Repair Safety First

[mykey]

It occurs to me that while teaching so many to work on tube amps and even solid state amps that so little has been said about safely working on these amplifiers.
A shock as little as 32 milliamperes can stop your heart from beating; and with so many killed by electrocution every year, now is the opportunity to add your safety tips and rules.
First let me remind you that a tube amp stores quite a bit of power in it's filter capacitors, long after the amp is unplugged. Before working on one of these amps the power supply(s) should be discharged- learn how BEFORE starting your repairs.
So then with that in mind, please take electrical safety seriously; and not as a bothersome formality. Many have categorized safety rules as a joke.
I would hope that the more experienced techs post their safety tips here, and that the learning techs read and follow these safety tips.

[mykey]

On Jun 14, 8:10 am, Jim <a...@beforeyousend.com> wrote:
> Patrick Hoelscher wrote:
> > I am interested in learning to build my own guitar amp. Possibly a tube amp
> > or hybrid. I am a Computer Science graduate with a small amount of
> > electrical engineering knowledge. What is the best way for me to learn? Buy
> > a couple different used amps and take them apart?
>
> A very good way to start is to buy an Epiphone Valve Junior and modify
> it. You can also buy an old hi-fi or PA and modify it. Old stuff needs
> more attention, such as replacing filter caps.
>
> DO NOT proceed until you understand HIGH VOLTAGE safety issues. Filter
> capacitors can store enough energy to KILL YOU, even with the amp off
> and unplugged! Always discharge the filter capacitors first, and I keep
> a lead between preamp plate and ground while working on a dead circuit.
> Learn the "one hand rule" when working on a live circuit. It's WAY
> different than computers or modern electronics.
>
> I have two yahoo groups that might interest you. The Epiphone Valve
> Junior mods site is by invitation only. You need to post your real
> E-mail and request an invitation. In that invitation, you must convince
> me you know the safety rules.
>
> The "open enrollment" site is:http://launch.groups.yahoo.com/group/guitar_amplifiers/
>
> ALL are welcomed at this site. General discussion on guitar amplifiers
> WITHOUT personal attacks, religion, politics (the usual alt.guitar.amps
> NONSENSE).

[Amp Kat]

Certainly you want to practise safety in amp repair but it's kinda like an Indy race car driver. Before they get in the car they are aware that they can be killed at any time or any wrong move. Same here so if your scared you may be shocked don't work on it. I also stated a while back that different people have different resistances and capacitances in there body. So it may kill this guy but not kill the other and of course othere medical issues that could cause something fatal totally objective in someone else. That being the case I should be dead because I've been hit with everything from 1 to 1000 AC & DC and I'm still here but what is does is give you respect for where your touching and what your grounding yourself on. I'd be willing to bet that there are some damn experienced techs out there that still get shocked even with the precautions and experience they have. And don't even tell me you never get schocked because if you do I know better. You are aware that some capacitors will recharge themselves even after discharged if there aren't discharge resistors across them so discharging isn't always as safe as it seems but yes it is a good idea and very easy and quick to do. Still not a reason to keep newbies from working on their amps but most of the ones I've seen are aware of the dangers before they do it. Do you have some numbers on the people that get killed every year working on guitar amps ?

[Rob Mercure]

Mykey,

You might want to review the Forum archives and the old Ampage archive to see how strongly we've stressed safety over the years - especially with one topic that reoccurs periodically: working on/restoring old line connected guitar amps. I dunno, I do you hope take this in the spirit I offer it but you seem to offer us strong opinions/facts intended to be helpful - and they are - without realizing/researching that we've addressed these again and again over the past decade +.

As far as electrocution hazard it really isn't the voltage but, as you noted, the current flow that kills and some folks with naturally low skin resistance have been killed by as little as 60 volts accidentally applied to a wet chest - and all of us who have serviced TVs sometime in the past have a tale, or a dozen, describing how we were "knocked" across a room when we accidentally encountered the ultor of a CRT or, if old enough, the HO plate cap, et al. KB noted how caps can "recharge" themselves over time and since a CRT is nothing but a big cap anyway when unpowered I've been "bitten" more than once by 30 KV, or so, that developed across a picture tube when the clip on the bleeder slipped off.

My personal gremlin has to do with safe grounding and I won't let an amp leave my shop unless it has an intact properly grounded cord with both the fuse and the switch in the hot side of the line.

Oh, and I'm glad you mentioned the "one hand rule" - can't stress that often enough (especially since I work in hot electrical panels often). But I really worry more about an inexperienced "technician to be" forgeting to unplug the piece of equipment under repair and electrocuting themselves across the mains input than I do from any residual "B" supply. The "newbie" probably has some idea that tubes involve higher voltages and may be a bit on guard while completely forgetting that his little solid state practice amp is pretty directly connected to his friendly neighborhood lightwater reactor.

