Subminiature Construction Techniques for the Home Builder
February 1957 Radio & Television News
suppose the term "Subminiature" as it applies to electronics components
is as relative as the word "Modern" is in book titles. They might be
accurate at the time of the writing, but passage of time renders them
ambiguous. Subminiature in 1957 meant anything other than full-size
vacuum tubes, huge power transformers, multi-layer wafer switches, and
hookup wire larger than AWG 20. The advent of peanut tubes, very early
versions of transistors and solid state diodes, and ever-higher operational
frequencies permitted component sizes to be shrunk by a factor of two
or more. Rather than using a pistol-style soldering gun or a soldering
iron designed for assembling copper guttering, a precision pencil-type
iron could be used and greasy tools from the garage no longer sufficed
for turning screws and nuts. A lot of the material in this article is
still useful for hobbyists and even electronics professionals in the
See all available
vintage Radio News
Subminiature Construction Techniques for the Home Builder
By Louis E. Garner, Jr.
The availability of these tiny parts
makes it possible for hobbyists with limited space to build electronic
For some time, the average home builder or electronics experimenter
has followed a rather general path as he acquired greater skill and
ability in his hobby. Starting with simple one- and two-tube receivers
or amplifiers, the builder would work into more and more complex (and
larger) equipment, eventually building such items as ten- to fifteen-tube
communication receivers, complex test equipment, hi-fi audio amplifiers,
and television receivers. Almost every hobbyist has, at one time or
another, reached the point where "what to build next?" becomes an important
Fig. 1 A representative selection of the subminiature
components that are now available at parts jobbers. Parts illustrated
are fully described in the article.
Today, however, with the increasing availability of
miniature and subminiature components, an entirely new construction
field has opened for the experimenter - assembling subminiature equipment.
Nor is this field restricted only to the experimenter; hams will find
that the construction of subminiature receivers and transmitters offers
a real challenge to their skill, hi-fi "bugs" can apply subminiature
construction techniques to the assembly of compact preamplifiers, and
technicians will find that subminiature test equipment reduces the weight
and space requirements of equipment that must be "toted" to a customer's
In addition to the advantages just outlined, assembling
subminiature equipment is the ideal solution to the problem of the experimenter
who lives in an apartment and has limited space at his disposal. The
equivalent of a "shop full" of tools may be carried in a large briefcase.
Enough components to assemble several receivers and amplifiers may be
easily stored in a cigar box and a coffee or end table offers ample
working area for most projects.
Recently subminiature components have become widely available
at local radio-electronic wholesale supply houses. Previously, such
parts were so difficult to obtain that the would-be builder soon gave
up in disgust. Today most of these components may be purchased across
the counter as stock items. A few still have to be obtained on special
order, but delivery time is now reckoned in days rather than in weeks
A sample selection of components that were purchased
at a local radio wholesaler is shown in Fig. 1. The 6" rule in the photo
illustrates the sizes of the components shown.
Across the back is shown (from left to right) Burgess No. 7 and type
Z penlite cells and Mallory RM-12 (RM-1200) and RM-4000 mercury cells.
These cells are generally used as "A" batteries in subminiature equipment.
Also shown is a Burgess U10, 15-volt hearing aid battery and a Burgess
U20, 30-volt hearing aid battery. Batteries of this type are used as
Fig. 2. Some of the tools that the builder of subminiature
gear will find useful. Shown are power tool, jeweler's screwdrivers
and pliers, soldering pencil, etc.
On the left side is shown (from top to bottom),
a Centralab PC-201 3-stage printed circuit audio amplifier, two types
of "subminiature" tubes with their corresponding sockets (a Sylvania
type 1E8 and a Raytheon type CK512), a printed circuit "Couplate," a
standard 1/2-watt carbon resistor, and a miniature volume control (Centralab
type B16-224). The miniature volume controls are available in a number
of resistance values and both with and without switches.
the right side of the photograph is shown a small iron-core audio transformer,
a miniature ceramic coil form, a miniature tuning capacitor, a miniature
"metallized" paper capacitor (size shown is .25 µfd., 200 volts),
a miniature tubular paper unit, and a disc ceramic capacitor. The small
iron-core audio transformers are available from a number of manufacturers,
although the UTC SO (Sub-Ouncer) and SSO (Sub-Sub-Ouncer) series are
the ones most often carried in stock by local suppliers.
manufactures the miniature ceramic coil form shown and can supply these
forms with either adjustable iron cores or brass slugs as stock items.
