• WARNING: Tube/Valve amplifiers use potentially LETHAL HIGH VOLTAGES.
    Building, troubleshooting and testing of these amplifiers should only be
    performed by someone who is thoroughly familiar with
    the safety precautions around high voltages.

Safety Practices, General and Ultra-High Voltage

Why not use the parallel windings?

I guess I didn't write it clearly. The transformer in question is a guitar amp transformer that has extra secondary circuits of a different, unneeded voltage.

Unused wires should go to an isolated terminal that is not
connected to anything else. Never use electrical tape or heat shrink tubing instead of a terminal.

That's what I was thinking. Use a barrier terminal strip to terminate the unused wires. I was even thinking about potting the terminal after connecting the wires so that someone would have to cut the wires to get burned.
 
I'm not averse to clean cutting the end of an unused tap wire, and then heatshrinking the end, and then coiling/securing (preferably under a bell-end to minimise stray coupling). That can free up terminal strips, and reduce risk of accidental touching or leakage or coupling.

If the tap wire comes from an internal solder tab (eg. Under a bell end) then better to remove the tap wire.

Future diy user can then use those taps if ever needed.
 
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Do you think a retired welfare guy has enough cash to afford all the equipment needed to fully and properly service your TV/VCR?

There are some retired welfare guys who made their living teaching the folks that built those TVs and VCRs how to build them, and how to repair them.
When it comes to tubes, and the HV supplies for them, I would only trust 100% the guy who graduated with a degree in electrical engineering, in the '40s or '50s, and then spent the better part of his career in that field. so that narrows it down just a tiny bit. ;)
 
Put a drain load across all significant charge-storage devices

I have no idea how to do this

In simplest terms, its called “a resistor”. In my secret-squirrel engineering days, we just always specified that “large can capacitors” have hard-wired an appropriate resistor across their terminals. For instance:

C = 10,000 μF at 200 V (which is really large), and the target is to get it to discharge at more than 10 volts per second.

There's a relationship:

ΔV = it/C

or change-in-voltage ΔV is current (i) times interval time (t) over capacitance (C). We want:

10 = it/0.01 …
0.1 = it … where i is unknown and t = 1 sec
0.1 = i

Since we know Ohm's Law: (V = IR … R = V/I) then

R = V/i
R = 200 ÷ 0.1
R = 2,000 Ω.

Since we also know the power law (P = IV) then

P = IV
P = 0.1 × 200
P = 20 W

So now we know that it needs to be a 20 watt resistor. Is losing 20 watts continuously a problem? Too much? Not what you want? Well, work backward from the voltage and power, and accept a higher value resistor that'll drain away a lesser amount of power. Then calculate out the time-constant, and also accept that instead of 10 volts/second, your drain plug might only deliver 1 volt per second.

There is no free lunch here, either. You can optimize the drain-resistor idea by using a constant current device to maximize the total time to discharge the capacitor; you can even use a more sophisticated constant power device (which are hard to come by) to totally minimize the capacitor's retained charge time. But … compared to the bûtt-ordinary simplicity and utility of an ordinary power resistor, well … sometimes ordinary beats the competition.

GoatGuy
 
Old, Bold Pilots

There are some retired welfare guys who made their living teaching the folks that built those TVs and VCRs how to build them, and how to repair them.
When it comes to tubes, and the HV supplies for them, I would only trust 100% the guy who graduated with a degree in electrical engineering, in the '40s or '50s, and then spent the better part of his career in that field. so that narrows it down just a tiny bit. ;)

OK, that was fun.
I “kind of agree” re: the need for an educated person to handle HV.

But I don't really agree that it has to be “degree in electrical engineering” plus “better part of career in field”. It certainly does take a mighty fear of self-electrocution and the safety of others to do HV safely. But its also a practical thing that only a year or two of apprenticing gives a life's worth of experience and expertise.

I — as a for-instance — am pretty dâhmned good at not self-immolation. I'm also 59, and have a life's worth of HV hacking. Wasn't my career, but just an avocation that never dissipated along with my bad teeth and gnarly knuckles. I don't trust very many people in doing HV at all. But the ones I meet that I do trust … gain that trust in only a few minutes of talking, and watching them work. You can tell a 'safe' HV guy (or gal!) in less than 5 minutes.

They don't TOUCH things.
They often have ONE HAND behind their back.
They LOOK DOWN at the floor often, and stand on a rubber mat.
They WEAR PROTECTION for the really HV stuff.
Their TOOLS are impeccably maintained.
And they LOVE thick layers of electrical tape.

Anyway - there you are.

As helicopter pilots say, “there are old pilots, and there are bold pilots. There aren't any old, bold pilots.” Same for HV expertise. Timid rules.

GoatGuy
 
I read all the stories and fighting about safe H. V. Working and I just wonder were we more careful when i started as apprentice in 1966 or did 400 volts became more dangerous today than then? I am more likely think that the more junior guys are so used to low voltage Solid State the t we want to scare dear life out of them working on Hi Fi in Vacuum Tubes? We must work safe but common sense and safety knowlege is what I was taught. Okay hi powered transmitters working with kilovolts and kilowatts is a different story and I hate insulation tape. I grew up with the old tar impregnated linen tape that after a while absorbed moisture and loose stickyness after some time. Shellac sleeving or called spaghetti we preferred ehere possible. ( I still wonder why we called it spaghetti and not macaroni?)
If live vo!tage chechecks have to be done I use crock clips and turn power off connecting my instruments.����

REMBER THE SAFEST MAINS SWITH IS PULL THAT MAINS PLUG OUT!!! It cannot climb into the socket and leak power due to faulty switch or accidentally turned on
 
Recent news article: this is actual text, with somebody's quote..

