Getting micro shocks from new harmonic percolator build

[mfairbridge]

Hi there first time posting. I've recently had a go at some of my first circuits and I think I've got the bug. I'm still a massive novice but I'm learning. I've made some bazz fusses with no issues and have just completed a harmonic percolator based on iviark's layout. Sounds awesome and fired up first time. Had a bit of high pitched oscillation to tame but I managed to do this trying a couple of caps to ground. These mods are mentioned on the forum here http://tagboardeffects.blogspot.com/2010/07/harmonic-percolator.html The 1n cap parallel to diodes mod tamed the osc. some so its usable now and the 1n5 cap in parallel to 20k resistor didn't do anything that I was aware of so removed it. Very happy with the sound now, it's pretty wild with a ge AC125 and si 2n5088. My current (pun intended) problem is that when I'm holding the metal end of a lead plugged into my turned on amp in one hand and the pedal itself in the other (as you do when plugging a pedal in lol) the pedal is micro shocking me and I can feel a warm, tingling current. Especially when I touch the switches, jacks etc. This lessened when I removed the 1n5 cap I mentioned earlier (this went from Q1 Collector to ground) but is still there. As soon as the lead is plugged into the output it's gone. Any ideas if this is an easy fix/don't even worry about it or is there something more sinister at play here? Grounding issue? Happy to give photos or transistor voltages if needed. Cheers all!

[idy]

Welcome to the forum!

Be careful! Almost certainly the problem is not the pedal but your amp! Unless you have done something very weird indeed there is not more than 9v in your stomp box. There may be hundreds of volts in the amp! Nothing more common than leaky caps, one of which can put a DC potential on the input jack. Do you have a multimeter? You can carefully plug a cable into the amp and carefully test for voltage (DC and AC, separate measurements) between the tip and sleeve of the other end of that cable.
Did I remember to say be careful?
We don't want to loose a new percolator or the monkey that made it.

[GibsonGM]

Holy crap....if you can FEEL it, it's dangerous, buddy...it's not 'micro' at that point!  IDY is correct, carefully test for voltage on the input cable alone plugged into the amp.  You can set it up with jumpers so that the meter is ready, then you turn the amp on, take reading, and shut it off before touching the input cable again...we're assuming you know how to use your meter here...

[deadastronaut]

sounds well dodgy ...do what the guys have said immediately


be careful man...

[antonis]

What is your amp mains supply configuration..??
(meaning, is it straight from wall socket or via a 1:1 isolation transormer..??)

[GibsonGM]

I too would like to know more - what is the make and model of your amp?

[anotherjim]

I suspect the added cap didn't stop it oscillating but shifted the frequency out of hearing range and now it's a microwave percolator.

If you think there's a voltage between tip and sleeve of the jack - measure it. Could be AC or DC. If you get shocks off the sleeve only, then the amp might be a suspect, but wouldn't you have noticed that without an effect pedal?

[iainpunk]

I suspect the added cap didn't stop it oscillating but shifted the frequency out of hearing range and now it's a microwave percolator.

If you think there's a voltage between tip and sleeve of the jack - measure it. Could be AC or DC. If you get shocks off the sleeve only, then the amp might be a suspect, but wouldn't you have noticed that without an effect pedal?

with a high enough frequency, you can feel down to 1.5 volt (source, my experience with powerful oscillators) but the most tingle (also the highest output current) i felt was around 4kHz, which is well withing hearing frequency, and going higher, the current also drops quickly due to the 'skin effect'. if it gets worse with the cap, it would suggest that the cap would bring the frequency more towards 4kHz, instead of out of our hearing range, so i think the cap just lets through more power supply ripple through the person to ground. is the amp actually grounded? is the pedal (and/or its power-supply) actually grounded? these would be my 2 cents on this ''mysterious case of the ghostly current.''

cheers

[R.G.]

I agree with the comments and questions about the amp. You need to sort out any possible mains leakage from the amp before worrying about mystery oscillations in the pedal. Any issues with the mains connections can kill you. A typical pedal's 9V power battery can't.

And there's another issue. >>> IS <<< your pedal powered by a battery, or by an AC power adapter? If it's an AC powered 9V source, that could be leaking AC mains current too.

