Considering that not much more than a year before this article
was written that the transistor had been invented, it is impressive
that already Raytheon was producing a commercially available
CK703 'crystal triode.' That nomenclature was a natural
extension of the preceding crystal diode already being widely
adapted in circuit design. If you have wondered how the transistor
schematic symbol came to be as it is, you will learn why here
where the emitter and collector symbols actually both have arrows
on the ends that contact the base, indicating the 'point contact'
physical arrangement of the semiconductor junctions. Shortly
thereafter the arrow on the collector port was eliminated, primarily,
I suppose to avoid confusion when the E, B, and C labels are
A Crystal Receiver with Transistor Amplifier
By Rufus P. Turner, K6AI
The recent appearance of the transistor on the commercial
market opens new fields to the radio experimenter. One of its
uses is discussed here.
Fig. 1 - Top view of receiver. The crystal
triode a.f. amplifier strip is mounted on rear. One section
of tuning condenser is used. Fig. 2 - Crystal triode a.f. amplifier
strip. All parts except the electrolytic "cathode" condensers
are mounted on top of the Bakelite.
The introduction of Raytheon's new CK-703 crystal triode
opens up a new field of interest for the radio experimenter.
Recommended for use at frequencies up to the video range, this
commercial transistor can be used as a medium mu a.f. or r.f.
amplifier, oscillator, or control "tube." Simple, small-sized,
and lightweight electronic equipment may be built around crystal
triodes. Enterprising radio men undoubtedly will find a host
of applications for this device.
This article describes a simple crystal diode broadcast receiver
with a crystal triode audio amplifier. Although this set will
not operate a loudspeaker, it will give a walloping good signal
in high-impedance headphones, provided it is connected to a
good outside antenna and ground. A great deal of satisfaction
may be derived from its operation, and some experience with
crystal triodes gained as well.
Fig. 3 - Under chassis view of the receiver
showing components and simple wiring.
Before describing the receiver, however, we will explain
briefly the characteristics and construction of the crystal
Features of Transistor
The transistor is a germanium crystal device. It differs
from the familiar crystal diode, of which the 1N34 is a well
known example, chiefly in that the transistor has two cat whiskers
instead of one. One whisker, called the emitter, acts in a manner
comparable to the grid of a triode vacuum tube. The point of
the second whisker, called the collector, touches the surface
of the germanium crystal extremely close to the place of contact
of the first whisker and acts like the plate of a triode tube.
The crystal itself (called the base in crystal triode terminology)
is comparable to the cathode of a tube. Unlike a tube, however,
the emitter (grid) of the crystal triode is biased with a low
positive voltage, while the collector (plate) receives a high
negative voltage. Another important difference is that the crystal
triode has a low-impedance input and a high-impedance output.
The recommended circuit symbol for the transistor is shown in
The CK-703 (See Figs. 5B and 7) is a small-sized unit, being
only 0.78 inch (maximum) long and 0.255 inch in diameter. The
housing of the CK-703 is a cylindrical brass shell to which
the crystal is connected internally. This shell serves as the
base (cathode) terminal. The emitter and collector whiskers
are connected to two nickel contact pins which extend through
an insulating disc-plug in one end of the brass shell. Fig.
5B identifies these pins in bottom view. The CK-703 has an input
(emitter) impedance of 500 ohms and output (collector) impedance
of 10,000 ohms. It is rated at 2 milliwatts average power output
with an average power gain of 16 db. The power rating is on
the basis of a 50-microwatt signal applied to the emitter. Table
1 gives the important electrical characteristics of the CK-703.
From the impedance ratings, it is evident that a conventional
amplifier circuit employing crystal triodes must provide for
an impedance step-down between stages. This means that interstage
transformers must have a step-down turns ratio.
Fig. 4 is the complete schematic for the receiver. This circuit
is entirely conventional except for the employment of a 3-stage
crystal triode audio amplifier.
Fig. 4 - Complete circuit diagram and parts
list for the crystal-transistor receiver.
The input coupler, L1-L2 is a standard
broadcast antenna coil of the type used in midget receivers
(J. W. Miller 20-A). This coil has been center tapped in order
to match the impedance of the crystal detector more effectively
and to improve selectivity. The center tap must be provided
by the builder, since this coil normally is not supplied with
one. Tuning is accomplished by means of a midget 365 μμfd.
variable condenser, C1.