""A 34.5 kilovolt supply line went out for 10 seconds at 9:38 a.m., said city Department of Water and Power spokeswoman Carol Tucker. The line serves 26 industrial customers at the airport.

Apparently, a bird came in contact with the line and at the same time touched a cross-arm or some other grounded device, the DWP said in a statement. The line re-energized moments later.

"But we didn't find the bird on the ground, so it might have flown away,"""

Hmmmm...might have flown away...

Perhaps someone had a dustpan and brush available to remove the evidence..

Cheers, John

At my work we used to have a huge 50 horse 3 or 4 phase electric motor that powered an irrigation pump. There is only so many lucky charms, crossed fingers, incantations, etc, that one can do before pushing the "on" button on the relay for that motor. Once, there was a fault i think in the ground leg at the junction box, and fortunately no one was injured or anything. But i remember that before they cut the power at the utility pole you could feel like a buzzing sensation through your shoes the closer you got to the junction box. That was pretty scary ****!
 
Thanks for starting this thread. If it develops well, we'll make it permanent.

Just some general comments that I've made before:

When you decide to work on high voltage circuits, spend time around experienced guys. Never work alone. For ultrahigh voltage stuff (high power supplies at 1kV and beyond), all the rules are different and you would be INSANE to the level of suicidal to tackle such a thing without the direct supervision of an experienced guy who wishes you no harm.

Too paranoid is much preferable to not paranoid enough.
Copy that....For some reason I detected luck in survival during my mixed high voltage life, now 60 yrs of it....but once in my earlier days nearly came a cropper when a field engineer asked me out to inspect /opinion on a very high 420 KVolt MW substation transformer which also handled AM signal sensoring for tap changing, and was making peculiar noises..It was huge, bigger than a shipping container. One has to take anything metallic off wedding rings etc due to induction fields, and although I was proficient in the maintenance work, at 50 yds the inconsistent lamination noise made me darned frightened and the case paint was smelling. To me it came across as a fifth sense and I said 'no way' without all being disconnected; and as we both turned around and backed off a hundred yards, it all blew up. Thanks for the visit......There are plenty of transformer exploding videos on YTube.

After that unforgetful event I've always double-eyed those massive installations with the utmost respect considering many are in residental areas!

Those of us HAMer's who have the kudos for high voltage, fine when the personal rules are observed. My rule is don't take on what one cannot handle and worse.... Don't guess. Never have both hands on an active chassis as the link via Cardiacs becomes the perfect last call.

rJ
 
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I don't know if this was posted before, but NEVER EVER WORK ON THE AMPLIFIER IF YOU ARE TIRED. Not even soldering.

I remember I was quite tired but really wanted to complete the build that night. Finally I put everything together. Turned it on and bang - bias power capacitor blew up. Of course put it in a wrong polarity. I took another one put it in place. Turned on the amp and bang - same thing. Looked closer and of course I put it the wrong way one more time. Fortunately at this time I was out of capacitors so stopped working on it till the next day.

In this case nothing bad happened, but it could be much worse.
 
Hi,

When you have to measure at switched-off equipment, ALWAYS ground the HV to chassis, I saw accidents caused by people switching on equuipment not knowing that someone was testing!!

Grounding circuits and locking switches are mandatory here especially when the the switch/fuse is remote and not visible from the eqipment site.
To avoid someone else turn the circuit on while working on site/equipment , I got these professional warnings signs. Written in german saying " Do Not switch ! Danger!"

Here:


They stick magnetically to frames of amps /transmitters etc and at the door of the fuse box.

Fiddling in HV circuits is always danger. Working on it need to be always conscious with a clear mind. The best is reduce the work on powered equipment to a minimum. The most of alignment and trouble shooting works with only heaters and drive power while the HV is turned off.

voltages above 1kV are serious danger , every one work with it shoud exactly know what he do. Otherwise... :RIP:


73
Wolfgang
 
wg_ski if you are quite concerned (and rightly so) then it is well worthwhile measuring the creepage and clearance distances, and then trying to find compliance distances for your countries electrical safety standards.

You will likely find the allowable distances (from a compliance perspective) relate to the operating environment you have - especially if dust and debris (pollution) can build up over time. The compliance distances are a good reminder of what companies have to meet to keep you safe when you buy and use commercial equipment, and imho that is a good view to have in mind for diyers.

Google is your friend for creepage and clearance, and standards compliance details can often be found with a bit of effort - eg. pcb routing software may have design guides.
 
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I would not trust them, particularly if they are bakelite and you live in an even slightly humid climate or are a messy solderer. (And I build lots of stuff out to 1kV or so, don't use them anywhere.)

Ceramic turrets or the mounting strips Tek used in their old scopes are suitable for use to voltages > 500V.
 
The ones in question happen to be made of red fiberglass and have over twice the stand off height of the classic ones you see in point to point wiring with 300V B+. But the same spacing between terminals. I used them to make the rectifiers for load testing, but I was wondering if the final construction needed to be different altogether or if I could get away with more voltage just by moving things apart. On a related note - which is safer as a screen regulator, a full-pack TO-220, or a TO-247 which requires an insulator but has more space between drain and source. Of course, similar drain voltages and I do want to heat sink the transistor. I’ve got a dozen of both types of hexfet to work with.