Vacuum tube audio amplifier
PARTS AND MATERIALS
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One 12AX7 dual triode vacuum tube
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Two power transformers, 120VAC step-down
to 12VAC (Radio Shack catalog # 273-1365, 273-1352, or
273-1511).
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Bridge rectifier module (Radio Shack
catalog # 276-1173)
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Electrolytic capacitor, at least 47 �F,
with a working voltage of at least 200 volts DC.
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Automotive ignition coil
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Audio speaker, 8 Ω impedance
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Two 100 kΩ resistors
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One 0.1 �F capacitor, 250 WVDC (Radio
Shack catalog # 272-1053)
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"Low-voltage AC power supply" as shown in
AC Experiments chapter
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One toggle switch, SPST ("Single-Pole,
Single-Throw")
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Radio, tape player, musical keyboard, or
other source of audio voltage signal
Where can you obtain a 12AX7 tube, you ask?
These tubes are very popular for use in the "preamplifier"
stages of many professional electric guitar amplifiers. Go
to any good music store and you will find them available for
a modest price ($12 US or less). A Russian manufacturer
named Sovtek makes these tubes new, so you need not rely on
"New-Old-Stock" (NOS) components left over from defunct
American manufacturers. This model of tube was very popular
in its day, and may be found in old "tubed" electronic test
equipment (oscilloscopes, oscillators), if you happen to
have access to such equipment. However, I strongly suggest
buying a tube new rather than taking chances with tubes
salvaged from antique equipment.
It is important to select an electrolytic
capacitor with sufficient working voltage (WVDC) to
withstand the output of this amplifier's power supply
circuit (about 170 volts). I strongly recommend choosing a
capacitor with a voltage rating well in excess of the
expected operating voltage, so as to handle unexpected
voltage surges or any other event that may tax the
capacitor. I purchased the Radio Shack electrolytic
capacitor assortment (catalog # 272-802), and it happened to
contain two 47 �F, 250 WVDC capacitors. If you are not as
fortunate, you may build this circuit using five capacitors,
each rated at 50 WVDC, to substitute for one 250 WVDC unit:
Bear in mind that the total capacitance for
this five-capacitor network will be 1/5, or 20%, of each
capacitor's value. Also, to ensure even charging of
capacitors in the network, be sure all capacitor values (in
�F) and all resistor values are identical.
An automotive ignition coil is a
special-purpose high-voltage transformer used in car engines
to produce tens of thousands of volts to "fire" the spark
plugs. In this experiment, it is used (very
unconventionally, I might add!) as an impedance-matching
transformer between the vacuum tube and an 8 Ω audio
speaker. The specific choice of "coil" is not critical, so
long as it is in good operating condition. Here is a
photograph of the coil I used for this experiment:
The audio speaker need not be extravagant.
I've used small "bookshelf" speakers, automotive (6"x9")
speakers, as well as a large (100 watt) 3-way stereo speaker
for this experiment, and they all work fine. Do not use a
set of headphones under any circumstances, as the
ignition coil does not provide electrical isolation between
the 170 volts DC of the "plate" power supply and the
speaker, thus elevating the speaker connections to that
voltage with respect to ground. Since obviously placing
wires on your head with high voltage to ground would be
very hazardous, please do not use headphones!
You will need some source of audio-frequency
AC as an input signal to this amplifier circuit. I recommend
a small battery-powered radio or musical keyboard, with an
appropriate cable plugged into the "headphone" or "audio
out" jack to convey the signal to your amplifier.
CROSS-REFERENCES
Lessons In Electric Circuits, Volume
3, chapter 13: "Electron Tubes"
Lessons In Electric Circuits, Volume
3, chapter 3: "Diodes and Rectifiers"
Lessons In Electric Circuits, Volume
2, chapter 9: "Transformers"
LEARNING OBJECTIVES
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Using a vacuum tube (triode) as an audio
amplifier
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Using transformers in both step-down and
step-up operation
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How to build a high-voltage DC power
supply
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Using a transformer to match impedances
SCHEMATIC DIAGRAM
ILLUSTRATION
INSTRUCTIONS
Welcome to the world of vacuum tube
electronics! While not exactly an application of
semiconductor technology (power supply rectifier excepted),
this circuit is useful as an introduction to vacuum tube
technology, and an interesting application for
impedance-matching transformers. It should be noted that
building and operating this circuit involves work with
lethal voltages! You must exhibit the utmost care while
working with this circuit, as 170 volts DC is capable of
electrocuting you!! It is recommended that beginners seek
qualified assistance (experienced electricians, electronics
technicians, or engineers) if attempting to build this
amplifier.
