Floating input

[woodyc]

They don't make em like this anymore.

You can draw about 4mA at 60Hz. I accidentally tossed my electrical safety class notes, but isn't that close to the no-let-go threshold? And for a bonus, there is about +9V DC on the input, which depends on the current in the preamp tube.

Actually now that I think about it, its not that bad. It can provide 4mA or 120V, but not 4mA at 120V.

[Steve Conner]

Well, the legal limit is something like 50 microamps.

[woodyc]

Another wake-me-up I came across:


Steve, who established that (the 50uA) standard?

[woodyc]

This one isn't dangerous, just kind of retarded:

[Steve Conner]

Sorry, I got it wrong. The limit for leakage current is 250uA for Class 2 (ungrounded) and 3.5mA for Class 1 (grounded).

It's called touch current and the relevant standard is IEC 60950.

[J M Fahey]

Actually now that I think about it, its not that bad. It can provide 4mA or 120V, but not 4mA at 120V.

Sorry but it isnīt read exactly like that.
The amp will provide 120V, no load (thus no current) because thatīs whatīs available at the wall plug.
As soon as you touch it, *you* close the circuit and current starts passing through your body.
Voltage across you will be less than 120V, sure, because thereīs a potential divider made out of the deviceīs and your internal resistance.
But voltage will lower because current is already passing through you !!!
What actual voltage will you drop on contact?
It depends on many factors: wet/dry hands, contact pressure, shoes you wear/wood/cement floor, voltage really is not the important factor; what will kill you is so many mA through your heart.
Usually estimated in 10 to 20 mA or more.
4mA is uncomfortably close to that.
Biological parameters vary wildly (we are not SMT parts mounted by a robot ) and I can easily imagine that a certain percentage of people may be hurt by the "permissible" 3.5 mA.
Gauss bell statistic distribution and all that.
Iīd prefer a much higher safety rating myself, such as a 1/10 the "killing" current maximum allowed.
As a side note, I already posted somewhere else that a long time ago I was working in a very hot humid basement (close to a steam leaking boiler) , repairing a 24V battery powered control equipment.
I was covered in sweat and felt a tingling in my forearm when it brushed the +24V line (I was holding the metallic case and the whole controller was on a wooden table.
24V DC !!!!!!

[jmaf]

Anytime you see a tube prefix above 12 you should be concerned. 50C5 for example, on the first schematic.

Here they call these circuits "hot tails", for obvious reasons. Well not so obvious reasons, there are two historical takes: one that the wiring was so thin it'd get warm to the touch, the mains lead was the hot tail. Another version is that it shocked the heck out of folks when they touched the back of the bakelite radios. I think the second version is more accurate.

Like an uncle's GE fridge by the swimming pool, we used to come out wet with bare feet and go grab a soft drink and instead we got a shock. My cousin landed on his back, meters away once. The handle was one of those chromed "pull to unlock types" I'll never forget it.

[Steve Conner]

The mains lead was the "hot tail" right enough, but the reason why it got hot was even worse than you think.

These radios were 120V designs, and when exported to 230V countries, a special mains lead made of resistance wire was fitted to get rid of the extra unwanted voltage.
Line Cord Resistors and other Heater Droppers

And set fire to any soft furnishings as a bonus

[jmaf]

The mains lead was the "hot tail" right enough, but the reason why it got hot was even worse than you think.

These radios were 120V designs, and when exported to 230V countries, a special mains lead made of resistance wire was fitted to get rid of the extra unwanted voltage.
Line Cord Resistors and other Heater Droppers

And set fire to any soft furnishings as a bonus

Didn't know they called them hot tails elsewhere as well - should've guessed. As for the resistive mains lead that is news to me. Here we've got a mess of 110 and 220 VAC mains, I bet the resistive tail is common here as well, I've just never come across one.

[J M Fahey]

I remember those resistive cords, because in Argentina we are 220V everywhere so they were needed.
I still have stashed somewhere my Fatherīs old AC/DC radio sporting such a power cord.
It looked like a pressing iron cord: thick, rubber insulated with a heavy mesh outer cloth covering.
It was always warm to the touch.

[woodyc]

Sorry but it isnīt read exactly like that.
The amp will provide 120V, no load (thus no current) because thatīs whatīs available at the wall plug.
As soon as you touch it, *you* close the circuit and current starts passing through your body.
Voltage across you will be less than 120V, sure, because thereīs a potential divider made out of the deviceīs and your internal resistance.
But voltage will lower because current is already passing through you !!!...

Exactly. All I meant to say was that you can't get 4mA from the Cadet because the current will be 120/R where R is its 30K impedance plus whatever resistance you have. If you are wet, it could be over 3mA, but most likely it will be much less. If you get it on your lip from a microphone, its obviously going to be something you will remember anyway.

At 60Hz and less than 500V it comes down to your resistance. Here's a relevant graphic from a class I had with a guy named Lloyd Gordon:

He does a lot of hazard analysis and some of his stuff is on the web.

[J M Fahey]

This is a very useful post, I humbly request to moderators that it becomes a sticky.
I would even go as far as making it part of the "becoming a member" conditions.
A random question might be asked as "what happens of so many ohms resistance touches such and such voltage?"
Heīd have to find the value graphically.
Not difficult for a human, impossible for a spam robot.
Or at least name what, say, the third diagonal line reads.
jm2c