Your take on the death cap issue

[Mars Amp Repair]

Hello folks,
This will probably 'spark' much conversation ;- ]...

I am wondering what the consensus is on the Death cap in amps that have a grounded 3 prong cord & the polarity switch with death cap...or even with just the cap.

Of course all amps should be grounded for safety & also to have a solid signal ground for the chassis, but if you remove the death cap & the player happens to be in a situation where the ac does not have a decent ground at the outlet, the amp will buzz sometimes even if you reverse the cord...or should the player just be left to 'deal' with those situations...I just worked on an old Champ Amp where this was the case.

Interestingly I've noticed that even many 2 prong solid state stereo hi-fi type amps from the 80's have a 'death cap' in them, too.

Is there any situation such as the above where it would be 'ok' to leave the cap in there for ac signal grounding when grounding is not available?

Can you reduce the value of the cap to reduce the effective capacitive reactance to the chassis (Xl)? What's the collective take on this issue....thanx, glen

[Mark Black]

I feel conflicted about simply removing the caps too, for the very reason you mention. That being said the "poor ground" condition should not exist anywhere by modern electrical codes and actually points to a glaring fault with an installation or an electrical system sorely in need of an upgrade. A commercial installation with this kind of problem leaves the owner liable for a nasty lawsuit and/or a lifetime guilty conscience if someone is killed. There are however still a lot of private residences in the USA with un-grounded outlets and no requirement to upgrade.

So maybe the type of "safety" capacitor discussed here is an appropriate answer: http://www.justradios.com/safetytips.html

Or not...

It will be interesting to see what others on the board think.

[Alex R]

- forgive my ignorance, but do you in the US have a 0v/240v or 'neutral' and 'live' setup on the two mains AC lines like we do in the UK?

I have a loose uinderstanding that we remove the death cap over here because our neutral line is already related to ground, or earth as we eccentrically call it. Would appreciate more info on this issue.

[cbarrow7625]

It's really only a "Death Cap" because someone has to touch the switch, right? But only if that switch malfunctions or there is a short, etc. etc.

Why not get rid of the switch? Just hard wire a .1uF cap to both AC lines? Then you no longer have to worry about which way the outlet is wired? While you are at it add one going from line to line as well.

The cap is not the "death" part, it is the switch. We should be calling it the "Death Switch". not the "death cap".

Chris

[TD_Madden]

hmmmm....why would touching the switch be any different from touching the chassis or touching the strings on your guitar?

I think the cap is the problem....when it shorts there'll be a direct connection to the chassis from whichever side of the line the switch has selected.

[drewl]

But won't that blow the fuse?
Unless of course the outlet isn't grounded.

One of the first things I did when I moved into this house was ground all the ungrounded outlets.

[J Martin]

I have been leaving that cap in just in case someone uses a ground lift switch on the mains feed to the amp.

In the usual three wire grounded line cord situation, if the cap shorts it will either blow the fuse or vaporize the cap, or both if its on the hot leg of the AC supply. This will not be a hazard since the grounding wire will conduct the fault current to earth ground. If its on the neutral side it will connect the neutral and ground together. While this is not proper, it probably won't be an issue unless the neutral is heavily loaded from other parts of the circuit thus causing a voltage differential depending how far you are from the service equipment. The difference between the white and green wires is that the white (neutral) is grounded, and the green is grounding.

There is/was a standard for how large those caps can be if someone wants to do the reseach.

[Enzo]

Standard house wiring in the USA. The mains come in as 240 with a CT. So 120-0-120. We use either half of that as the 120 in the wall outlet. The CT is earthed at the house service panel. The wires to each wall outlet are 120v hot and the neutral line back to the CT. There is a separate ground wire back to the service panel as well. The ground wire is connected to the same place as the neutral wires. A true earth ground rod is pounded into the soil near the panel, and a strap from that is connected to the neutral/ground bus in the panel. So the ground and neutral are more or less redundant.

The point of the cap is to provide a path for noise to earth from the amp chassis. It has nothing to do with the signal path. If you remove it the amp still works fine, but might hum more.

Without the cap, the only potential on the chassis is whatever leakage there is from the transformers and other parts, OR whatever comes in on the signal or speaker cord grounds/shields.

