Amp Has No Power Transformer

[bobloblaws]

I have a small amp here which has no power transformer. From what I have read this is unsafe and it is recommended to use an isolation transformer. I have a few questions in this regard.

1. The info I've read says that if there is a fault and the chassis becomes connected to the hot side of the mains power and if my body is at ground potential, when I touch the chassis I am providing a return path to ground for the electrical current. The amp has a three prong plug installed with ground wire connected to the chassis. Does this not eliminate my body as a path to ground or is it that in this case there would simply be two paths to ground as opposed to just one?

2. Again with the idea of my body being at ground potential, is this typically the case? Is my body always at ground potential, or put another way, is my body always a viable path to ground, or does it matter where I am standing etc?

3. Why is it safer to have a transformer isolating my body? The info I read seems to imply that if I touch a 300V leg of a PT secondary with only one hand I will not get shocked. This is news to me as I seem to recall having done this or similar a few years back during a momentary lapse and getting bit hard.

- BL

[Enzo]

In a three wire system, the ground prong connects to chassis. The two main wires are hot and neutral. If the outlet is wired correctly, the chassis winds up lined to ground via the neutral and a resistor and cap.,, if the outlet has hot and neutral reversed, then the 120v TRIES to get to the chassis through those parts. Essentially they are now across the mains. If you are yourself grounded, this should not be an immediate threat since the chassis is grounded also,You would indeed be a parallel path, but your body has millions of ohms while the power cord ground has virtually zero. So any current through you would be too small to measure. But what if the outlet ground prong is not wired? I have seen many outlets where they replaced the two holer with a three. No grounding but at least you could plug in a three prong plug. But zero protection.

Your body is at ground potential if you are connected to ground. Like standing on damp soil or a damp basement floor. or touching something grounded like a water or drain pipe. it isn't always so obvious though, so you cannot throw caution to the wind. It you were hanging by a nylon rope, you would not be grounded. So you could get away with touching the mains wires. But getting away with something is not the same as a good idea.

I like to say it isn;t about the parts, it is about the circuit. You could touch one end of a 300v PT and possibly not get a shock, however, many times there is a center tap involved. OR if it is connected to a rectifier, the rest of the circuit can fool you. But there is also induced voltages, leakage currents, other ways to get a tingle. But we are not worried about a 300v secondary, we are worried about the mains. If you touch the mains, it is dangerous. You are directly connected to the power lines. By using an isolation transformer, the amp circuits are no longer directly connected to the power lines.

[Randall]

As usual, Enzo speaks truth in easily digested form.

[darkfenriz]

The requirement is to have all the accessible conductive parts (chassis, knobs, whatever the strings are electrically connected to) suitably insulated from hazardous mains.
In that case a single fault of either lack of proper grounding or short between mains and grounded chassis does not result in a hazardous voltage at the chassis (and all the conductive parts).
In that case it takes both bad grounding and internal fault to put the user at risk of electrocution.

This is usually done with power transformer, which allows the circuitry to be ground-referenced and insulated from mains, but in theory it does not have to be the only way to achieve safety.

[cjenrick]

that amp wants an iso xfmr, unfortunately those death traps are usually cheap Silvertone card board cabinet jobs that have little room and get top heavy by the addition of a 3 lb transformer, so you mount it on the bottom of the cabinet, run the three terminal pwr cord into the amp chassis, install a fuse holder and fuse if it does not have one, then run two wires down and two wires back to the xfmr thru a nylon cable bundle thing a ma jig doohookey and you are done. your gonna need some butt splice conns because the blk pri wires will be too short,

[nosaj]

that amp wants an iso xfmr, unfortunately those death traps are usually cheap Silvertone card board cabinet jobs that have little room and get top heavy by the addition of a 3 lb transformer, so you mount it on the bottom of the cabinet, run the three terminal pwr cord into the amp chassis, install a fuse holder and fuse if it does not have one, then run two wires down and two wires back to the xfmr thru a nylon cable bundle thing a ma jig doohookey and you are done. your gonna need some butt splice conns because the blk pri wires will be too short,

-1 on butt splice connectors. Do it right butt splice solder with heat shrink over it.

OP what amp is this?

nosaj

[bobloblaws]

As usual, Enzo speaks truth in easily digested form.