The detector diode is the new Sylvania 1N54 high-efficiency
unit. The cathode of this diode is connected directly to the
high end of a 500 ohm load resistor, R1, and to the
emitter terminal of the first CK-703 socket.
Table 1. Electrical characteristics of CK-703.
In order to dispense with emitter batteries in the amplifier,
the author has employed base or "cathode" resistors, R2
R3 and R4, in each stage. Collector current
flowing through each of these resistors develops a voltage drop
which is applied as a small positive potential to the emitter
terminals. The 500 ohm value is correct for R2 R3,
and R4. Reducing this resistance decreases power
output and increases distortion. While there has been some criticism
regarding this method of operation, it has given satisfactory
operation in practice.
The interstage transformers, T1 and T2 employed by
the author are U.T.C. sub-ouncers Type SO-2. These transformers
are connected backward. That is, the normal secondary (high
impedance) is connected to the collector output of one crystal
triode, while the primary (low impedance) is connected to the
emitter input of the following crystal triode. While these transformers
were not designed specifically for this application, they have
approximately the proper turns ratio for the required impedance
transformation and are very small in size. To improve fidelity,
an individual builder may prefer to shunt-connect the transformers
to keep d.c. out of their windings.
The headphones are connected in series, through the output
jack, with the collector of the last crystal triode and the
negative battery terminal. Operation of a pair of 2000 ohm headphones
has been satisfactory in this circuit.
In the event that crystal headphones are used it will be
necessary to use a choke and condenser coupling arrangement
to keep the d.c. out of the headphones.
Fig. 5 - (A) Circuit symbols for the transistor. (B) Details
of CK-703 crystal triode.
A single 45 volt battery is employed for power and is bypassed
by a 10 μfd., 150 volt electrolytic condenser, C6.
There is sufficient voltage drop across the high-impedance windings
of the two transformers to reduce the collector-to-base d.c.
voltage in each amplifier stage to about 37 volts .
It is absolutely imperative that proper polarities of the
crystal diode and of the three crystal triodes be observed.
For this reason, the cathode terminal of the diode has been
labeled in Fig. 4, and so have the emitter (E), collector (C),
and base (B) terminals of each crystal triode. The builder must
be careful to ground the positive terminals of electrolytic
condensers C3, C4, and C5 and
also to connect the positive battery terminal to ground. It
will be easy to make a mistake in these connections, since radio
technicians are well-schooled in the opposite procedure of connecting
"B-minus" and the negative terminals of cathode bypass condensers
to ground. The crystal triodes definitely will be burned out
if the battery polarity is reversed.
At this writing, no sockets have been manufactured for the
CK-703. For this reason, the author made his own. Fig. 8 gives
constructional details of the homemade socket, and Fig. 6 shows
two completed sockets. Undoubtedly, the sockets could have been
made smaller in size. The reader who is mechanically proficient
may prefer to follow an entirely different design.
Fig. 6 - Home-made wafer-type CK-703 sockets.
Bottom view shows transistor plugged into socket with the two
base pins in contact with the wire springs. Brass shell for
gripping the CK-703 case is shown at right.
The basic requirement in the transistor socket is that good
contact be made separately to the outer metal shell of the CK-703
and to each of the base pins. Whatever type of contact is used,
it must grip the shell and pins firmly but without bending,
twisting, or denting. Soldering to the CK-703 is not advised.
The author's socket (See Figs. 6 and 8) consists of a ring-shaped
base shell for clamping the CK-703 shell, and two short lengths
of spring wire for pressure contact against the side of each
base pin. The wafer is a rectangular piece of thin Lucite, but
can be made of Bakelite or other phenolic. The CK-703 is inserted
into the tight-fitting base shell, and its pins pass through
small-diameter clearance holes in the wafer to be contacted
by the straight wire springs mounted underneath. Small soldering
lugs are provided for circuit connections to the three contact
members of the socket.
Fig. 7 - Three new CK-703 crystal triodes.
The contact pins are the collector (plate) and emitter (grid)
terminals. Brass shell is connected to the crystal and is the
base (cathode) terminal. Triode is 0.78" by 0.255".
Since the contact pins protrude just below the center line
of the CK-703 base (See Fig. 5B), the unit can be plugged into
the socket only in the correct position. Thus, all danger is
removed of inserting emitter or collector pin into the wrong
The entire audio amplifier is built on a strip of 1/16-inch-thick
Bakelite, 4 7/8" long and 1 3/4," wide. Fig. 2 is a view of
this amplifier strip. All components are mounted on the strip,
except condensers C3, C4, and C5
which are mounted directly under the main chassis (See Fig.
