Subminiature Construction Techniques for the Home Builder
February 1957 Radio & Television
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 lab.
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 problem.
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.
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 and months.
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 "B" supplies.
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
On 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.
Millen 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.).
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.
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 a penlite.
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 time.
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.
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 condition.
Fig. 4. One technique for working with subminiature
parts. A jeweler's screwdriver should be used on small machine
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
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 preference.
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.
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.
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
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.
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.
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,
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