So do some detective work. It could save your life from electrocution. Get your voltmeter out, and set it to measure AC volts.
1. This one is first, as it presents the most danger to you. Is your AC mains socket properly grounded? Some are not wired correctly, and can make any plugged-in device be an electrocution hazard in some circumstances. The building itself could be your problem. You need to find a true earth ground and measure AC volts from the earth ground to the chassis and signal ground of your amp, and to all the metal knobs, jacks and switches of your amp. Anything more than 1-2 volts says that the amp can be leaking AC, or that the AC socket might be improperly wired.
This is not very likely, but it can happen, and it's so dangerous that you need to eliminate it first.
2. If you are powering the pedal from a battery, you can then proceed with tinkering caps and stuff. There's not enough power or voltage in a battery powered pedal to really hurt you. Yes, I'm aware of 9V battery powered shock devices on ebay and such, but those require some special setups to make the high voltage to make shocks. The pedal you describe is almost certain to not be able to produce a dangerous shock.
3. If you're powering the pedal from a plug-in AC to DC adapter, it could have the same shock issues as described in number 1 above for the amp. Do the same AC volts measurement on the pedal and a cable plugged only into the pedal, looking for AC volts between any metal you can touch to a known good earth point. A leaky AC to DC adapter plugged into AC mains voltage can kill you too.
4. Only after eliminating any possibility for the AC mains power to kill you continue messing with the circuit.

[fowl]

Keith Relf

[mfairbridge]

Hi all I'm still alive. Ok so I'll do some investigation tonight. To be honest I'll go check it all out at another power point in my house as its possible the less than favorable power situation (extension cable from house into power board) to my shed where the gear is might be the culprit. I also have to say this has never happened with my amp (6 month old Marshall Origin 50 head) or any other pedals I've made or bought hence my thinking its the harmonic/microwave perc. I'm powering it with a 1spot adaptor (no battery snap) so might try my truetone c7 supply as well. And yes I will be careful.

Might as well clear this up while I'm here too, the 1spot I noticed if I touch the end of it when plugged in I get a similar tingling current sensation, is this normal? and before anyone says anything I accidentally touched it and noticed this. I'm not trying kill myself.

Thanks

[R.G.]

That helps. If it's a Visual Sound/Truetone 1Spot - and you're not using "1Spot" in the generic sense like I've seen - I happen to know a little about it. There is an EMI-regulations required capacitor in the incoming AC line circuit that can let through a small AC current. It's designed to not be hazardous. In use, the adapter is usually connected to earth ground by being plugged into the cable and then into a nominally grounded amp. This shunts the leakabe current to ground. However...

In any given 1Spot (or any other AC-DC adapter) it is possible that something failed inside. That could allow the small unavoidable current to increase to a possibly dangerous level. So the same advice applies: get out your voltmeter and measure for dangerous voltages. And there's another step that tells you more information. Use a 20K or 22K resistor to bridge from the places you're measuring voltage to the known earth ground. Measure the voltage across that. The resistor is a quick and dirty "human body model". If the voltage across that is still more than minimal, the leakage current can be dangerous. If the measured voltage is low, it's not dangerous, but is still very annoying. I don't remember the exact voltages, but I'll go look them up if you run into this.

Good on you for thinking about the extension cord as a possible problem If you're using a three-wire extension cord, it's common for the third pin to take mechanical abuse and get flakey. That sets up a not-grounded-at-all situation on three-wire equipment and can be anywhere from annoying to dangerous.

[PRR]

Dangerous stray electricity--

Assume 'normal' skin (not a surgical suite, not immersed in water).

25 Volts is perceptible but "generally" assumed to be safe.

12 V is perceptible when sweaty.

(Cats can die chewing on 3V wires.)

Both V and A matter, but skin R is very variable and current is often assumed to matter most.

5 milliAmps is generally assumed "safe" long-term. Shock preventers "GFI/RCB" usually trip on a time-curve: 50mA for a milliSecond or 5mA for a whole second.

Interestingly the suggested 22k "dummy body" at 5mA is 110V.