WARNING: do not touch any wires or
terminals while the amplifier circuit is energized! If
you must make contact with the circuit at any point, turn
off the "plate" power supply switch and wait for the filter
capacitor to discharge below 30 volts before touching any
part of the circuit. If testing circuit voltages with the
power on, use only one hand if possible to avoid the
possibility of an arm-to-arm electric shock.
Building the
high-voltage power supply
Vacuum tubes require fairly high DC voltage
applied between plate and cathode terminals in order to
function efficiently. Although it is possible to operate the
amplifier circuit described in this experiment on as low as
24 volts DC, the power output will be miniscule and the
sound quality poor. The 12AX7 triode is rated at a maximum
"plate voltage" (voltage applied between plate and cathode
terminals) of 330 volts, so our power supply of 170 volts DC
specified here is well within that maximum limit. I've
operated this amplifier on as high as 235 volts DC, and
discovered that both sound quality and intensity improved
slightly, but not enough in my estimation to warrant the
additional hazard to experimenters.
The power supply actually has two different
power outputs: the "B+" DC output for plate power, and the
"filament" power, which is only 12 volts AC. Tubes require
power applied to a small filament (sometimes called a
heater) in order to function, as the cathode must be hot
enough to thermally emit electrons, and that doesn't happen
at room temperature! Using one power transformer to step
household 120 volt AC power down to 12 volts AC provides
low-voltage for the filaments, and another transformer
connected in step-up fashion brings the voltage back up to
120 volts. You might be wondering, "why step the voltage
back up to 120 volts with another transformer? Why not just
tap off the wall socket plug to obtain 120 volt AC power
directly, and then rectify that into 170 volts DC?" The
answer to this is twofold: first, running power through two
transformers inherently limits the amount of current that
may be sent into an accidental short-circuit on the
plate-side of the amplifier circuit. Second, it electrically
isolates the plate circuit from the wiring system of your
house. If we were to rectify wall-socket power with a diode
bridge, it would make both DC terminals (+ and -) elevated
in voltage from the safety ground connection of your house's
electrical system, thereby increasing the shock hazard.
Note the toggle switch connected between the
12-volt windings of the two transformers, labeled "Plate
supply switch." This switch controls power to the step-up
transformer, thereby controlling plate voltage to the
amplifier circuit. Why not just use the main power switch
connected to the 120 volt plug? Why have a second switch to
shut off the DC high voltage, when shutting off one main
switch would accomplish the same thing? The answer lies in
proper vacuum tube operation: like incandescent light bulbs,
vacuum tubes "wear" when their filaments are powered up and
down repeatedly, so having this additional switch in the
circuit allows you to shut off the DC high voltage (for
safety when modifying or adjusting the circuit) without
having to shut off the filament. Also, it is a good habit to
wait for the tube to reach full operating temperature
before applying plate voltage, and this second switch
allows you to delay the application of plate voltage until
the tube has had time to reach operating temperature.
During operation, you should have a
voltmeter connected to the "B+" output of the power supply
(between the B+ terminal and ground), continuously providing
indication of the power supply voltage. This meter will show
you when the filter capacitor has discharged below the
shock-hazard limit (30 volts) when you turn off the "Plate
supply switch" to service the amplifier circuit.
The "ground" terminal shown on the DC output
of the power supply circuit need not connect to earth
ground. Rather, it is merely a symbol showing a common
connection with a corresponding ground terminal symbol in
the amplifier circuit. In the circuit you build, there will
be a piece of wire connecting these two "ground" points
together. As always, the designation of certain common
points in a circuit by means of a shared symbol is standard
practice in electronic schematics.
