Compactrons: Advance in Tube Design
October 1960 Electronics World
article titled "One
Receiver - All Bands" that appeared in the January 1963
Popular Electronics was a single tube design, but the trick
to using a single tube was that the it was actually three tubes
in one - a 6AF11 compactron. It contained two separate triodes and
a pentode within the same glass capsule. A modern equivalent would
be to use an IC package that contains three or more opamps in the
same package. This article from the October 1960 edition of
Electronic World reported on the engineering behind compactrons.
October 1960 Electronics World
of Contents] People old and young enjoy waxing nostalgic about
and learning some of the history of early electronics. Electronics World
was published from May 1959 through December 1971. All copyrights are hereby acknowledged.
Electronics World articles.
See all the available
Electronics World articles.
Compactrons: Advance in Tube
Refinement of multi-function techniques results in advantages
in cost, reliability, and equipment size.
E. Hatfield, W9GFS
Receiving Tube Dept., General Electric Co.,
that semiconductor devices would eventually replace all vacuum tubes
have been heard since the demonstration of the first operational
transistor. To tube design engineers with the General Electric Co.,
however, such forecasts have seemed arbitrary. These men saw certain
advantages in tubes, actual and potential, that
presaged a continuingly important
Acting on this faith, they embarked on a
re-appraisal of the factors involved
in tube design to see whether they could not exploit possibilities
to a greater extent than had heretofore been realized. Their success
has been such that they are reluctant to
call the new devices tubes. Thus
the vacuum devices now emerging in this separate category are being
a compactron? How does it differ from its vacuum-tube predecessors?
Part of the answer appears on this month's cover, which features
one of the first compactrons to be developed flanked by the three,
conventional, miniature tubes (four tube
(functions) it replaces. Designed
for use in a.c-d.c. table radios, this
single envelope houses a power-supply rectifier, an audio-output
pentode, a detector diode, and first -audio
voltage-amplifying triode (providing the combined functions or the
35W, 50C5, and 12AV6 found in many
Combining it with one more compactron that comprises a pentagrid
section (converter) and an r.f. pentode
(i.f. amplifier), we can put together
a tiny, two-compactron radio.
In fact, G-E engineers have done
just this. The two compactrons shown to the right in Fig. 1, do
the work or five conventional miniatures shown to the left in the
same photograph. Equivalent diagrams for these units appear just
below the tubes and compactrons themselves. A developmental mode
of the radio, beside its cabinet, appears in
Fig. 1 All functions of the "All-American Five" used in a.c.-d.c.
radios (left) included in the envelopes of just two compactrons,
at the extreme right.
Fig. 2 Horizontal mounting of internal structures facilitates
low seated height in this multi-function device.
Fig.3 An engineer experiments with layout of prototype for two-compactron
radio. Cabinet is to the right. A working model has since been
believes that the advantages which will make compactrons
attractive include the following: they will be smaller than
tubes, will outperform tubes and transistors, will feature high
reliability and life, and will be less expensive than either tubes
or transistors. In a stereo hi-fi
amplifier, 7 compactrons will do the work of 10 tube or 26 transistors.
In the home radio mentioned, 2 compactrons are equivalent to 5 tubes
or 7 transistors. In a black-and-white TV receiver, 10 compactrons
will match 15 tubes and 3 diodes or 24 transistors and 11 diodes.
From the foregoing, a compactron simply would appear to
be a single-envelope device in which the technique of housing as
many tube functions as possible has been advanced to on exceptional
degree. Yet this definition does not take into account all the distinguishing
features that make compactron design possible. It also fails to
account for the fact that some single-function units are included
in the compactron line. To understand this advance in vacuum-tube
technology broadly, we must explore specific characteristics one
at a time.
Beginning at the bottom, we find that the all-glass
envelope is designed around a 12-pin circle whose diameter is 3/4
of an inch, larger than that of any conventional tube type. The
number of pins, of course, is to take care of multi-function types,
but the circle offers advantages even where single-function units
are involved. The device will be solidly seated, and it adapts well
to printed wiring in that there is adequate space to make connections
to all pins. By assigning heater connections to pins 1 and 12, there
is space to bring heavier printed wiring to these points if it should
be necessary to carry higher current.
The wide 12-pin circle
also provides a good foundation (or supporting internal structures.
For the most part, points of support fall directly under the electrodes
to which they are attached. In addition, the extra spacing between
pins makes welding of internal connections easier, therefore more
reliable. Simplified fabrication wll be passed on to compactron
users in the form of reduced costs. The development or interelectrode
shorting or microphonism during use would appear to be reduced.
The compactrons thus far developed use a T-9 bulb with a
diameter of 1 1/8 inch, allowing space for multi-function structures.
