Vacuum Fluorescent Display Amplifiers For Primitive Radio
H. P. Friedrichs, AC7ZL
December 10, 2008
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Vacuum Fluorescent Display Amplifiers For Primitive Radio
By, H. P. Friedrichs, AC7ZL
Vacuum tubes represent an electronic technology
whose roots extend a century and more into the past. With
the exception of their continued use in certain niche applications,
they have all but vanished from contemporary electronics, having
been replaced by an endless array of solid state switching, amplification,
and display technologies. Yet, despite all of this, it seems that
interest in vacuum tubes and the "hollow state" technology
that makes them work refuses to die.
A Typical Vacuum Tube
Iíve done a fair bit of tinkering with vacuum
tubes, even to the extent of building a few of my own. I
actually wrote a book about it. This article, however, comes
from a different vantage point. In essence, Iíve identified a piece
of contemporary electronic junk that, while not intended to be a
vacuum tube in the classic sense, shares the necessary features
of radio tubes. It can be induced to amplify feeble audio signals
from primitive radio gear like crystal radio sets. Such experimentation
is a lot of fun, and the end result is actually useful.
That said, let us not overlook the fact that
most vacuum tube experiments, including the ones to follow, involve
the use of high voltages. Besides the obvious dangers associated
with electric shock, miswired batteries can overheat and short circuits
can produce fire. My presumption is that the reader is familiar
with basic electrical principles, competent with regard to safe
practices, and endowed with some level of common sense. Remember,
you are responsible for your own safety.
Before I describe what Iíve been playing with,
it is worthwhile to engage in a quick review of what makes a classic
vacuum tube what it is. It would actually take volumes to discuss
this in a comprehensive manner, but letís summarize things this
way. We start with a common light bulb, that is, a wire filament
contained in a glass bulb from which all the air has been evacuated.
If we add a metallic plate to the interior of the bulb, and
place a positive charge on that plate, electrons will actually leave
the surface of the heated filament, float through the space between
the filament and plate, and strike the plate. In other words, an
electric current can be made to flow between the filament and plate.
Current will not flow from the plate back to the filament, making
such an instrument a one-way check-valve, or diode.
The diode form of tube has numerous applications,
not the least of which is the extraction of audio from amplitude
modulated (A.M.) signals. But letís put that aside for the moment.
Once we set things up in the tube described
above, the current between the filament and plate is constant. However,
if we introduce a wire mesh or "grid" between the filament
and the plate, and then apply varying electrical charges to that
grid, we find that the flow of current between the filament and
plate will also vary. This is the essence of amplificationóthe idea
that a tiny signal can be used to control the flow of a more powerful
The triode (From Vacuum
Tubes in Wireless Communication, Bucher, 1919)
The Vacuum Fluorescent Display
Many of the electronic gadgets in the modern
world speak to us through information displays of some kind. All
sorts of display technologies are used to communicate functional
status and numeric values like time, voltage, temperature, weight,
and mileage. Among these is the vacuum fluorescent display, or VFD.
VFDs are luminescent displays, that is, they
glow. VFDs typically emit a pleasing blue-green light, and the glowing
images produced by the VFD can be fashioned to represent numbers,
letters, bar graphs, icons, or other symbols. Where can one find
VFDs? Everywhere! They are commonly used as displays in appliances
like microwave ovens. They are used in industrial electronics like
digital panel meters and thermometers. Iíve seen them in toys, bathroom
scales, stereo equipment, and office phones. The information display
for the entertainment system in my 2008 Ford is a VFD.
Internal Structure of the VFD
Like a vacuum tube, a typical VFD consists
of a glass container from which all the air has been removed. Because
VFDs are display devices, the glass envelope tends to be somewhat
flat and rectangular in shape. The display characters-- the symbols,
words, and segments that actually light up-- are composed of electrodes
called anodes that are coated with phosphorescent chemicals. Typically,
these chemicals contain zinc, selenium, sulfur, and other trace
Stretched across the interior of the container,
suspended above the display characters, are several filament wires.
These wires are powered by a low-voltage supply to heat them. Although
these filaments lie between the observer and the characters, they
are composed of exceedingly thin wire and are not heated to more
than a very dull red, so you wouldnít even notice them unless you
actually looked for them.
If we apply a source of low voltage to the
filament, it will heat up and generate a cloud of electrons. If
we then apply a high-voltage positive charge to the characterís
electrodes (the anodes), electrons produced by the filament will
be attracted to this positive charge, and will strike the phosphorous
chemical coating on the characters. When stimulated in this fashion,
the characters glow.
Of course, when the display is in use, we
donít want all of the display items to be glowing at once. Rather,
we want to be able to control the display features independently,
and turn them on and off as necessary. To enable this, VFDs contain
another structure: a thin mesh or grid that lies between the filament
and each phosphorescent display feature. If we apply a positive
charge to the grid, electrons from the filament are encouraged to
move toward the phosphors and induce them to light. If a negative
charge is applied to the grid, electrons from the filament are repelled,
they fail to reach the phosphors, so the display feature goes dark.
