Weird problem with tube preamp - is it dangerous?

[markusw]

I have built an Alembic preamp clone that works at 370 VDC. It has one transformer to get from 230VAC to 12VAC and a second one to get up to the plate voltage. Basically, it works fine and sounds great.  However, when I connect it to main power in our music room there is some weird phenomenon: even if the power switch is off I have a "vibrating" feeling when I touch the enclosure. This is not the case at home. So I though it might be a grounding issue in our music room. The circuit´s ground as well as the enclosure is connected to earth via the power cable. Any ideas what´s happening??

[David]

"Vibrating" ?

You're getting shocked!  Not good, not good at all!

Not having any experience with tube circuits, I can't offer any solutions.  It does sound, however, like there's some leakage.

[R.G.]

***||| IT IS DANGEROUS |||***

That tingling is AC line voltage leakage coming out of the pedal through your body. It is possible that it could KILL you.

Since the action is different in two places, at least one of the two places is wired incorrectly as regards line, neutral and safety ground; however, your pedal should NOT have the tingling if it is wired correctly also.

[sir_modulus]

RG's right. Thats the feeling you get in India when you touch appliances such as laptops, that have grounded metal connectors (like USB) except to a much larger extent. That's real bad, as you going to kill things inside (and may not be good for you) real quick. Thats due to either two things, Faulty ground, or Really miswired AC, which is against CSA regulations. Do you have a line tester (like one of those neon lamp screwdrivers?) Try testing the socket first, then try testing the pre amp case. Don't touch it again, as It could become worse :!:

Not good man

Good luck, and don't shock yourself bad,

Nish

[markusw]

Thanks. Sounds scary. I´m sure the tingling is not 230 VAC. Unfortunately I know how it feels since I touched an open light switch in a friend´s apartment. It´s more some kind of vibration I feel, not when simply touching it but when I softly run my finger over the surface of the enclosure.

So, if I got right, there is an AC leakage which  - if safety gnd is properly connected - simply runs to earth. If the main power cable is not connected properly the leakage is not able to run to earth and I feel that vibrations.  The funny thing is I can feel the vibrations even when the power switch is off, so both line and neutral are in an off state. Thus, the only connection between the enclosure and the main power system is through the gnd line. Since I am pretty sure that at least the switch is wired correctly I really do not have any idea what´s giong on. Is it possible that in our music room someone mixed up earth and neutral or might this be also explained if the gnd line in the wall is simply not connected to earth?

Markus

[R.G.]

I´m sure the tingling is not 230 VAC. Unfortunately I know how it feels since I touched an open light switch in a friend´s apartment. It´s more some kind of vibration I feel, not when simply touching it but when I softly run my finger over the surface of the enclosure.

What you are feeling is just a ...little... power line voltage. The actual voltage across your own personal body may be reduced by the fact that your shoes don't conduct much, or that you're standing on rugs, etc. which reduces the applied voltage. In cases like that, the act of simultaneously the amp and a well grounded object - like a microphone plugged into an amp that does happen to be well grounded - will then put the full power line across you, and you could get dead.

The vibration is power line voltage. Fortunately so far you haven't gotten much.

Find out what is wrong. If it's the AC wiring in the room(s) where you feel it, that's one thing, and you should feel similar vibrations from correctly grounded electrical equipment. If it's a problem in the AC wiring in your self constructed equipment, you're perilously close to earning a Darwin Award.

Find out and fix what is wrong now - the correct amount of AC power line leakage is ... zero....!

[Lonestarjohnny]

First off, if you feel voltage from any appliance, and you can't re-engineer the dam thing to keep it from shocking you, you have a serious problem,
don't care who's design it is are was, it's got problem's that need to be corrected, If i shock 1 of the people I work for, I loose my job, so this is a very serious subject with me, I have seen some preamps that had this problem because of badly designed power supply's, but if this circuit is known and no one else has had this kind of problem you for sure need to check the circuit for a mistake that would cause this problem, and i'll say this 1 time, DON'T USE this until you find the problem.
Johnny

[Peter Snowberg]

Since you had this problem in one place and not another, I'm wondering if your amp and the preamp were plugged into the same outlet?

You might have been feeling a ground loop or the effect of a voltage induced into the ground of one of the outlets. One of Neil Young's amp techs said he's measured 150VAC on stage in such situations :!:  :shock:

I would ALWAYS plug the preamp and power amp into the same outlet.

Even better, I would remove the hard wired ground from the preamp and replace it with a .022uF cap (@ 600V or greater) at the largest. The cap allows it to still work as somewhat of an AC ground while eliminating any DC in that path. The amp should provide the ground here and adding another one is just asking for trouble in many situations.

I would remove the circuit ground from the case ground if you used isolated jacks, otherwise I would add the cap as above, or remove the ground all the way from the mains plug.

Grab your DMM and measure the resistance between the case and the power and neutral mains connections. They should be infinite. If you see any resistance, you have a SERIOUS problem that could cause death.