While bulb height will vary, it will be kept down by reason of the
fact that the exhaust tip. which usually extends about 5/16 inch
above the top of a standard miniature, is placed at the bottom of
the compactron between the pins, where it is not wasting space.
Thus a receiver designer would have to allow considerably less height,
for the most part, than he would for conventional tubes. Compare
the height of the compactrons in Fig. 1 with that of the tubes they
replace. With a shorter unit, the space occupied will approximate
a compact tube.
In some cases, like that of a single-function,
horizontal-deflection amplifier now being developed, an envelope
with a wider diameter (1 1/2 inch, in this case) may be used. However,
the pin circle will be unchanged. The wider bulb permits higher
power dissipation. Even with the standard diameter, the width permits
an interesting possibility. Flexibility in design and other manufacturing
advantages may sometimes be obtained by disposing internal structures
horizontally, as in Fig. 2, instead of using the vertical mounting
common to most tubes. Horizontal mounting also often retains the
desirable feature of low tube height.
The advantages or
the generous pin circle are not exclusively structural. By connecting
a plate (or other high-voltage electrode to one pin and leaving
two unused pins on either side, a voltage isolation in the order
of 10,000 volts can be achieved. This will permit the economy of
removing the top cap in many designs (such as a horizontal amplifier,
for example), since the relatively simple connection to the base
can be made with safety.
Thus, even with compactrons limited
to a single function because of the high voltages or power dissipation
involved, there will be advantages. Improved reliability with reduced
cost and size are anticipated.
However, most compactrons
will be multi-functional devices. Which means lower cost per function
will be obtained by the elimination of extra bulbs, extra stems,
and extra evacuation procedures. Not the least significant cost-reducing
factor is the use of a single heater to activate all of the cathodes
(as many as three) in a single envelope. This not only reduces initial
cost to the equipment designer but, by keeping heater power requirements
and heat dissipation down in use, will further improve reliability.
Take the case of the two-compactron radio. The unit shown
on this month's cover has a tentative heater rating of 70 volts
at 100 milliamperes. Its companion in the a.c.-d.c. radio (the pentode-heptode)
is tentatively rated at 40 volts and 100 milliamperes, with the
heaters of both compactrons in series across the line. The standard
five-tube complement which the pair replaces draws 150 milliamperes
- a 50 percent increase in total heater power.
this reduction in required heater power is made possible by the
use of a copper-base, aluminum-clad, iron plate material in compactron
design. The copper layer efficiently reflects heat from the plate
back to the cathode, where it is needed. Also, the use of new alloys
in making the cathodes increases heat transfer from heater to cathode,
improving efficiency further and cutting warm-up time.
much for the features that characterize the compactron type. To
what extent have results already been achieved? With many types
in various stages of development, G-E has already announced pilot
production of six units, available to equipment manufacturers on
a sampling basis. In addition to the two for the radio, there is
a compactron with two triodes and two diodes. It is intended for
TV use as a combined horizontal oscillator, phase detector, and
A compact beam-power pentode, for use as a
horizontal-deflection amplifier in TV, is one of the few single-function
types. Here are some of its characteristics: plate dissipation,
12.5 watts: perveance, 320 ma. at 60 volts; maximum plate voltage,
6500 volts; heater voltage, 6.3 volts; and heater current, 1.2 amperes.
A companion "single" type is a damper diode, for use in television,
with an average damper current of 165 ma. and a maximum heater-cathode
voltage rating of 5000 volts. Last of the six initial units is a
combined vertical oscillator and deflection amplifier (two dissimilar
triodes) for TV.
Now in the works are many other types.
These include an alternate unit for use as the TV horizontal oscillator,
phase detector, and a.f.c. correction circuit that will consist
of two diodes, a triode, and a pentode. Nor are equipments other
than TV receivers being neglected. Compactrons especially designed
to meet the requirements of auto-radio design are being developed.
All of these are expected to be in production within the next year.
And this may simply be the beginning.
With growing technical
expertise, it is hoped that the advantages already established will
be enhanced even further as new designs come out. One possibility
that design engineers are actively evaluating is that of including
within the glass envelopes circuit elements that are usually added
(resistors, capacitors) externally in the case of tubes. If this
is in fact done, it can further enhance miniaturization and cost
savings. Inclusion of such elements inside the vacuum may also pay
noteworthy dividends in the form of improved reliability.
A possible disadvantage is the replacement cost when a multi-function
compactron, Instead of a single tube, becomes defective. However,
G-E engineers feel that other savings may at least cancel this out.
Lower initial equipment cost, cost per function, and reliability
will be factors.
Ultimate success or the line is probably
in the hands of equipment manufacturers, whose use of compactrons
in less expensive, more efficient designs will be a key factor.
Posted August 22,