Below is a photomicrograph of a section of
a typical display. In this image one can see a display feature/anode
(the word "TEST"), a honeycomb-like control grid that
covers the word, and one of the displayís five filaments, which
appears as a thin, white, horizontal line suspended above the grid.
Internal features of the
While the purpose of a VFD is very different
than that of the average radio tube, there are similar structures
in both. Both contain an electron source, the filament. Both contain
a positively charged target. In the radio tube, this electrode is
called a "plate." In the VFD, this electrode is called
an "anode," and takes the form of a phosphor-coated word,
symbol, or display segment Finally, each device contains a
grid structure, which is used to control the flow of electrons from
the filament to the anode/plate. The similarity between VFDs and
the types of radio tubes Iíve been used to playing with prompted
me to wonder if a VFD could be used as an amplifier. Interestingly,
the answer is a definite yes.
Wiring Up a VFD
Wiring up a VFD is a simple matter. The key
is knowing which terminals on the VFD do what. This information
can be arrived at in two ways. If one is lucky, a Google search
of the part number for a display may yield a manufacturerís datasheet.
A datasheet will indicate which pins are connected to which internal
structures, and it will also provide useful information regarding
the proper voltage to apply to the filament. The other way to derive
hookup information is to study the display visually, make a few
measurements with an ohmmeter, and deduce what one needs to know
from these observations. My VFDís were salvaged from junk,
so I took the latter route.
The first step is to identify the filament
terminals. The filaments appear as long, thin, hair-like wires that
run parallel to the long axis of the display. I would expect to
find at least three, or possibly more, but the exact number depends
upon the type and dimensions of the display. The ends of each filament
wire terminate at little metal bars or tabs. The tabs feed electricity
to the filaments, support them, and in some cases, maintain tension
on them so that they wonít sag. The first goal, then, is to study
the VFD and its internal structures to determine which pins are
connected to the filament.
The VFDís internal structures are connected
to external pins through strips of thin metal or conductive traces
that are applied to the glass inside of the VFD. Knowing this, one
can simply follow the path of these traces with the naked eye.
Traces on the backside of
Once a pair of filament terminals have been
identified, they can be verified with a multimeter set to read ohms.
If one probes between suspected terminal pins, one can expect to
see conductivity and a resistance reading one the order of 10ís
Next, one must identify the pins associated
with each character or display feature, the VFDís anodes. This is
purely a matter of visual inspection. One chooses a display feature--
a word, symbol or perhaps a segment of a numeric figure-- and then
tries to identify a signal path from that feature back to one of
the external pins. This determination must be done for all of the
display features. As my VFD was a reasonably complex device, I recorded
which feature/anode connected to which pin on a sheet of lined paper.
Finally, all of the grid connections must
be identified. This is also a visual exercise. One must choose
a grid, study the VFDís internal construction, and identify a path
from that grid to one of the external pins. This must be repeated
for each of the grids in the VFD. Again, I found it helpful to record
my findings on paper as I studied the device.
Lighting Up the VFD
Lighting up the VFD requires two power supplies,
a low-voltage supply for the filament, and a high-voltage anode
supply to light up the characters.
Filament power requirements differ depending
upon the specific VFD. However, of the parts Iíve tinkered with,
the lowest filament voltage I saw was on the order of 3 volts, and
the highest appeared to be 6 volts. I would suspect that other VFDís
have requirements that will fall in this range.
Filament voltages are easily achievable with
flashlight batteries. Two "D" cells wired in series will
get you 3 volts. For a 6-volt filament, 4 "D" cells in
series are the answer. Batteries work just fine, but I preferred
to take advantage of one of the variable power supplies on my bench.
In any event, the supply is connected to the filament terminals
that were identified earlier. When all is well, the filaments should
barely glow when viewed in a darkened room. If the filaments are
bright red, this is indicative of filament voltage that is far too
high. Continued operation at this level will eventually burn out
VFDís also require a high-voltage supply to
light the display features. In normal use, the "high-voltage"
supply can actually be quite low, i.e., I have lit VFDs with as
little as 15 volts on their anodes. However, in the radio application
I am about to describe, higher voltages work much better. In that
case, a D.C. power supply on the order of 90 volts or greater is
just the ticket.
As is the case with filament voltages, the
high-voltage anode current can be supplied with batteries. Ten 9-volt
batteries, connected in series, will deliver 90 volts. Given the
modest energy requirements of the VFD, batteries are a practical
power source, and ten batteries in series should last a long, long
A comment is in order, here. A 90-volt battery
is capable of inflicting an unpleasant shock. There is also sufficient
energy in such a battery to cause a fire if it is abused. I have
built high-voltage batteries of this type myself. I always prefer
to install them in an insulated container, which is then fitted
with binding posts or fahnestock clips. I usually insert a small
fuse inside the battery container, wired in series with the batteries,
to safely limit the current in case Murphy strikes and a mistake
is made. In this particular instance, I avoided having to build
a high-voltage battery by using a variable power supply I constructed
a few years ago.
To light up the VFD, we first activate the
filament as described above. The negative terminal of the high-voltage
supply is connected to one of the filament terminals (it doesnít
really matter which) and the positive terminal of the supply is
connected to one of the VFD pins that feeds a character or symbol
anode. Finally, the grid (corresponding to the feature we desire
to light), is connected to the positive side of the high voltage
supply through a large resistorósomething on the order of 4.7 megohm.