While 5mA may be "safe", in real life we often have a voltage source (current not really limited) and a varying skin resistance. If current continues to flow, the skin cells break down, weep juice, skin resistance soon falls way off. Even to 1kΩ. On 120V source that is 120mA which is far beyond "safe".

[R.G.]

Yep. I think the idea for the 20K was that if you got more than 5ma, you're really in trouble, but 5ma or less you'd probably survive.

It's kind of like the idea of LD50 for toxins. That stands for Lethal Dose in 50% of the test subjects.    A few volts across 20K is a much safer threshold.

[pinkjimiphoton]

Welcome to the forum!

Be careful! Almost certainly the problem is not the pedal but your amp! Unless you have done something very weird indeed there is not more than 9v in your stomp box. There may be hundreds of volts in the amp! Nothing more common than leaky caps, one of which can put a DC potential on the input jack. Do you have a multimeter? You can carefully plug a cable into the amp and carefully test for voltage (DC and AC, separate measurements) between the tip and sleeve of the other end of that cable.
Did I remember to say be careful?
We don't want to loose a new percolator or the monkey that made it.

sounds more like a leaking coupling cap at the first stage of your amp letting dc leak thru. amps are more dangerous than effects are, so be careful nosing around in it. if you're uncomfortable,  take it to a tech.

odds are its not the pedal messing up... its dc backfeeding into it from the amp messing it up. try in a different amp and i bet there won't be a problem.

please don't mess around and get that amp checked out asap. unlike a 9 volt stompbox, even a low wattage guitar amp could kill you under the proper circumstance. there's no way the PEDAL could be zapping you as it  would need a ground to flow from it... that would be you... but there's no appreciable voltage to a percolator.

its the amp, dude. get it straight. do a google, keep one hand in your pocket, and while you're in there, change out the power supply filter caps. but its gonna definitely be the cap at the input to the amp at fault here, its the only place dc could leak thru that you'd feel.

IF its one of them little vox modeler amps, say the valvetronix 30 or 40, that tingle is "normal" according to vox... but... it only takes about 5ma to kill you dead in the right circumstance.

don't @#$% around.

[mfairbridge]

Hi all still alive and kicking. Well I tried my 1spot c7 to power the harm perc and no more tingling/current sensation from the pedal when I'm holding it and plugged into power and holding the amp lead. So I guess it could've been the 1spot 9v adaptor? I'll be taking my amp to get new tubes so I'll mention this problem and see what happens. Cheers all for the replies.

[Rob Strand]

Hi all still alive and kicking. Well I tried my 1spot c7 to power the harm perc and no more tingling/current sensation from the pedal when I'm holding it and plugged into power and holding the amp lead. So I guess it could've been the 1spot 9v adaptor? I'll be taking my amp to get new tubes so I'll mention this problem and see what happens. Cheers all for the replies.

It's not 100% conclusive but that is one possibility.

Another possibility is:

The 1spot 9V has no ground pin but the 1spot c7 has an IEC plug with a mandatory ground pin.   While I suspect the there is no DC connection between mains earth and the 1spot c7  0V output there may be an AC path from 0V back to mains ground.

That means your amp still could be leaking just that now there is a low enough impedance from the 0V (which connects to the amp ground via the pedals) to mains earth that the current not longer wants to flow through you.  In other words an AC path to ground inside the 1spot c7 could mask an issue with the amp.

Does your amp chassis connect to ground? can you measure a low impedance from the earth pin on the mains plug to the amp chassis (for example the ground on the audio input sockets).     If your amp is floating it can promote leakage currents to flow through the audio input cable.

You would need to set-up a test configuration where you get a tingle from the 1spot 9V  without the amp being involved.  A PC might not work because it is grounded.   It can also depend on how you touch things ie if you are between the 1spot 9V and ground.

[pinkjimiphoton]

what rob just said ^^^^^^

[Rob Strand]

what rob just said ^^^^^^

IF its one of them little vox modeler amps, say the valvetronix 30 or 40, that tingle is "normal" according to vox... but... it only takes about 5ma to kill you dead in the right circumstance.

don't @#$% around.

I agree with what you said before as well 

It's not a good idea using youself as the tingle test box.