[markusw]

Wow, thanks a lot to you all!!!

I'm wondering if your amp and the preamp were plugged into the same outlet?

Yes the were.

Grab your DMM and measure the resistance between the case and the power and neutral mains connections. They should be infinite. If you see any resistance, you have a SERIOUS problem that could cause death.

I did this and the resistance between the enclosure and the power and neutral mains connections is infinite. The resistance to gnd mains is 1-2 ohm.

So in my opinion it´s rather unlikely that there is a problem with the preamp itself. Even more since the vibrations (they for sure are not 230VAC, actually you can just feel them if you know they are there) persist with the power switch in off state.

You might have been feeling a ground loop or the effect of a voltage induced into the ground of one of the outlets.

Could it be that I feel a voltage induced into the gnd line of the outlet by the paralleling power line if the gnd cable of the oulet stops after several meters in the wall because there is a faulty connection?

[Paul Perry (Frostwave)]

I think it is AC being capacitively coupled to the shell of the amp.
I believe there is a physiological phenomenon, where you can detect a suprisingly small AC voltage, by stroking a metal plate gently.

:shock:  :shock: potentially fatal advice coming up, but inquiring minds want to know...
If you hold one lead of the DMM in your hand and stick the other on the box, what do you see? (oh, set the DMM to max AC voltage!!)

[markusw]

I think it is AC being capacitively coupled to the shell of the amp.

How does this happen??

I believe there is a physiological phenomenon, where you can detect a suprisingly small AC voltage, by stroking a metal plate gently.

I think I read something like this in an article somewhere in the www about old devices 230VAC that do not have a separating trafo, but I do not remember where it was. Does it make any sense?
Interestingly, a long time ago I observed the same phenomenon with an old floor lamp. It also had a metal shell.

potentially fatal advice coming up, but inquiring minds want to know...
If you hold one lead of the DMM in your hand and stick the other on the box, what do you see? (oh, set the DMM to max AC voltage!!)

I will check.

[Brian Marshall]

check the outlet at your practice space.  it is possible that the hot and neutral are reversed..... or the ground may not be connected.  

Im not sure actually if canada uses a hot and a neutral though... Ive heard that some power systems in some countrys dont use a true ground they use two hots out of phase and a ground that also acts as a neutral.

[Paul Perry (Frostwave)]

Imagine a  power transformer inside a metal box.
And a secondary winding that connects (directly or via circuitry) to the box.
Now the primary winding of the transformer ios say 100 AC at one end and 0 at the other.
Every part of the primary winding has some capacitance to the secondary winding, and if the box is totally isolated, the effect will average out to look like maybe 10 or 20 pf of capacitance.

And then you close the circuit when you touch the box (your own body looks like a capacitor as well).

Part 2 of this may help come to grips with the idea: http://members.ozemail.com.au/~tabbler/GndRule/GndRule.html

[markusw]

OK. I did this:

f you hold one lead of the DMM in your hand and stick the other on the box, what do you see?

I just connected the preamp to the main outlet, left it turned off, no guitar or power amp plugged in and I got around 30V AC.

Then I did the same directly with the gnd connection of the main outlet: again 30V (at home I get close to 0 VAC, as it should be I suppose). Between phase and neutral I had 230 VAC but: between neutral or phase and gnd both I get around 115VAC (at home phase gives 230VAC against gnd, neutral around 0VAC).

Now I´m pretty sure the bug is somewhere in the outlet or in the main lines and not in the preamp. However, I have no idea where these 30VAC do come from.

Maybe I will solve the issue after going through this:

Part 2 of this may help come to grips with the idea: http://members.ozemail.com.au/~tabbler/GndRule/GndRule.html

Looks really comprehensive.

Thanks again for your replies!

[Paul Perry (Frostwave)]

Between phase and neutral I had 230 VAC but: between neutral or phase and gnd both I get around 115VAC (at home phase gives 230VAC against gnd, neutral around 0VAC).

Looks like something is SERIOUSLY wrong in the music room! the ground circuit is broken somewhere. That is going to have to be fixed!
Might be a break  at the wall socket, might be elsewhere. It isn't safe. :shock:  :x

[Gilles C]

Play it safe and fix it.

Find where it's coming from.

My brother-in-law almost got kill just by connecting 2 cables together. He had to go to the hospital because his earth wasn't beating steadily.

So make sure it is really safe before you use it. I mean, make sure you find the real problem, not just eliminate it where you normally use it.

If you take it somewhere else someday, it could hit you if you didn't fix the real problem.

Gilles

[markusw]

Looks like something is SERIOUSLY wrong in the music room! the ground circuit is broken somewhere. That is going to have to be fixed!
Might be a break at the wall socket, might be elsewhere. It isn't safe.

Thanks for your help. Next time before playing we will check the gnd circuit, for sure. Actually, I am happy that we found the bug before something serious happened.

Markus