The tuning capacitor shown is also a stock item and is manufactured
by the E. F. Johnson Co. A type 20M11 is shown (2.6-19.7 µµfd.).
Large-capacity, small-size metalized paper capacitors are made
by a umber of manufacturers and stocked by most suppliers. A Pyramid
type MT capacitor is shown in the photograph as representative.
In addition to the above the photo shows a miniature two-conductor
jack Walsco type 791) and a midget ear-set (Telex No. A4680). Again,
both of these are stock items.
In addition to the items shown,
:here are a number of other miniature and subminiature components available
that might appeal to the specialized builder. Such items include miniature
sensitive relays hardly bigger than a standard miniature tube (such
relays are manufactured by Potter & Brumfield). However, the items
shown in the photograph are those most likely to be encountered as "stock"
items at local distributors.
The builder of subminiature equipment will find it worthwhile to obtain
tools that fit in best with his hobby. A typical assortment of tools
is shown in Fig. 2.
The collection shown includes a Casco power
tool with an assortment of accessories, including drills, brushes, slitting
saws, etc. A set of Moody jeweler's screwdrivers is shown in the background.
On the right is a set of jeweler's type pliers manufactured by Kraeuter
and purchased by the author at a local radio wholesalers. Included are
"dikes," "long-noses," and other useful pliers.
In the foreground
are shown an Ungar pencil iron with small tip, a magnifying glass, a
small triangular file, a small rat-tail file, a pair of tweezers, and
Another useful tool, not shown in the photograph,
is a small bench or hand vise. If a vise is not available, a small C-clamp
may sometimes be used instead.
Some of the materials used in the assembly of subminiature equipment
are illustrated in Fig. 3. Included are service cement, Bakelite cement,
Scotch tape, Scotch electrical tape, rubber cement, a small aluminum
box (Bud "Minibox"); a small plastic box, sheets of Bakelite and plastic,
plastic tubing and rods, aluminum foil (used for shielding), small wire
and spaghetti tubing, and small size machine screws and nuts (the "large"
screws are size 4-40, the smaller ones size 2-56).
Fig. 3. "Accessory" items used in assembling small equipment.
Service cement, Bakelite cement, Scotch tape, aluminum and plastic
boxes, foil, tubing, etc. are shown.
Not all the
material shown may be available at local radio supply houses, and it
may be necessary to go to a plastics "hobby" shop to obtain a supply
of plastic sheets, rods, and tubing. Aluminum foil may be obtained at
many drug stores and at most supermarkets. Rubber cement can be obtained
at dime stores or stationery stores. Small machine screws and nuts may
generally be purchased at hardware stores. The prospective builder will
find it convenient to purchase corresponding taps and dies at the same
In addition to the material shown, the builder will find
it worthwhile to check through the small parts assortments offered in
packages by most radio parts wholesalers. Assortments of small brackets,
springs, and similar components often prove handy when assembling equipment.
Small diameter wire is likely to prove a little difficult to
obtain in some cases, for many radio wholesalers do not stock wire smaller
than #20 or #22 gauge, and these sizes are considered as "bus bars"
in some types of subminiature construction work. However, it is often
possible to "manufacture" small sized wire by removing individual strands
from large size stranded wire and using small diameter plastic spaghetti
tubing as insulation.
Although the basic methods used when designing
subminiature electronic equipment are the same as those used when designing
conventional sized equipment, there are certain special considerations
that must be kept in mind.
First, since most subminiature equipment
is self-powered, using small batteries, it is necessary that the current
drains be kept to a minimum to insure long battery life. Tubes or transistors
should be selected so that optimum performance at minimum battery drain
is obtained. As few parts as are necessary to accomplish the builder's
minimum requirements should be used.
An example is in the design of audio amplifiers. While most conventional
audio amplifiers are designed to have considerably more than the average
gain needed, so that gain controls are left near "zero" in most work,
it is not unusual to find a subminiature audio amplifier with the gain
control usually adjusted almost to "full gain" as a normal operating
Fig. 4. One technique for working with subminiature parts.
A jeweler's screwdriver should be used on small machine screws.
This requirement makes it imperative to avoid the
use of power amplifiers unless absolutely necessary, and here only to
drive an earphone. While it is possible to build a subminiature amplifier
with sufficient output power to operate a loudspeaker, this may result
in the production of signal distortion.
