Is it dangerous ... (dot dot dot)

[Gaz]

I've actually been shocked badly twice from touching the sleeve of a speaker cable because the OT common wire was not ground referenced. This was in two separate amps, and a high AC voltage appeared on the sleeve (with signal applied of course). Ground referencing the OT fixed the "problem."

I had previously been under the impression that it was not necessary to ground reference the OT common wire unless the amp used global NFB (Aiken), but now I always do regardless after being shocked.

[Chuck H]

Sure... There are low voltages (but highish current) at the speaker terminals. I don't see a risk of shock there (and B+ on the sleeve of the OT output is probably indicative of another problem) but the OT primary to secondary short (thank you Enzo) is also a possibility (though I've never seen it) so grounding the output sleeve (and therefor the - speaker terminal) is definitely a "best practice" idea. If hum issues come up they can usually be dealt with via grounding scheme.

[Gaz]

Sure... There are low voltages (but highish current) at the speaker terminals. I don't see a risk of shock there...

But I got shocked there!

...(and B+ on the sleeve of the OT output is probably indicative of another problem)

It wasn't B+ on sleeve it was a high (over 100 volts IIRC) AC voltage. The amp functioned perfectly otherwise. I would've mentioned if it didn't. Since I was able to fix the problem by ground referencing the common wire, I really never investigated it further.

[J M Fahey]

Mmmmhhhhhhhhhh, strange.
The way you tell it, it was AC voltage directly related to audio signal, very different from +B leaks through faulty insulation which is what we have been discussing so far.
Problem is, to get shocked, you need to touch *two* points which have a voltage differential between them.
If both OT_to_speaker wires are floating, touching any of them with a finger will not cause any current to flow through you.
If you grab each end with a hand, yes.
What I *did* experience was getting mildly shocked by fumbling with the *unplugged* end of a speaker cable, trying to plug it again , on stage, in the dark, and with sweaty hands.
In that case a 100W into 8 ohms amp produces around 40V peak; if tubed (and unplugged) , the inductive kick out of the OT may reach easily over 100V peak, quite shocking.
But it's not directly related to grounding:
if conventionally grounded, in theory it's actually easier to get shocked, because I'm already grounded to the amp through the guitar in my hand, I only need to touch the plug's hot end.
If not grounded, I may still grab the plug with a sweaty hand, bridging tip and sleeve.
How do you know you had around 100VAC?
Where did you apply your test probes?
Thanks.

[EFK]

I should add that while it is a constant struggle for me to remember to fasten that shoulder belt, I grit my teeth a bit and keep an eye upwards every time I drive under an overpass or railroad bridge.

[Chuck H]

Mmmmhhhhhhhhhh, strange.
The way you tell it, it was AC voltage directly related to audio signal, very different from +B leaks through faulty insulation which is what we have been discussing so far.

Right. If it was a constant voltage it would need to a diferential between the guitar (grounded) and the output sleeve. But not between the - output sleeve and the + output otherwise the speaker would be humming (and probably failing).

Problem is, to get shocked, you need to touch *two* points which have a voltage differential between them.
If both OT_to_speaker wires are floating, touching any of them with a finger will not cause any current to flow through you.
If you grab each end with a hand, yes.

+1

In that case a 100W into 8 ohms amp produces around 40V peak; if tubed (and unplugged) , the inductive kick out of the OT may reach easily over 100V peak, quite shocking.

Yup. I considered that a 100W amp would have enough juice to kick you hard but left it out.

if conventionally grounded, in theory it's actually easier to get shocked, because I'm already grounded to the amp through the guitar in my hand, I only need to touch the plug's hot end.

Right. I mentioned that in post #2. It's about the voltage differential, not ground.

Still odd to have a voltage differential of 100V between the output sleeve and ground rather than a voltage differential between the output sleeve and the output + lead.

Was this a field coil speaker?

[Chuck H]

Haha... I always get nervous when there are kids standing in the middle of an overpass as I go under. Mostly because when I was a kid others should have been nervous to see ME standing in the middle of an overpass

[J M Fahey]

Haha... I always get nervous when there are kids standing in the middle of an overpass as I go under. Mostly because when I was a kid others should have been nervous to see ME standing in the middle of an overpass

Well, the classic kid "weapon" is spit.
The mother of all WMD would be pee (in that context).
Neither of them deadly, of course, although if you ride a convertible or the open back of a pickup truck ... you'd better worry.

[tubeswell]

But I got shocked there! It wasn't B+ on sleeve it was a high (over 100 volts IIRC) AC voltage.

The current on the OT secondary is typically between ~1-3A, and it only takes a few dozen mA to defibrillate your heart.

[J M Fahey]

The current on the OT secondary is typically between ~1-3A, and it only takes a few dozen mA to defibrillate your heart.

The OT current *through the speaker* may be around the 5A mark, in a 100W/4ohm amp , but that's quite unrelated to the current *through you*, which depends on the voltage you receive.
The human body has a very high resistance, mainly at the skin surface, in the high thousands of ohms, as anybody can check by holding a multimeter's probes in his bare hands.
It's also highly non-linear, diminishing rapidly above a certain threshold voltage.
That's why 100/110V is considered reasonably safe, and 220V up is quite lethal.
But current through the rated load is not the main concern here.
We are talking "usual" guitar amps of course, not 2000 to 7000W PA monsters.

[EFK]

So I was messing around today with one of my little single-ended Supros which does not have the negative speaker lead grounded. I can notice a distinct difference between stock (no ground wire back to chassis) and running a ground wire back to a ground point; it's not dramatic or anything, and I do have to listen for it, but it is quite audible. Adding a ground wire seems to add just a touch "more." I don't know whether I'm hearing a fuller frequency response, or a bit more volume, or what, but there is a definite difference for the better. Anyone have any ideas as to the big WHY?

[Gtr_tech]

And it is not just bowling balls, frozen turkeys are just as lethal. However, frozen turkeys are less likely to get you on the bounce.


And let us not even talk about live turkeys. "As God is my witness, I thought turkeys could fly."

Great episode of WKRP. Mahn that was a great show.....

I like the "phone cops" one too. Johnny Fever is onna my heroes...