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.
[Table 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.
As time permits, I will be glad to scan articles for you. All copyrights
(if any) are hereby acknowledged.
Electronics World articles.
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.
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.
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 demonstrated.
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.
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.
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.
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