The compact construction
necessary in subminiature equipment requires small coils with, usually,
high distributed capacities to ground. These factors lead to low "Q"
circuits. Because of this, r.f. circuits selected for subminiature assembly
must be capable of operating satisfactorily with low "Q" coils. With
subminiature-type tubes, "B" voltages as low as 15 volts may often be
used, while a "high B" voltage is 45 volts. This makes it necessary
to select circuits which can operate with very low "B" voltages, and
sometimes makes an additional stage necessary to obtain sufficient over-all
The builder of subminiature equipment will also find that
"high fidelity" is actually difficult to achieve except in resistance-coupled
stages due to the small amount of iron used in the miniature audio transformers.
Better results with subminiature r.f. circuits will be obtained
if the builder chooses parts designed specifically for the type of work
contemplated. Check through the characteristics carefully before picking
a particular component.
A small "pencil" type soldering iron with a 1/8" tip will be
found satisfactory for most subminiature wiring. The iron should be
kept well tinned.
Most connections are made by pre-tinning the
components to be wired and using simple "lap" joints, applying the iron
only as long as is necessary to flow the solder. Care should be taken
not to overheat subminiature components.
Some builders prefer
to clamp the soldering iron in a small vise and to hold the subminiature
assembly or components to the iron tip, as illustrated in Fig. 5. Note
that this technique is just the opposite to that employed in wiring
conventional sized equipment. If this method is used, be careful when
clamping the iron in the vise. Use enough pressure to hold the iron
securely, but remember that too much pressure may damage the iron.
Point-to-point wiring is generally used in subminiature construction
in order to reduce overall space requirements.
Tubes are sometimes
wired directly into the circuit while, at other times, the special subminiature
sockets are employed. The choice of which to use is a matter of individual
The builder contemplating subminiature construction should spend a little
time becoming familiar with the tools used (Fig. 2). Somewhat greater
skill and patience is required when working with subminiature components
than is necessary when working with "full-sized" equipment.
Special care is required when working with small machine screws and
the use of a "jeweler's" screwdriver is almost mandatory. One technique
for using such a screwdriver is illustrated in Fig. 4 (the author is
left-handed). Note that this technique makes it virtually impossible
to apply very much twisting force to the screwdriver handle, thus avoiding
any tendency to bend the screwdriver blade or to "strip" the threads
of the small screws used.
Fig. 5. Pencil type soldering-iron is ideal for subminiature
electronic construction work. To avoid damage to components,
iron can be clamped in a vise - with part brought to the tip
instead of usual technique.
It is customary to mount many parts
(even including small iron core transformers and complete printed circuit
amplifiers) simply by cementing them in place, using either Duco cement
or regular radio service cement.
When it is desired to keep
a plastic surface free from scratches during construction work and layout,
plain white paper may be cemented to the surface using rubber cement.
All layout may be made directly on the paper, using a lead pencil. Once
the construction is completed, the paper may be "peeled" off, with excess
rubber cement removed simply by rubbing it with a finger.
equipment is more often assembled in plastic cases than in metal because
of the greater ease of mounting parts (using cement) and of avoiding
wiring shorts. Where shielding is necessary, two techniques may be employed;
one is to mount the plastic cased assembly inside a larger metal case,
the other is to cement aluminum foil to the plastic case as a close
fitting shield. "Heavy" grade aluminum foil should be used.
When working with plastics, an ordinary scratch awl makes an admirable
"center punch" for small holes. Hand pressure is generally all that
is needed to make a good sharp impression in most plastic materials
to guide the drill.
It is generally not practical to "punch"
large holes in plastic. Rather, a large drill or a series of small holes
around the circumference of the large hole may be used instead, with
the final "sizing" obtained with a file or pocket knife. Still another
technique (generally used by the author) is to layout the desired hole
and then to use one of the "carving" bits of the electric power tool
set to cut out the plastic.
Subminiature construction offers interesting possibilities to the
electronics experimenter, ham, or home builder. Very little space and
no "heavy" equipment is required.
As far as cost is concerned,
the builder will find that individual items are generally somewhat more
expensive than their counterparts in conventional sized equipment; however,
the total cost of components for a finished piece of equipment is likely
to be no higher than its full-sized "equivalent." This is due primarily
to the use of batteries in place of expensive a.c.-operated power supplies
and filter circuits.
Construction projects are numerous, being
limited primarily by the ingenuity and interest of the builder. Typical
projects are hearing aids, small preamplifiers, "vest-pocket" broadcast
receivers, miniature transceivers, and transmitters (for licensed hams),
vibration pickups an" amplifiers, etc.
Posted July 17, 2013
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