You will note that the schematic diagram
shows a 100 kΩ resistor in parallel with the filter
capacitor. This resistor is quite necessary, as it provides
the capacitor a path for discharge when the AC power is
turned off. Without this "bleeder" resistor in the circuit,
the capacitor would likely retain a dangerous charge for a
long time after "power-down," posing an additional shock
hazard to you. In the circuit I built -- with a 47 �F
capacitor and a 100 kΩ bleeder resistor -- the time constant
of this RC circuit was a brief 4.7 seconds. If you happen to
find a larger filter capacitor value (good for minimizing
unwanted power supply "hum" in the speaker), you will need
to use a correspondingly smaller value of bleeder resistor,
or wait longer for the voltage to bleed off each time you
turn the "Plate supply" switch off.
Be sure you have the power supply safely
constructed and working reliably before attempting to power
the amplifier circuit with it. This is good circuit-building
practice in general: build and troubleshoot the power supply
first, then build the circuit you intend to power with it.
If the power supply does not function as it should, then
neither will the powered circuit, no matter how well it may
be designed and built.
Building the
amplifier
One of the problems with building vacuum
tube circuits in the 21st century is that sockets for
these components can be difficult to find. Given the limited
lifetime of most "receiver" tubes (a few years), most "tubed"
electronic devices used sockets for mounting the tubes, so
that they could be easily removed and replaced. Though tubes
may still be obtained (from music supply stores) with
relative ease, the sockets they plug into are considerably
scarcer -- your local Radio Shack will not have them in
stock! How, then, do we build circuits with tubes, if we
might not be able to obtain sockets for them to plug in to?
For small tubes, this problem may be
circumvented by directly soldering short lengths of 22-gauge
solid copper wire to the pins of the tube, thus enabling you
to "plug" the tube into a solderless breadboard. Here is a
photograph of my tube amplifier, showing the 12AX7 in an
inverted position (pin-side-up). Please disregard the
10-segment LED bargraph to the left and the 8-position DIP
switch assembly to the right in the photograph, as these are
leftover components from a digital circuit experiment
assembled previously on my breadboard.
One benefit of mounting the tube in this
position is ease of pin identification, since most "pin
connection diagrams" for tubes are shown from a bottom view:
You will notice on the amplifier schematic
that both triode elements inside the 12AX7's glass envelope
are being used, in parallel: plate connected to plate, grid
connected to grid, and cathode connected to cathode. This is
done to maximize power output from the tube, but it is not
necessary for demonstrating basic operation. You may use
just one of the triodes, for simplicity, if you wish.
The 0.1 �F capacitor shown on the schematic
"couples" the audio signal source (radio, musical keyboard,
etc.) to the tube's grid(s), allowing AC to pass but
blocking DC. The 100 kΩ resistor ensures that the average DC
voltage between grid and cathode is zero, and cannot "float"
to some high level. Typically, bias circuits are used to
keep the grid slightly negative with respect to ground, but
for this purpose a bias circuit would introduce more
complexity than it's worth.
When I tested my amplifier circuit, I used
the output of a radio receiver, and later the output of a
compact disk (CD) player, as the audio signal source. Using
a "mono"-to-"phono" connector extension cord plugged into
the headphone jack of the receiver/CD player, and alligator
clip jumper wires connecting the "mono" tip of the cord to
the input terminals of the tube amplifier, I was able to
easily send the amplifier audio signals of varying amplitude
to test its performance over a wide range of conditions:
A transformer is essential at the output of
the amplifier circuit for "matching" the impedances of
vacuum tube and speaker. Since the vacuum tube is a
high-voltage, low-current device, and most speakers are
low-voltage, high-current devices, the mismatch between them
would result in very audio low power output if they were
directly connected. To successfully match the high-voltage,
low-current source to the low-voltage, high current load, we
must use a step-down transformer.