And thanks for your "invitation" - I'm sure that we all appreciate it. One thing that I've treasured about this group/Ampage from the start is the lack of flames, ego dogfights, personal attacks and emphasis on solidly backed technical information instead long rants in support of whatever fad modifications are in vogue.

Take some time and get to know us - while we can be a bit "laid back" if not lethargic now that summer has heated up for those of us north of the equator - we do strenuously emphasize safety related issues and tend to be "one big happy family" (especially since that last lobotomy "took" - was running out of scalp - <grin>).

Rob

[mykey]

no, I won't pretend that experienced techs don't get shocked because we know they do. and a large portion of those killed are the pros who should know better and have let their guard down.
the tendency of a capacitor to charge itself is referred to as the chemical charge. the capacitor will slowly charge itself up, even after sitting unused for years, and even when there are discharge resistors across it.
what are some of the favorite methods for discharging power supplies before working on an amp?

[mykey]

Mykey,

You might want to review the Forum archives and the old Ampage archive to see how strongly we've stressed safety over the years - especially with one topic that reoccurs periodically: working on/restoring old line connected guitar amps. I dunno, I do you hope take this in the spirit I offer it but you seem to offer us strong opinions/facts intended to be helpful - and they are - without realizing/researching that we've addressed these again and again over the past decade +.

I am glad you have stressed safety, and I am bringing it up again just to keep the thought of this fresh in people's minds. And to remind those who have just started the adventure of amp repair and tweeking.
In fact there should be a separate safety category in this forum, just to remind everyone, every day.
I have repaired amps for 34 years, especially tube amps, and we need to pass what has been learned, along to those in particular who have just started doing this.
repair safety cannot be a part time job, it must be job #1.

[Steve Conner]

Well, Ampage may be a big happy family, but for better or worse, it's also part of the Internet. The Internet is huge and open to anyone with a computer and a phone line. You don't have to sit any kind of exam to get on there.

So on Ampage, like on any forum, you will encounter a bewildering range of people, from hardened techs like Enzo all the way down to the kind of simple folks who would happily clean their sneakers with gasoline by the light of a candle. While smoking a cigar.

What I'm trying to say is that many people who read this board are probably so dumb that they will ignore any safety warnings we post and kill themselves anyway, and then their moms and dads will sue TBoy. I read the other day about some guy who climbed into a power substation with a hacksaw and tried to cut off some 132,000 volt busbars to sell for scrap. While they were turned on. Did he notice the Danger Of Death signs and 15 feet of razor wire? Did he have the slightest conception of what 132,000 volts at a couple of thousand amps does to human flesh?

In the light of this, I don't even bother with safety threads on forums. I think they are more of a liability than an asset: what if someone posted safety information that was wrong? or could be spun as wrong by a cunning lawyer?

If I were TBoy, I'd post a prominent disclaimer saying that all information on Ampage is provided without warranty and any liability is disclaimed, and leave it at that. In fact he probably did that already, I hope.

Your safety is not someone else's responsibility. It's yours. Use common sense at all times. Educate yourself. Don't do something just because you read it on the Internet.

[MWJB]

Jumper wire from a preamp tube pin to chassis - as long as you can see it there (and it's been there more than 30 secs) you know you are safe, no worries about caps recharging.

If you KNOW that the amp you are working on has first stage filters wired in series with smoothing resistors/as a totem pole (if you don't know, refer to previous paragraph), always verify that the standby is in the "ON" position. Leave for a short while (I put the kettle on and make a cup of tea, by the time it's boiled, you're safe. Coffee works just as well). Again, easily visually confirmed, caps won't recharge and much reduced risk of suprises later.

[drewl]

I like getting zapped.....wakes me up.

[R.G.]

the tendency of a capacitor to charge itself is referred to as the chemical charge. the capacitor will slowly charge itself up, even after sitting unused for years, and even when there are discharge resistors across it.

Actually, if you think about it for a minute, this is impossible. If capacitors could recharge themselves to any significant extent with appropriately sized discharge resistors across them, it would in effect constitute a source of neverending electricity.

Capacitors do have dielectric absorption and chemical effects absorption, but both of these are successfully kept to below any reasonable danger level by properly sized bleeder resistors.

What is "properly sized" is the next question, obviously.

I designed power supplies for a living for a long time. We were required to demonstrate that every capacitor in the power supply was safely under 42Vdc and 10J of stored energy within 10 seconds of the main power being cut off irrespective of the normal load. That's a pretty stiff requirement. But 42V and 10J was what the international safety regulations said for our commercial computing equipment.

F'rinstance, if you have 47uF at 500V on your B+. Voltage out of a cap declines at V= Vo*e^(-t/tau) where tau is the time constant, R*C. So we have to have 42=500*e^(-10/R*C), which gives us ln(42/500) = -10/R*C, or
R = -10/C*ln(42/500) = 85.898K
Since E = C/2*(^2), the cap at full voltage has E = ((47E^-6)/2)*(500^2) = 5.9J, we don't have to sweat the stored-energy requirement.

