On 1/6/2017 5:45 PM, Mark Bitterlich wrote:
The fact that some young man or woman might decide to build a KW amp a
week after getting their ham ticket (and coming here for help to do
so) does not bother me at all, in fact it shows some common sense.
Mark
wa3jpy
I don't want to be argumentative, but I think the above perspective is
dangerously misguided. I've been homebrewing ham equipment since I was
twelve, starting with a Heathkit AT-1. At age thirteen, a ham buddy and
I were playing "Hot Potato," by tossing a charged ten microfarad 1000V
oil capacitor back and forth. He caught it wrong and the shock knocked
him to the ground and sent him running crying to his mom. Stupid. At
sixteen, I built a homebrew amp running four 6AG7s, stealing the HV by
running a piece of hookup wire clipped to the plate cap of the 6146s of
my AM rig, across the desktop, to the amp. Also stupid. At seventeen, as
a senior in high school, I unscrewed the coax connector to my pair of
813s, not realizing that the coax braid was the ground return to the
2000V power supply. I was thrown across the room and dislocated my
shoulder when I crashed into the wall.Stupid, maybe. Ignorant, very.
Fast forward six decades, and I'm still homebrewing. My most recent
project (a duo-band 80m/160m amp running three GU74Bs), took three years
to design, build, and debug. The design and circuitry is my own, and the
enclosure, metalwork, printed circuit boards, etc., are homemade. The
transformers were custom made, and with a few exceptions all the parts
were new and unused. I've not added up the cost, but I probably spent
$3000 on it, spread over three years. It's probably the best thing I've
ever built. I mention this, not to show off, but to make the case that
I've paid my dues by spending most of life reading and studying and
experimenting and building electronics. A year or so ago, I wrote an
article for QST/QEX (one of many over the years) on one of my HV power
supplies. In it, I summarized my personal safety rules which have
probably saved my life several times. Here they are. Please note,
especially, number two in the list:
1. Don’t let your reach exceed your grasp. This is not a project for
beginners. You should not attempt to build [a high power amplifier]
unless you’re a seasoned builder who has experience with high voltage
circuitry.
2. Young amateurs should not attempt this project. Working with high
voltages requires the maturity and patience that come with age and
experience.
3. Never work around high voltage when you are tired, stressed, or in a
hurry.
4. Never work around high voltage after drinking alcohol. Even one beer
or glass of wine can impair your judgment and make you careless.
5. Before working on a high voltage power supply, always follow these
three steps: /Unplug/ (the AC power cord), /discharge/ (the filter
capacitors) and /verify/ (that the output voltage is truly zero).
Time-honored practice is to use a “chicken stick” (a wooden dowel or PVC
tube, with one end attached to a grounded wire) to make sure filter
capacitors are completely discharged.
6. When working on a high voltage power supply, remember that a
dangerous time is after the power supply has just been turned off, but
before the filter capacitors have fully discharged. A 50 µF capacitor
charged to 4000 V holds a potentially deadly 400 Joules of energy. Even
with bleeder resistors, it can take a minute or more to discharge fully.
7. When removing a recently discharged filter capacitor from a power
supply, tie the two terminals together with wire. Large high voltage
capacitors can self-charge to dangerous levels if the terminals are left
floating.
8. Don’t stake your life on the expectation that bleeder resistors,
fuses, circuit breakers, relays, and switches are always going to do
their job. Even though modern components are very reliable, it is safe
practice always to assume the worst.
9. Don’t build this power supply if you don’t understand how the circuit
works. High power amplifiers and power supplies are not “plug-and-play”
projects with step-by-step instructions. Builders must be knowledgeable
enough to improvise, make component substitutions, and implement design
changes.
10. With high voltage projects, it doesn’t pay to be “penny wise and
pound foolish.”Use high quality components throughout and save your
forty-year-old junk box parts for projects where safety and reliability
are not paramount requirements.
So I don't know about the rest of you, but this is what I've learned
from the School of Hard Knocks.
73,
Jim W8ZR
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