October 1960 Electronics World
Wax nostalgic about and learn from the history of early electronics. See articles
Electronics World, published May 1959
- December 1971. All copyrights hereby acknowledged.
An article titled "One Receiver - All Bands" that appeared in the January 1963 issue
of Popular Electronics was a single tube receiver 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 integrated
circuit (IC) package that contains three or more opamps in the same package. In
fact, variations on compactrons that included internal biasing components were referred
to as integrated circuits. This article from the October 1960 edition of Electronics
World reported on the engineering behind compactron vacuum tubes.
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).
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.
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. 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
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 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.
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
will 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.
In part, 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. unit.
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
Posted August 29, 2019