Agreed, thanks Enzo. However, something I still don't get. Why is mains power inherently more dangerous than the higher voltage on the secondary side of a transformer?

[Tom Phillips]

Agreed, thanks Enzo. However, something I still don't get. Why is mains power inherently more dangerous than the higher voltage on the secondary side of a transformer?

Because it is referenced to things around you such as water pipes, other properly grounded equipment and the very earth you stand on. In addition, it can supply massive amounts of current.

[bobloblaws]

-1 on butt splice connectors. Do it right butt splice solder with heat shrink over it.

OP what amp is this?

nosaj

It is one of the Canadian amps made by Pepco in Montreal, possibly brand name Pine, but has no tags to indicate make or model. It has a 35Z5 rectifier, a 50L6 and a 12AX7, tone and volume.

[bobloblaws]

Because it is referenced to things around you such as water pipes, other properly grounded equipment and the very earth you stand on. In addition, it can supply massive amounts of current.

The danger inherent in massive amounts of current I understand, but not why water pipes and the earth are inherently more dangerous as a voltage reference. Forgive my ignorance, this is obviously electricity fundamentals that I don't understand (but really want to). Is there a resource you could point me to that might help me?

[eschertron]

The danger inherent in massive amounts of current I understand, but not why water pipes and the earth are inherently more dangerous as a voltage reference. Forgive my ignorance, this is obviously electricity fundamentals that I don't understand (but really want to). Is there a resource you could point me to that might help me?

The North American grid has all the voltage produced (from 10s of kV industrial power, on down to 120vac residential mains) referenced to the ground we walk on. It's how generating stations 100s of km apart can concurrently feed the grid from different sources. Sometimes referred to as the "one true ground" (I think I read that here). So the water pipe, garage floor, etc., can return ANY AMOUNT of current fed to it from a voltage source. GFCI outlets rely on that ability to sense when a mains voltage fault-to-ground is occurring and trip the circuit.

You are connected to the secondary side of a PT when you pick up a guitar. You are at chassis potential because the input is referenced to chassis. If that chassis potential ALSO happens to be mains line voltage, now you and 'true ground' are at odds. This is what we are trying to prevent. With a non-PT amp, there is the possibility of that happening. Details above

[Justin Thomas]

While I can certainly imagine a possible failure scenario of "primary shorted to secondary" or something wildly outlandish, which would be no better than a shorted "death cap," does the (pretty much proven statistically) fact that transformers are among the most reliable and trustworthy components in electronics (well, amps anyway) have something to do with the fact that we stress the isolation provided by a tranny so much more than isolation provided any other way?

Justin

[Enzo]

OK, try this. You plug your guitar into the amp. You touch your strings or cord. You are not electrically speaking also touching the chassis. If that chassis is hot, not anything around you that is grounded makes YOU the circuit to ground.

Say your bass player has a nice newer grounded amp. his axe is grounded to his chassis , but his chassis is earthed, yours is not. SO if you guys go face each other during a solo or just to talk between songs, and you touch any part of his rig, while touching your own axe, YOU are the path to ground from your hot chassis through his earth. The 300v inside the amp in a transformered amp is not going to find a earth path outside the amp.

That is the difference. All those high voltages INSIDE the amp don't come outside. But on a hot chassis the mains voltage is ON the outside of the amp. If that doesn't sound dangerous, consider a power cord not wired to anything, but plugged into the wall outlet. The hot wire is stripped to bare the copper. Now leave it sitting around the room.

[Tom Phillips]

The danger inherent in massive amounts of current I understand, but not why water pipes and the earth are inherently more dangerous as a voltage reference. Forgive my ignorance, this is obviously electricity fundamentals that I don't understand (but really want to). Is there a resource you could point me to that might help me?

Yes it's an unfamiliar concept until you study it. It's all about the dangers of allowing a potential across body parts which causes a current to flow through the body. A couple of images may help.
1) This one illustrates the unsafe condition

2) This one illustrates a safer condition

There are many resources which explain this. The photos above came from the web page at Electrical and Fire Safety - Clinical Anesthesia, 6th Edition.
You can give it a read to obtain additional explanations.

Cheers,
Tom

[The Dude]

A picture is worth a thousand volts.

...... well, at least a hundred.