18 March 2018
You can often take these old
capacitors to a local utility or recycling
center for proper disposal. Some
locations have hazardous waste
disposal events where they will take
these old components. It should be
obvious that you don’t want to use
them if the oil is leaking from the
seals or if the enclosing can is dented
or punctured. Once discharged, it’s
good practice to install a shorting wire
across the capacitor to avoid
self-charging to hazardous voltages.
Vacuum- and air-variable
capacitors are often used for tuning
or adjusting high power RF circuits.
Vacuum capacitors have voltage
ratings of 10 kV or more and values
up to 2,000-3,000 pF. The electrodes
are copper cylinders, and the variable
models move one cylinder with a
threaded shaft attached to flexible
Air-variables have somewhat lower
voltage ratings. Semi-circular plates
connected in parallel are interleaved to
create the necessary area. One set of
plates is attached to a rotating shaft to
vary the amount by which the plates
overlap to change the capacitor’s
These capacitor types are non-polarized, meaning
they can be used with AC or either DC voltage polarity.
However, you should pay attention to which electrode is
closest to the outside of the capacitor’s insulation. This is
often marked with a line on the component’s body.
The reason this matters is that many capacitors are
connected with one electrode tied to circuit common or
chassis ground. If the capacitor fails shorted, connecting
the outer electrode to the lower voltage point reduces the
If you’re going to connect capacitors directly to the AC
line, you should use X- and Y-type self-healing metalized
film capacitors. X capacitors are connected from line to
line, and Y capacitors are connected from line to ground.
www.tecategroup.com/capacitors/applications/ac-safety-capacitors.php for an overview of these capacitors.)
These capacitors are safety-tested to minimize fire or shock
Dust and Dirt
Dust, dirt, and grime are the mortal enemy of high
voltage electronics. Not only do they make it harder for a
component to dissipate heat, they can be conductive too.
High voltages also attract other particles such as oil mist or
water vapor. The result can be a surprisingly large buildup
inside your equipment.
Once foreign substances get inside, they are
electrostatically attracted to surfaces, wires, and
components by the high voltages. Eventually, enough
A Tube-Based Bench Supply
for Tube Projects
If you like building tube gear, you’ll probably have to
build your own power supply. Not too many manufacturers
are selling adjustable regulated HV supplies these days!
Bryant Julstrom KCØZNG (an ARRL Handbook contributor
and prolific builder) designed just such a power source in
his August 2014 QST article that is downloadble at www.
The supply output of 150-260 VDC is regulated up to 50
mA. A 6. 3 VAC output for filaments is also available. Just
the thing for getting your “hollow-state” designs up and
n FIGURE 4. Cleaning high voltage equipment can be done with a small
paintbrush and home vacuum crevice cleaning attachment. Knock the dust
loose with the paintbrush into the vacuum cleaner. Avoid blowing dust away
as it will only get into hard-to-reach places or into other equipment. (Photo
courtesy of ARRL.)