Safety Questions

[sonolink]

I apologize if these matters have been discussed in the forum or if this isn't the right place to post this, but I couldn't find a thread dedicated to the subject. If anyone knows where this thread is, please don't hesitate to point me to it.

1)
On premise that when you want to use a Multimeter to check i.e. voltage, you should connect the ground first and then the live lead keeping one arm behind your back, my question is: which ground? Are all grounds the same? Are "power ground" and "audio ground" the same? If I have a star ground shall I just keep the black lead plugged to it and use the red lead around?

2)
I built a cap discharge tool as recommended by Jakob on his G9 page. I have also read that it is also recommended after bleeding the caps, to leave the Multimeter on the circuit while working on it. Where shall I "leave" it? Where do I connect it?

3)
Although this is not personal safety, it does relate to components safety. Is it a good idea to fire up a circuit for the first time with empty sockets (no tubes, no opamps, etc) to avoid frying them? If so, do I have to bleed the caps before placing the tubes and opamps in their sockets? How do I know the circuit is ok to place them in their sockets?

Again, I apologize if this has been covered already or it's the wrong place to post. I am about to finish a couple of builds and I really want to be sure about some things before hitting the power buttons and diving in. ;D
Of course, I appreciate any other safety measures/advices you might think of.

Thanks in advance for your time, input and help.
Cheers
Sono

 

[W DeMarco]

You have two things going on, one is using the DMM with the circuit powered and the other is using the DMM with the cicuit unpowered.  Due to the nature of capacitors they often hold a charge for hours and even days when unpowered.  The reason you drain the caps is so you dont get shocked and leaving the DMM verifies that they are drained.  You most certainly want the DMM to be measuring after the big power caps as this is where your charge is gonna be stored. 

If you used a star ground all of your grounds should reference a single point on your build, usually near the transformer at the chassis.  I usually go with my ground, black, lead to the chassis then probe with the red lead for voltages.  Another simple tool you'll want is a wooden chopstick.  The wood does not conduct so it makes a practical tool to probe wiring without the chance of creating an accidental short. 

Alot of folks will build up their power supply first without other components to verify voltages before they begin other sections of the build.  It would be helpful for you to review the build threads for your new pieces to see if it is advised to do your first fire up with or without tubes, opamps or transistors.  Another great thing to do before you fire it up is to inspect everything very carefully, are the electro caps installed correctly, Is the interconnects all going to the right places, are your heatsinks installed correctly with isolation, Is your iec input wired correctly on primary and secondary of tranx.  Check the flipside of your pcb with a magnifier to check for solder bridges and poorly soldered components...  I have been careful on my builds but always find a tweak or two before I fiire it up. 

I have read a good idea for working on live gear is to keep one hand in your back poket so you minimize the possibility of frying yourself.  Check the meta thread Im sure there is more safety info there.  And approach your teching with respect for the gear and voltage, it can kill you...beware.

 

[AK79]

another important point is to always use a safety switch between the unit you are working on and the power outlet. Especially when you power up a finished unit for the first time this is essential to have.

 

[sonolink]

Thanks a lot for your replies.

@W DeMarco
Could you explain a bit further the chopstick thing? I don't understand very well how you use it

AK79
By safety switch do you mean something like a light bulb current limiter?
http://tubenewbie.com/light_bulb_current_limiter.html

 

[Kingston]

@W DeMarco
Could you explain a bit further the chopstick thing? I don't understand very well how you use it

Sometime you're going to want to move wires or other objects around in your project when it's live. Talk about 370VDC live medium current wires that will kill you on contact. Move things around with chop sticks only for safety.

Stuff like wires interfering (noise) with each other, or maybe you want to stick your multimeter into a more difficult spot and you have to make room with chop sticks.

Be safe!

Light bulb current limiter, http://www.repairfaq.org/samnew/tvfaq/tvtslbt.htm

 

[lassoharp]

When working with tube circuits I would highly advise using 2 DMMs.

Leave one set to constantly monitor the supply output voltage (B+).  This way you can always see what's going on in the most dangerous part of the circuit.  Alligator clips on your probe leads will aid greatly in securing a good ground connection.  This connection should be made between ground and the final PS filter cap just before it feeds the plate circuit of the output stage tube.  Your ground, star(audio) or safety (fault current) should be tied to the chassis back at the power supply.  Secure your DMM ground probe there.

Use the second one for taking routine measurements - plate, cathode voltages, filaments etc. .

Bleeding the big caps should always be done before poking around.  With a DMM set to monitor, you will never be in doubt about how much is left or how long to bleed.