With the cap, the cord plugged in one way has the cap from neutral to chassis, thus grounding the chassis. Flip the mains plug over, and now the cap connects the 120VAC to the chassis. The full current of the mains is not available, but it can still poke you. The polarity switch it there so you can select which side of the thing gets the cap to chassis. This is a convenience so you don't have to find the plug and turn it. Turn the plug or flip the switch, the result is the same.

In a three wire mains situation, the cap is irrelevant until a ground lift is present - intentional or not. AS someone mentioned, heavy mains draw could create a potential on the neutral wire, and any noise there would be abated to a degree by a cap from neutral to ground, but the chassis should alredy be grounded. And if the cap runs from hot to chassis, since chassis and neitral are bonded at the paneel, this is equivalent to putting a cap across the incoming mains - which is often done.

If the ground is severed, then we are back to the classis two wire situation. except you cannot now turn the plug.

it is a death cap, because if it shorts, and the switch is set for cap to hot, then the chassis is directly connected to the 120VAC. Touch the chassis and anything else that is grounded, and you just grabbed a live wire. Flip the switch then and the chassis in no longer hot.

When I three wire an older chassis, I tend to just leave them there as irrelevant. I do clip them out if they are in the way of my working though.

[Steve Conner]

So the ground and neutral are more or less redundant.

This is technically true. But in civilized places like the UK, we use the ground wire to ground things for safety and hum reduction, and the neutral wire to return our spent electrons to the power company. We never mix the two functions, except once it gets to that neutral/ground strap in the breaker box that Enzo mentioned, which we also have in many installations, and try not to think about too much...

The death cap is considered bad because its purpose is to let you use the neutral wire as a kind of half-assed ground, which is as bad as throwing tea into Boston harbour

Two death caps, like another poster suggested, would leave you with half the line voltage on your chassis, and half the level of hum and shocks you'd get with the death switch in the wrong position.

With a three-wire power cord, the death cap and switch are redundant, not to mention a violation of most modern electrical codes, and should be taken out. You can use the hole where the switch used to be for some sort of mod.

[Enzo]

I dunno, that tea thing worked out OK for us. And the water in Boston harbor is STILL brown.

Every now and then someone wires up an outlet with ground and neutral reversed. For most three wire stuff, this works out, since both go to the same place on the service panel, and can go without notice for a long time. But then you plug in another device and the ground loop is terrible.

I meant redundant in the sense they both go the same place, not as an invitation to use either one as a return.

[cbarrow7625]

These caps are standard issue on just about all RFI supressor circuits in existence. You know those really expensive power strips you buy for your home theater equipment? Caps & coils across and in series with the AC line to supress RF interference.

Also, you have to use poly caps. They are self-healing. Electrolytics would be a disaster. If a poly truly shorts, you are going to have bigger problems.

[cbarrow7625]

Steve, you will not have the "mains" on your chassis with the cap. Without a ground wire attached there is the potential that you will put some voltage on the chassis, but not the entire 120VAC. The cap is acting as a high pass for the 60 Hz AC line voltage and could potentially pass a percentage of that voltage to the chassis at a much lower current. The only way you get the "mains" on the chassis is with a shorted cap.

Since, in residential electric in the States, ground & neutral are tied together to the same ground buss back at the breaker panel, the best thing to do is probaly to add a cap across the Hot to neutral and leave the chassis out of the whole mess.

Let's not lose sight of the fact that it is not these caps that are the issue. It is faults that occur that cause this cap to be the issue. The biggest fault of all being a missing, or lifted, ground. Electrical equipment without a shielded chassis KILLS PEOPLE. Period. It is never safe to operate any equipment without a shielded chassis. Bring an outlet tester with you to gigs to check for grounded outlets before plugging into anything. The cap is a reasonable thing to have in the equipment with the assumption that the chassis is a 0 volt (or very close) potential. I just think we're placing the blame in the wrong place for this issue. Treat the sickness, not the symptoms and there will be no more problem.

[Enzo]

I regularly measure AC volts from chassis to earth in the shop here. I see plenty of older amp with two wire cords, and believe me I see the full 120VAC on the chassis. it is not a direct connection, the cap will limit current, but all that voltage is there. Of course if thecap shorts, there WILL be a direct connection to 120VAC.

CB, I think if you review this thread you will see that a failure in the cap is exactly what we are discussing, it is the source of the name "death cap."

We find death caps in vintage equipment, no one is suggesting the new construction of something with one.

[trevorus]

Just update your stuff with a true 3-wire setup, don't risk a zap. It sucks.