An article titled "One
Receiver - All Bands
" that appeared in the January 1963
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
Compactrons: Advance in Tube Design
Refinement of multi-function techniques results in advantages
in cost, reliability, and equipment size.
By Philip E. Hatfield, W9GFS
Receiving Tube Dept., General
Electric Co., Owensboro, Ky.
Promises 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 role.
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 called "compactrons."
What is 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 radios).
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.
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. 3.
Fig. 2 Horizontal mounting of internal structures
facilitates low seated height in this multi-function device.
The manufacturer 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.
Fig.3 An engineer experiments with layout
of prototype for two-compactron radio. Cabinet is to the right.
A working model has since been demonstrated.
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.
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.
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
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.
So 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.f.c.
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.
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, 2012
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.