If you have a variac with an ammeter these are invaluable for saving a trainwreck (major or minor).  If not here's a general checklist to follow before throwing the juice.

1)  Make sure the mains fuse is wired correctly and contains the right value fast acting fuse.

2)  Double check pinout wiring on the tube sockets .  Turning the circuit on with no tubes in the sockets shouldn't harm anything - you can do this and at least verify that the B+ is going to the plates, and filament voltage to the filaments.

3)  Double check filament wiring.  You can use a DMM and do a continuity check from where it leaves the power transformer to each tube.  The worst thing that can happen here is having the leads short together which could eventually melt the PT winding, or having an open which would leave the tubes cold.  HV on cold tubes could cause damage to the tubes.

4)  Make sure the big PS electrolytics (and cathode bypass) are wired in with the correct polarity.

5)  Use the safety switch AK79 advised with a quick finger and quick eye on the PS monitoring DMM as you fire it up.  Be weary of a B+ voltage that is way low - like 65V or something.  This would suggest something is drawing too much current( often bad).  If the B+ goes and stays really high it would suggest you're drawing little or no current (minor problem).

You shouldn't need to do much prodding around for basic operation. Closely watching the PS and other circuit voltages will tell most everything you need to know.

 

[zebra50]

It has been hinted at above, but it is a good idea with newly built or newly acquired vintage gear to bring it up to speed using a variac whilst keeping an eye on the voltage. That way you can spot disasters occurring before your gear melts (like plugging 110V gear into 240V!)

The 'safety switch' referred to would be a 'power breaker' or residual current circuit breaker. I keep one of these handy and tend to use it on equipment until I trust it. Of course you should have circuit breakers on your mains supply anyway - (at least we do in Europe, if your wiring meets modern standards)

A good set of 1000V insulated electricians screwdrivers is a godsend.

 

[sonolink]

@Kingston
Oh I see what you mean.... mmm I've always been a disaster eating with chopsticks...I guess this will make good practice to improve my skills!! ;D

@lassoharp
Thanks for the checklist and general advice, but there's a couple of things I'm not sure I understand (sorry, I'm a bit new to this and english is not my main language)

Leave one set to constantly monitor the supply output voltage (B+). 

Is B+ the voltage feeding the tubes? (In the case of the G9 that would be the 2nd toroid primaries output before BR1)
What is the difference between pinout wiring and filament wiring?

Finally, which one: variac or bulb limiter?
Thanks to all for the advices and tips.

 

[lassoharp]

@Kingston
Oh I see what you mean.... mmm I've always been a disaster eating with chopsticks...I guess this will make good practice to improve my skills!! ;D

@lassoharp
Thanks for the checklist and general advice, but there's a couple of things I'm not sure I understand (sorry, I'm a bit new to this and english is not my main language)

Leave one set to constantly monitor the supply output voltage (B+). 

Is B+ the voltage feeding the tubes? (In the case of the G9 that would be the 2nd toroid primaries output before BR1)
What is the difference between pinout wiring and filament wiring?

Finally, which one: variac or bulb limiter?
Thanks to all for the advices and tips.

The B+ on the G9 looks to be right after R37(470R) - 245VDC.  Strap the positive probe there. Neg to ground of course.  Yes, B+ feeds the tubes.

Pinout just means make sure the correct lead is tied to the correct pin on the tube socket whether it's filament or plate or other.  If you're working with a pre-fab PC board it shouldn't be an issue.  If you're manually wiring it point-to point you have to be more careful.

In your readings I would suggest checking out the US Naval Training manual for tubes - aka NEETS modules.  They have an excellent section on the basics of tube power supplies.

Hopefully this link will work:

http://www.rarmy.com/coleman/neets/

Module 6 = tube PS,  Module 7 = sol state PS

 

[sonolink]

Thanks for the link Lasso 

One final question if you don't mind: variac, bulb limiter, circuit breaker? Is there a real difference or advantages between them or is it just a matter of taste or personal preferences? Thanks again for your time.
Cheers
Sono

 

[lassoharp]

The circuit breaker is going to act like a fuse and is a necessity for protection.

The variac with ammeter(some don't have them) is maybe a bit more versatile than the bulb for it's purpose but they both will work.

The variac/light bulb tells you when you need to turn something off - the circuit breaker is preset to do the turning off when you're not quick enough. That's the difference. One just spots the trouble slightly beforehand, the other does something to prevent it from reaching meltdown.