Since the vacuum tube circuit's Thevenin
resistance ranges in the tens of thousands of ohms, and the
speaker only has about 8 ohms impedance, we will need a
transformer with an impedance ratio of about 10,000:1. Since
the impedance ratio of a transformer is the square of
its turns ratio (or voltage ratio), we're looking for a
transformer with a turns ratio of about 100:1. A typical
automotive ignition coil has approximately this turns ratio,
and it is also rated for extremely high voltage on the
high-voltage winding, making it well suited for this
application.
The only bad aspect of using an ignition
coil is that it provides no electrical isolation between
primary and secondary windings, since the device is actually
an autotransformer, with each winding sharing a common
terminal at one end. This means that the speaker wires will
be at a high DC voltage with respect to circuit ground. So
long as we know this, and avoid touching those wires during
operation, there will be no problem. Ideally, though, the
transformer would provide complete isolation as well as
impedance matching, and the speaker wires would be perfectly
safe to touch during use.
Remember, make all connections in the
circuit with the power turned off! After checking
connections visually and with an ohmmeter to ensure that the
circuit is built as per the schematic diagram, apply power
to the filaments of the tube and wait about 30 seconds for
it to reach operating temperature. The both filaments should
emit a soft, orange glow, visible from both the top and
bottom views of the tube.
Turn the volume control of your radio/CD
player/musical keyboard signal source to minimum, then turn
on the plate supply switch. The voltmeter you have connected
between the power supply's B+ output terminal and "ground"
should register full voltage (about 170 volts). Now,
increase the volume control on the signal source and listen
to the speaker. If all is well, you should hear the correct
sounds clearly through the speaker.
Troubleshooting this circuit is best done
with the sensitive audio detector described in the DC and AC
chapters of this Experiments volume. Connect a 0.1 �F
capacitor in series with each test lead to block DC from the
detector, then connect one of the test leads to ground,
while using the other test lead to check for audio signal at
various points in the circuit. Use capacitors with a high
voltage rating, like the one used on the input of the
amplifier circuit:
Using two coupling capacitors instead of
just one adds an additional degree of safety, in helping to
isolate the unit from any (high) DC voltage. Even without
the extra capacitor, though, the detector's internal
transformer should provide sufficient electrical isolation
for your safety in using it to test for signals in a
high-voltage circuit like this, especially if you built your
detector using a 120 volt power transformer (rather than an
"audio output" transformer) as suggested. Use it to test for
a good signal at the input, then at the grid pin(s) of the
tube, then at the plate of the tube, etc. until the problem
is found. Being capacitively coupled, the detector is also
able to test for excessive power supply "hum:" touch the
free test lead to the supply's B+ terminal and listen for a
loud 60 Hz humming noise. The noise should be very soft, not
loud. If it is loud, the power supply is not filtered
adequately enough, and may need additional filter
capacitance.
After testing a point in the amplifier
circuit with large DC voltage to ground, the coupling
capacitors on the detector may build up substantial voltage.
To discharge this voltage, briefly touch the free test lead
to the grounded test lead. A "pop" sound should be heard in
the headphones as the coupling capacitors discharge.
If you would rather use a voltmeter to test
for the presence of audio signal, you may do so, setting it
to a sensitive AC voltage range. The indication you get from
a voltmeter, though, doesn't tell you anything about the
quality of the signal, just its mere presence. Bear in
mind that most AC voltmeters will register a transient
voltage when initially connected across a source of DC
voltage, so don't be surprised to see a "spike" (a strong,
momentary voltage indication) at the very moment contact is
made with the meter's probes to the circuit, rapidly
decreasing to the true AC signal value.
You may be pleasantly surprised at the
quality and depth of tone from this little amplifier
circuit, especially given its low power output: less than 1
watt of audio power. Of course, the circuit is quite crude
and sacrifices quality for simplicity and parts
availability, but it serves to demonstrate the basic
principle of vacuum tube amplification. Advanced hobbyists
and students may wish to experiment with biasing networks,
negative feedback, different output transformers, different
power supply voltages, and even different tubes, to obtain
more power and/or better sound quality.
Here is a photo of a very similar amplifier
circuit, built by the husband-and-wife team of Terry and
Cheryl Goetz, illustrating what can be done when care and
craftsmanship are applied to a project like this.
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