3), and electrolytic condenser C6 which is "hung"
between the battery terminals. The small coupling transformers
are held to the strip by means of bands bent from 3/16"-wide
brass. (These mounting bands must not be used if the transformers
are mounted directly in contact with a metal chassis, since
they might form eddy current loops). The amplifier strip is
supported on the chassis (See Fig. 1) by means of four 1-inch
The entire set, as shown in Fig. 1, is built on a 7"x7"x2"
metal chassis. The tuning dial plate is home-made. The headphone
jack, which must be insulated from the chassis, is mounted along
the front lip of the chassis, as is the battery "On-Off" switch.
The author employed a 2-section tuning condenser (with only
one section in use), as shown in Fig. 1, simply because a single-section
unit was not immediately available. The antenna coil, L1-L2,
is mounted under the chassis directly below the tuning condenser.
This coil is provided with a slip-over primary (L1)
which may be slid off temporarily while a center tap is soldered
to the secondary coil (L2).
Center tapping the coil is a delicate operation, since the
coil is wound with rather fine wire. But it can be accomplished
easily, provided a reasonable amount of care is used. Locate
the center turn and, using a sharp razor blade, carefully scrape
about 1/4 inch of the enamel off the wire. Avoid scraping the
adjacent turns. Solder a length of No. 32 insulated wire to
this scraped portion, replace the primary, and run the tap lead
directly to the 1N54 crystal diode.
For simple mounting, the 1N54 pigtails are soldered to a
2-lug insulated terminal strip mounted near the coil. The r.f.
bypass condenser, C2, is mounted near the 1N54. Load
resistor R1 is mounted on the audio amplifier strip
near the first CK-703.
Hookup wire leads from the amplifier strip and battery pass
through clearance holes in the chassis to reach under-chassis
contact points. A stiff, bare lead extends from the tuning condenser
stator lug through a large chassis clearance hole to the coil,
L2 below the chassis.
Fig. 8 - Construction details of transistor
socket. The terminal markings (E, C, and B) indicated on both
of the lower drawings refer to the symbols given in Fig. 5A.
Testing and Alignment
Do not connect the battery until all connections in the circuit
have been checked and found to be correct. After verification,
connect the battery, plug in the high-impedance headphones,
and throw the "On-Off" switch to "On." A gentle rushing sound
should be heard, due to the inherent noise level of the crystal
triodes, but there should be no oscillation nor singing. If
there is oscillation, it will be necessary to re-run critical
leads and possibly to orient the transformers to prevent the
A modulated r.f. signal generator may then be connected to
the antenna and ground leads of the receiver and the dial calibrated
at fixed broadcast band points in the usual manner. Use the
signal in the headphones to indicate resonant points, or connect
a high-resistance a.c. voltmeter in parallel with the headphones
for visual indication. If a manufactured broadcast dial is used,
line-up first at the top of the band (1600 kc.) and bend the
outside plates of the tuning condenser to make the other frequencies
coincide with the dial graduations.
The noise level of the crystal triodes is somewhat higher
than with comparable triodes in the same circuit, but is not
objectionable in this arrangement. In the absence of a signal,
a soft steady hiss will be heard.
Current drain of the amplifier is low and the battery consequently
will give long service. However, the receiver should be switched
off whenever it is not in use. This will prolong the life of
the battery and of the crystal triodes as well. The set should
not be left connected to the outside antenna during a thunderstorm
unless an efficient lightning arrestor is attached.
No data is available at this writing regarding transistor
life. The crystal triode is not yet old enough to have undergone
prolonged life tests. However, it seems reasonable to expect
life comparable to that of germanium diodes which presently
are guaranteed up to 5000 hours.
The little receiver shown in this article is only an example
of successful crystal triode application. The author does not
intend to imply that this is the ultimate by any means. Certainly,
a much smaller set could be built and a smaller battery of the
hearing aid type could be employed. Higher power output can
be obtained by using 4 transistors in push-pull parallel in
the output stage. A crystal triode r.f. amplifier might be added.
An individual experimenter may prefer to employ fixed emitter
bias, possibly derived from a "lower than ground" point on a
voltage divider across the battery. Many applications, other
than receivers and audio amplifiers, will undoubtedly occur
to the reader.
Posted August 23, 2015