 

[Speedskater]

Light Bulb Tester Schematic:

 

[sonolink]

I guess the best thing would be to have both bulb limiter and breaker then
Thanks for all your answers and advice
Cheers
Sono

 

[Speedskater]

Yes the "Light Bulb Limiter Tester" is only for testing and troubleshooting activities, the circuit breaker (or Fuse) is forever.

 

[Mike Cleaver]

As someone who has worked on high powered radio transmitters (up to 50 thousand watts,) the "keep one hand behind your back" is THE golden rule.
After powering down a transmitter in the old days, the first thing you did was to take a "shorting stick," usually a wooden broom handle with a metal hook on the end that would fit between the terminals of the huge capacitors inside.
You would short all of them before even putting a finger inside.
We would usually stand on a thick rubber mat as well.
And you would never work on a high powered transmitter alone.
You always had someone with you, even a friend who could call for help if something happened.
Working on tube gear can be fatal as many here and elsewhere have pointed out.
Shorting or draining the power caps is a must.
If you are going to work inside a "live" chassis, use (as others have mentioned) a wooden chopstick or insulated probe.
Rubber gloves can work for you if you want to be extra safe and stand on a rubber mat.
Make certain your work area is clean and dry so nothing can "fall inside" a live piece of equipment and short.
Capacitors have been known to explode.
Wearing glasses or protective goggles can be an eyesaver.
Some may think all of this is overkill.
One good jolt is all you need.
I never power up any old tube gear without a variac.
And I replace power supply electrolytics first if the unit is more than 20 years old before I even think about plugging it in.
If there's no fuse, install one.
If there is no grounded power cord, install one.
Safety first means a long and rewarding career or hobby in electronics.

 

[sonolink]

Mike,
I appreciate your comments and sharing your experience. Precisely because 30 years ago I was electrocuted and had my left hand burnt because of it, although it was in an altogether different context than fixing or building amps, I have developped a compulsive obsession for "Health and Safety Regulations", whether on the place I work or at my place with my hobbies. Since I got into the pleasure of DIYing clones and such, one of my main concerns has always been safety, reason why I have opened this thread about safety. I am about to finish 2x SSL and 2x G9 clones. I chose them because they are proven builds. I have tried to be as meticulous and careful as I could think I could be, so hopefully they will present no problems. But, just in case, to be on the safe side, I wanted to have more information, especially on the "standard" or obvious procedures.
I would never build a unit without a fuse. Although I have circuit breakers in my house, I will install an extra one on my bench. I will build a bulb limiter and use it. I have already built a cap drainer following Jakob's specs on his page. I have 2 DMMs and will use them following the tips given in this thread. I still need to get the chop sticks
Incidentally, I have been looking at some variacs with ampmeter. They are kinda expensive...
Cheers
Sono

 

[zayance]

Good post of Sonolink, cool to have a Post on safety that tells it all, there is a lot of infos here and there but one main is better i think, so anybody wanting to mess up with HV will know what to do, personnaly i would go all the way safety as Sonolink, hand on the back, gloves, circuit breaker, safety switch etc......, and of course even after years of practice never think you know enough, that's where you can loose 

 

[Speedskater]

As Mike wrote - broadcast radio transmitters are very, very dangerous!!!
I know of three engineers that were killed while working on radio transmitters.
a) While building a new station, the engineer fell onto a large floor mounted transformer.  This was before the safety cage was erected.
b) While doing maintenance on spare transmitter, engineer did not upon a safety interlock. C.E. turned on the transmitter from a remote location even after being told of possible maintenance work.
c) A independent sales engineer was doing maintenance work on a customer's transmitter. The "Shorting Stick" was defective and did not discharge a very large capacitor.

 

[AK79]

Hi Sono,
actually I was talking about a circuit breaker which is switched between the device you are working on and the outlet. This detects voltage on the safety earth.


I´m using this one:

http://www.reichelt.de/?;ACTION=3;LA=444;GROUP=D7731;GROUPID=4280;ARTICLE=69709;START=0;SORT=artnr;OFFSET=16;SID=25xs0wMqwQARkAADzAHKE690b3f1d588ee11c76168d56f250c53a

I don´t know what´s the correct name in english and whether it is available in your country. The big advantage of this thing is that it is fully automatic.

cheers
Arne

 

[sonolink]

Arne,
That's very handy!
I did understand what you originally meant and was thinking of building one on my bench so that I could plug the device in it, but what you show on that link is much easier and cheap!! ;D
Thanks for the link
Cheers
Sono