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cathode itself can be heated by an electric current through a filament
like a light bulb or indirectly heated by a nearby filament.
A two-element tube or diode (“two-electrode”) can act as a rectifier,
which is basically a one-way current valve. Fleming’s diode was used
mostly as a detector for RF signals by turning their AC current into a DC-plus-audio current that could produce audible sound.
It was de Forest who created the triode by adding a third element:
a control grid in the space between the cathode and anode. By varying
the voltage between the grid and the cathode, the flow of electrons can
A positive voltage accelerates the electrons toward the grid but
most miss its tiny wires and reach the anode, creating plate current, IP,
through the tube. A negative voltage repels the electrons, reducing plate
PRACTICAL TECHNOLOGY FROM THE HAM WORLD
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High Voltage Safety
The following 10 items of advice from veteran builder,
Jim Garland W8ZR are adapted with permission from the
newly-released 2018 edition of the ARRL Handbook and
apply to any project involving high voltage — whether tube
circuits or some other project.
We are all besieged these days with verbose safety
warnings on mostly harmless consumer goods and have
become accustomed to solid-state circuits in which we
seldom encounter any AC voltage higher than 12V. It’s easy
to forget that some things really are dangerous, and high
voltage is one of them.
1. Don’t let your reach exceed your grasp. High voltage
is not for beginners.
2. Working with high voltages requires the maturity and
patience that comes with age and experience. If you are a
young builder, work with an experienced mentor.
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 any energy storage or filter capacitors; and verify
that the voltage is truly zero. A 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 4,000V 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 high voltage circuits 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
n FIGURE 1. A cutaway drawing of a triode tube showing the filament
heating the cathode, which is the source of electrons. The control grid
surrounds the cathode and the electrons must pass through it on the
way to the anode or plate. In a tetrode or pentode, there are additional
grids between the control grid and the plate. (Graphic by Svjo (Own work) [CC
BY-SA 3.0 ( http://creativecommons.org/licenses/by-sa/3.0)], via Wikimedia Commons.)