If the VFD is wired in this fashion, the experimenter will be rewarded
with blue-green light. If one applies the appropriate signals to
other grid and anode pins, other display features can be made to
A circuit to light up the
For the purpose of the experiments to follow,
the final preparatory step is to use a soldering iron and some wire
to connect all of the anodes together. Likewise, we must connect
all of the grids together. This leaves a VFD with what amounts to
four terminals: two filament terminals, one "super" grid
terminal (the sum of all grids), and one "super" anode
terminal (the sum of all anodes). In this "super" configuration,
a positive high voltage applied to the "super" anode (with
the 4.7 megohm resistor connected to the "super" grid)
should light up all of the display features simultaneously.
The VFD Amplifier
One of the first observations I made occurred
after I had lit my VFD in the "super" configuration. I
broke the anode connection and inserted a high-impedance headphone
into the circuit. I lifted the grid terminal and allowed it to "float."
Immediately, the headphone was filled with a deep 60-cycle buzzóthe
kind one hears if the patch cord for a guitar amplifier is left
dangling. It was obvious that the VFD was functioning as an audio
amplifier. This prompted me to wonder if there was benefit in connecting
this to a crystal radio.
The CDROM radio
Amplifying the output
of the CDROM radio. From left to right: Headphones, high-voltage
power supply, low voltage supply, CDROM radio, and in the center,
two different VFDs.
Simplified schematic for
the CDROM radio and VFD amplifier. Click
here to see it in higher resolution.
Above is an image of my "CDROM"
radio, a crystal radio set based upon the container in which
CDROMs are sold. I connected this radio to the VFD as represented
in the simplified schematic above. Music came blasting through the
headphones. Despite the fact that my antenna consisted of no more
than 15 feet of wire thumb-tacked to the ceiling, the amplifier
worked so well that the volume in the headphones was unpleasantly
loud. During this period of experimentation, I made note of the
a) The amplifier works when the audio signal
from the crystal set is positive with respect to ground. It is almost
completely non-functional when the detector diode is reversed and
the audio signal becomes negative with respect to ground.
b) The VFD can function as both detector and
amplifier. I verified this by bypassing the diode in the CDROM radio.
Volume was reduced, but the amplifier still worked just fine.
c) The apparent gain of the amplifier is dependent
upon the voltage applied to the "super" anode of the display.
I have a variable high-voltage power supply. The amplifier provided
useful amplification with an anode voltage of about 60 volts. At
90 volts, the amplifier worked much better. At 150 volts, the audio
in headphones was so loud that music and speech could be heard with
the headphones simply lying on the table.
d) Anode current never exceeded 6 milliamps.
While this limits the VFDís usefulness as a general purpose amplifier,
it is more than up to the task of driving high impedance headphones.
e) A voltage gradient, apparently caused by
the voltage drop along the filaments, causes a column of character
blocks to assume an interesting triangular shape.
f) Strong local stations cause the display
to modulate, i.e., become brighter and darker as the audio varies.
This is not only interesting to watch, it is potentially useful
as a tuning indicator.
Image of operating
display showing voltage gradient across the width of the display
g) A magnet applied to the face of the VFD
causes visible distortions in the display and appears to have some
affect on the gain of the tube, depending upon the magnetís strength
h) The planar construction of the tube makes
it susceptible to nearby electrical noise. Noise in the audio channel
can be greatly reduced by placing the VFD inside of a grounded metal
box, or by laying a grounded metal plate upon the face of the display.
i) At one point, I replaced the headphones
in the circuit with a small transformer, and used this to feed audio
into my desktop computer to make a recording.
Audio transformer used to
Image of characterization
(From Vacuum Tubes in Wireless Communication, Bucher, 1919)
The great success of the VFD as an amplifying
device prompted me to try to collect data to characterize it. To
do so, I used variable power supplies to apply a series of voltages
to the "super" anode and to the "super" grid.
This, I did while simultaneously monitoring the anode current. This
data was collected
in a spreadsheet and then graphed using Excel.
Plate current as a
function of plate voltage. Click
here to see it in higher resolution.
Plate current as a function
of grid voltage. Click here
to see it in higher resolution.
Tube characterization data.
Click here to see it in
On this basis of this data, it appears that
the particular VFD I was playing with, depending upon operating
point, offers amplification on the order of 15 to 20 timesóa respectable
amount of gain.
While the triode vacuum tube and VFDs share
comment internal features, it doesnít appear that VFDs can be wired
to take advantage of grid-leak bias. And, despite the significant
gain apparent in the VFD, I was not able to get it to function in
a regenerative circuit. This shortcoming, however, might possibly
be overcome with alternative biasing methods.
Samples of A.M. radio recorded with this equipment
can be heard by clicking the following hyperlinks: smilesmilesmile
I have also prepared a short video, showing
the VFD amplifier in operation, which can be found on Youtube
(This article was originally prepared for,
and appeared in, the newsletter of the Xtal
Set Society. Check them out!)