlets must be 4-wire. You can put a new stove on a 3-wire outlet, but twice now I have done the right thing and pulled new wire and outlet.

No, the "balance" is not critical. Most houses are wired 200 Amps main breaker. There are not enough 120V loads in a house to ever pull 200 Amps: that's like two-hundred 100W lamps or 20 toasters and microwave ovens. More likely worst-case is 50A one side 30A the other. This flows 50A in one leg, 30A in the other, and 20A difference in the "neutral" (so it isn't a true neutral). In older installations, N was sized one size smaller than the two hots, so would be good for 140A; 20A difference is nothing. On top of this a house in Arizona could be pulling 40A at 240V for air-conditioning. Now we have 90A/70A with the same 20A "neutral" current. (There's another reason why 200A boxes are common: older code limited the number of circuits in boxes but excepted "large" boxes. Recently they realized this was dumb and dropped it, but most fusebox makers would rather sell the 200A box than make a 100A with 42 spaces.)

Complicated? No. You wire the drop to the box the same on all routine jobs. You load the 2-pole circuits for range, dryer, A/C. Then you load the 1-pole 120V circuits. The box is wired so that every other space is alternate legs of the 240/120 split. So I put Furnace, Pump, Kit 1, Kit 2, Living, Dining, Bedroom, Bedroom. Furnace and Pump wind up "balanced", Kitchen 1 and 2 wind up "balanced". They never are: the furnace and pump don't run together, Kit 1 is a clock and Kit 2 is a toaster... but on average loads tend to split more-or-less balanced close-enough.

"Complicated" is emergency generator systems. In a power failure, a home engine-generator will back-feed the street, electrocuting the lineman fixing the failed utility wires. Actually you don't want to feed the street, so there is a cut-over switch. However wiring this correctly IS "complicated", even to someone who can pull grounds into kitchen outlets OK.

[DavenPaget]

ahh damn i was looking at the old code and got it reversed 

[markeebee]

..........when the country got sort of industrialized (trains, cow meat processing factories, wheat and cotton cropping and processing, etc.), it was the british who did it, to take advantage of our resources.

Um, yeah, us Brits have done awful, awful, things in the past, but we did give large parts of the world a halfway sensible electrical infrastructure.  My European colleagues always laugh about our clunky fused plugtops and seemingly random choice of transmission voltages, but it all makes pretty good sense really.

Big yay for us.

Oh, and we gave you soccer, so that Argentina can regularly embarrass us.

[artifus]

yay for jingoism! you'd have thunk we'd have gotten over all that by now, what with the moon landings and the internet an' all. but hey ho, on we go...



*edit* paranoia compels me to edit to ensure the acknowledgement of humour and the spreading of good vibes to all originally intended in this post. the clue was in the use of the word 'we' - in we would have. that is as a community, a race, a species, inhabitants of this one fragile planet of limited resource, doing our best to get along. maybe that should be co inhabitants. as 'one' if you will. anyways, let's all try try to get along, play fair and share a like. peace y'all! or maybe i just couldn't find/decide on the appropriate emoticon. doh.

[Morocotopo]

Paul, thanks for the explanation. I´ll try to digest it all. A couple of questions arise: The neutral is tied at SOME point to the safety ground, right? At the breaker box in the house, or at the power plant... that´s why you have to fuse in, let´s say an amp, the hot and not the neutral, because if the fuse blows in the neutral you could still get shocked by the hot/safety ground wires combo, right?

Um, yeah, us Brits have done awful, awful, things in the past, but we did give large parts of the world a halfway sensible electrical infrastructure.  My European colleagues always laugh about our clunky fused plugtops and seemingly random choice of transmission voltages, but it all makes pretty good sense really.

Big yay for us.

Oh, and we gave you soccer, so that Argentina can regularly embarrass us.

Actually currently we are in no position to embarrass anyone, our national team is, let´s say, hmmm, of dubious quality.Or, more to the point,  they suck big time!!
OK, enough about soccer. Let´s talk about something else, please. Please.

[DavenPaget]

Paul, thanks for the explanation. I´ll try to digest it all. A couple of questions arise: The neutral is tied at SOME point to the safety ground, right? At the breaker box in the house, or at the power plant... that´s why you have to fuse in, let´s say an amp, the hot and not the neutral, because if the fuse blows in the neutral you could still get shocked by the hot/safety ground wires combo, right?

Yes , pretty much yes .

[PRR]

> damn i was looking at the old code

Even if you are older than you look, in the US NEC there's no "hot white".

Pre 1900, all wires are black.

Sometime early in the 20th century, all services required to have one conductor groundED, "identified", with white paint or other Approved means.

When other colors became available, the wording was changed to reserve White and Green; "hots" may be any OTHER color.

BTW: the main point was to make the outside shell of our stupid light-bulbs less dangerous.

> you have to fuse in, let´s say an amp, the hot and not the neutral, because if the fuse blows in the neutral

This is a very picky detail.

Note a difference (in NEC) between groundING and groundED. The green/bare wire runs to fusebox then to dirt. The groundED conductor carries load current, and returns through the fusebox to the utility company. They are connected together but serve very different functions.

In the US, in building wiring, fuse in grounED conductor was once common (perhaps because we didn't know which if any wire was groundED). Later it was banned for the reason you give. However in appliance (and amplifier) wiring, again we do NOT know which wire is groundED. There are right ways and wrong ways to connect sockets and plugs. Most of the wrong ways will light the lights, and that's good enough, right? There is a growing trend in _appilances_ to double-pole switching and even double-pole over-current breakers.