For a cap with an 86K resistor across it, to hold the voltage to a dangerous level (i.e. over 42V) the cap has to supply 488uA FOREVER to get that much voltage to appear across the resistor. That's 20.5mW coming out of the cap FOREVER to get even 42V across the cap plus discharge resistor.

I promise you, that does not happen. If you remove the discharge resistor, either accidentally or on purpose, the cap may well recover some charge that had been stored in chemical or mechanical distortions of the dielectric; and they do. But the constant current-eating of a discharge resistor will keep up with the slow rate of recovery of the cap from this kind of recharge.

As a general practice, mounting a 100K bleeder resistor across every high voltage cap in a fashion that will not reasonably let it come loose will guarantee that the cap is bled down to safe if not zero voltage levels. Having a discharge resistor on each cap with each cap separated by a dropping resistor also gives protection against all single faults of bleeder and/or dropping resistor. 100K does not meet the 42V in 10S rule, but then we're not designing space shot stuff, and really it's difficult to get your hand on a cap within 10 seconds if you don't already have the thing on the bench in front of you.

That doesn't mean that bleeders make it safe. But they do remove some of the hiding-rattlesnake danger of charged caps.

By now you have guessed my favorite way of discharging caps - put a bleeder on them and wait.

[Enzo]

Hey, I have a hacksaw. Are power stations easier to rip off than construction sites?

[bob p]

As far as electrocution hazard it really isn't the voltage but, as you noted, the current flow that kills and some folks with naturally low skin resistance have been killed by as little as 60 volts accidentally applied to a wet chest -

Trivia time. The types of currents that most of us are familiar with fall into the range of what is defined as macroshock by the medical community. Thanks to the high impedance of dry skin, when you're looking at 120 VAC / 60 Hz power, it takes about 1 mA of current to reach the threshold of perception, and more than that to hurt you. 5 mA is regarded as the maximum harmless current intensity.10-20 mA falls into the "let go" range, where you still have voluntary muscle control and you can purposefully remove yourself from the current source. Any currents that are higher are bad. Very bad.

In contrast to macroshock levels of current, people can be killed by microshock, where currents of as little as 20 uA can cause ventricular fibrillation / cardiac arrest. Susceptible people would be those with pacemaker wires, or external conduits that are in direct contact with the heart. Those people are susceptible to electrocution from very "minor" problems, such as stray capacitance buildup on the chassis of a piece of equipment with a faulty ground.

For this reason operating rooms are required to have non-conductive floors, isolated AC power lines (with no direct connection to ground), all of the equipment has to be chassis grounded (to the supply side), and line isolation monitors are in place that will alarm if a faulty piece of equipment were to be plugged into the line. In the event that a faulty piece of gear were plugged into the isolated line, the decreased the impedance from the isolated line to ground would set off an alarm, and you'd have to start immediately disconnecting equipment until the offending piece of equipment was identified and the alarm condition was resolved.

I often wonder why people who are talking safety when working on tube amps don't mention using isolation transformers for the equipment under test, and clamping a dedicated ground lead onto the chassis when they go inside of the amp. A redundant low Z path from chassis to ground is a beautiful thing.

[Enzo]

I don't use an iso on the tube amp chassis because it won't have any effect on the high voltage getting to me or not. I can be just as dead from having the isolated 120VAC coursing through my inner fluids. And all the other voltages like B+ and stuff are with respect to chassis, so that doesn't change.

And I am not sure how the difference between voltage and current on the body helps. The point is to avoid contacting it, whether the voltage or the current is what actually kills you is - seems to me - irrelevant. I mean having that knowledge tells me to make exactly what changes in my procedures to make me safer?

[mykey]

Actually, it's happened with as little as 12 volts...don't even need 60. just depends again on skin resistance, how well the body "hooks up" with the current.

I often wonder why people who are talking safety when working on tube amps don't mention using isolation transformers for the equipment under test, and clamping a dedicated ground lead onto the chassis when they go inside of the amp. A redundant low Z path from chassis to ground is a beautiful thing.

The more experienced will preach isolation transformers, but more commonly used while working on TVs- for tube amps really an excellent suggestion. ground fault breakers not a bad idea either. This whole set up, variac, current meter, isolation tranny, GFI breaker, clamp on ground lead, could be built for about $200 with some scrounging. There are the GFI breakers with a higher trip point for motors that still afford you some increase in shock protection. A safe work area will have a rubber mat to stand on because after all your feet can ground to the floor just as easy as your hand touching the chassis.

[mykey]

Hey, I have a hacksaw. Are power stations easier to rip off than construction sites?

Forget the saw Enzo, be a man and use your TEETH.