[Morocotopo]

> damn i was looking at the old code

Even if you are older than you look, in the US NEC there's no "hot white".

Pre 1900, all wires are black.

Sometime early in the 20th century, all services required to have one conductor groundED, "identified", with white paint or other Approved means.

When other colors became available, the wording was changed to reserve White and Green; "hots" may be any OTHER color.

BTW: the main point was to make the outside shell of our stupid light-bulbs less dangerous.

> you have to fuse in, let´s say an amp, the hot and not the neutral, because if the fuse blows in the neutral

This is a very picky detail.

Note a difference (in NEC) between groundING and groundED. The green/bare wire runs to fusebox then to dirt. The groundED conductor carries load current, and returns through the fusebox to the utility company. They are connected together but serve very different functions.

In the US, in building wiring, fuse in grounED conductor was once common (perhaps because we didn't know which if any wire was groundED). Later it was banned for the reason you give. However in appliance (and amplifier) wiring, again we do NOT know which wire is groundED. There are right ways and wrong ways to connect sockets and plugs. Most of the wrong ways will light the lights, and that's good enough, right? There is a growing trend in _appilances_ to double-pole switching and even double-pole over-current breakers.

Yep Paul, in my amps (two so far) I use double pole switching, so switching both lines, to be as safe as possible.

[PRR]

I had not realized that in the Schuko system, "high" and "neutral" can be reversed.

"Schuko plugs and sockets are symmetric AC connectors. They can be mated in two ways, therefore live and neutral can arrive on either pin at the consuming device."
http://en.wikipedia.org/wiki/Schuko



While breaking either wire turns-off the light (or appliance), for personal safety you need to break both lines.

[R.G.]

Pre 1900, all wires are black.

It gets scarier than that. A friend of mine bought a house near the college campus in (pre-Katrina) New Orleans. Its electrical wiring was bare copper wrapped around glass/ceramic knobs in the attic. 

> you have to fuse in, let´s say an amp, the hot and not the neutral, because if the fuse blows in the neutral
This is a very picky detail.

It's one the safety testing guys will trip you on. Open fuse in the line side can't leave full line voltage lurking after the fuse. Open fuse in neutral can.

From the relatively few two-wire amps I've redone, I think all of them had the switch in one side and the fuse in the other. No good reason not to if you don't know which side is hot. And it gives you a terminal on each side of the incoming line to tie the line cord to without needing another terminal strip.

In converting amps to three wire, it is very convenient to replace with a DP switch that breaks both power wires to have a ready-made terminal to tie the incoming white/neutral to, because the fuse really needs to be moved to the hot side.

The groundED conductor carries load current, and returns through the fusebox to the utility company. They are connected together but serve very different functions.

I think that's one reason they changed the nomenclature to "neutral".

There are right ways and wrong ways to connect sockets and plugs. Most of the wrong ways will light the lights, and that's good enough, right?

Yep. I caught the "electricians" wiring the sockets all possible ways in my house building days. I checked 100% of the outlets for proper polarity. Caught about 5% wrong. In one case the lights in a bedroom would not stay on. The code at the time required arc-fault interrupters on all breakers servicing bedrooms. The arc fault interrupters kept tripping, and were blamed as flakey and unreliable until I found that the safety ground to the bedroom with the AFIs was mis-wired. The only hint of this at first was a flakey reading on the plug-in idiot light tester.

I harp on this a lot, but gigging musicians need to either take an outlet tester or an isolation tranformer with them to gigs.

There is a growing trend in _appilances_ to double-pole switching and even double-pole over-current breakers.

I applaud this heartily, even if it is probably to cut back on the requirements that people who do the wiring know what they're doing.

[PRR]

> wiring was bare copper wrapped around glass/ceramic knobs in the attic.

OK, trumped.

I've lived with abandoned knob & tube. My last old house had the knobs in the porch attic, and tube into the walls up from the cellar (so there may once have been exposed bare wire in a 6-1/2' cellar). But I've never lit a lamp with the stuff.

> the safety testing guys will trip you on. Open fuse in the line side can't leave full line voltage lurking

IMHO, they trust too much in their own regulations. _I_ say you can't know which is H or N.

> proper polarity. Caught about 5% wrong.

It was 60% H/N reversed in my last old house's kitchen.

80-90 years of "identified conductor" (wide-pin outlets go back to 1928) has gotten us to 5%-60% of lamp-shells _LIVE_. Your observation say we got safer; my bad-case observation says it's dart-throwing, and Murphy supplies the darts.

[amptramp]

I harp on this a lot, but gigging musicians need to either take an outlet tester or an isolation tranformer with them to gigs.

If you are building your own amplifier or a power supply for all your pedals, it may be a good idea to build in the outlet tester so you never lose it.  It consists of three neon lights that should show hot to neutral is on, hot to ground is on and neutral to ground is off.  Double pole switching and circuit breakers are good ideas.

Many amplifiers have a capacitor from line to ground that may be switchable from one side of the line to the other.  This should be replaced by a line-to-line filter using class X or class Y capacitors which are described here:

http://www.justradios.com/safetytips.html

or L-C filters using these components.

At some point, someone should suggest using MOV's (metal-oxide varistors) or semiconductor spike suppressors.  Noise caused by a bad neutral connection can easily cause up to double the voltage to appear at the power input.  I would put the suppression after the breaker just to ensure that continuous double voltage trips the breaker.