Low Plate-Potential Tubes
1957 Radio & Television News Article
radios in the family car became a standard accessory, a push ensued
to make them cheaper, more reliable, and service-free. A major Achilles'
Heel was the high voltage power supply required to energize vacuum
tubes. Known as vibrator power supplies due to using an oscillator
to convert the battery's 12 DC supply (some autos still used 6 V
systems) into AC that could be transformed up to the 300 volts used
by most tubes of the day, most early failures were attributed to
the circuits. They also caused annoying noise in the audio output
if careful filtering and installation was not performed. Having
been invented only a couple years earlier, transistors were being
designed into the power supplies, but low-voltage tubes were still
needed for the electronics. In response to the demand, low-voltage
tubes were created to fill the gap until acceptable transistors
became available for a fully solid-state radio. This article discusses
some of the problems with low voltage tube design and methods employed
to overcome them.
Low Plate-Potential Tubes
By C. E. Atkins
Tung-Sol Electric Incorporated
Re-evaluation of design parameters led to this new family of tubes
for direct operation from a 12-volt source. With a compatible power
transistor, they make the elimination of vibrator power supplies
in automobile radios possible.
With the transistor in the output stage, use of these new
tubes in auto radios makes possible a vibratorless supply.
The development of the transistor
by Bardeen, Brattain, and Shockley of Bell Telephone Lab in 1948
started a new phase in the technology of communication and electronic
instrumentation. The rapid development of transistors provides many
challenging opportunities to achieve results heretofore difficult
or impossible with vacuum tubes.
Of immediate interest are
those applications where the special advantages of the transistor
can be most fruitfully employed. A clear-cut example is the hearing-aid
field, where the transistor inroad is complete. Another intriguing
opportunity is in automobile radios, and this field received early
attention from application engineers. Many experimental, and a few
commercial, all-transistor automobile sets have been made. They
have many good features but are presently expensive and an adequate,
economically practical, supply of the varied transistor types needed
for volume production of such sets is not yet at hand.
the most attractive feature transistors afford in auto-radio design
is the chance to eliminate the vibrator power supply. Aside from
its cost and the jealously husbanded space it requires, this facility
is a cause of considerable field trouble. The car manufacturers
rank vibrators No.1 among the components needing attention during
the early period of service. Furthermore, in doing its job, the
vibrator does not suffer in silence. Vibrator noise which is both
mechanical and electrical (in the form of r.f. hash and low-frequency
hum) is an irritation to the set designer. His mastery of this problem,
often incomplete, also results in minor annoyance to the customer.
A single power transistor providing power output levels
that are out of the question with vacuum tubes operating at low
anode potentials allows one to discard the vibrator power supply;
provided the functions of amplification, heterodyning, detection,
and automatic gain control can be fulfilled with vacuum tubes at
low potentials. Whether provision of such functions was feasible
has been a hotly debated subject.
Tung-Sol took a lively
interest in this matter shortly after March 1952 when the Lamp Department
began work on 12-volt sealed-beam lamps. One of the motor car companies
had advised it was changing from 6 volts to 12 volts in its automobiles,
principally to obtain better ignition, but also to save some copper.
Later, the other car companies made the same switch and furthermore
adopted the convention of a negative ground connection. In the past,
many makes of automobiles had the positive terminal grounded. This
lack of standardization would have been awkward.
the writer has been unable to rationalize the choice of polarity
made by different car manufacturers, in the past, although he has
inquired extensively of automotive people. One quaint story was
to the effect that a certain ground polarity was required because
its opposite caused ground currents returning from the rear axle
via the drive-shaft to de-plate the crank-shaft bearings. In any
case, the trend just noted opened the way for a realistic consideration
of a hybrid receiver employing one power transistor and 4 or 5 vacuum
Most tube engineers were loathe to embark on a tube
program for low plate-potential service. There was a history of
unhappy consequences whenever set designers tried using tubes at
too low an anode potential. For instance, during the war the 6AJ5
was designed in a hurry because the 6AK5 proved unusable in production
communication equipment running at 28 volts, even though this type
had been used successfully in a pilot run. There was an old rule
of thumb to the effect that the mu of a tube must always be less
than the plate potential. Operating difficulties were sometimes
encountered with triodes having a mu of 100 operating in a.c.-d.c.
sets with anode potentials of 80 to 100 volts.
If one attempted
to use much bias on the tube, the plate current would be cut off.
This sometimes happens in practice. Because of this, some engineers
expressed the offhand opinion that practical tubes at 12 volts Ep
must have an amplification factor, or mu, of less than 10. They
felt that, since the grid bias had to be approximately -1 volt (due
to contact potential), there would be no plate current if the mu
were higher than this value. Although the mu decreases rapidly as
the plate voltage is reduced below 20 volts, it is possible to have
an appreciable plate current and Gm under these conditions.
In fact, one is surprised that the Ip is higher than
a prediction based on the tube's characteristics at 100 volts would
lead one to expect. From inspection of the plate families of the
type 12AJ6 (a diode-triode) at high and low plate voltages respectively,
it can be seen that the mu, which was 112 at 250 volts, drops to
50 in the neighborhood of 10 volts. Fig. 1 is a dramatic display
of this effect, where the mu of the 12AJ6 is shown to drop from
77 to 50 volts Ep to 15 at 1.5 volts Ep. The
plate current was held constant at the different values of Ep
in taking this data.
Fig. 1. Change in mu of triode section of the 12AJ6
as plate voltage changes.
Fig. 2. Control-grid resistance (RC1) affects
change of Ip VB heater voltage.
It is believed the reason for this
phenomenon is that, as the grid bias approaches zero, the potential
minimum which is normally at or very close to the, cathode moves
over in the direction of the grid plane. This changes the effective
geometry of the tube, thus lowering the mu. Also, the initial velocities
of the emitted electrons are more nearly comparable to the accelerating
field due to the plate at these low anode potentials. This undoubtedly
modifies the behavior of the tube.
Another serious pitfall
was thought to be contact potential. Since this would establish
the bias at which these low-voltage tubes would operate, many engineers
felt that variations of contact potential between different production
lots (or during life in the same tube) would seriously affect the
performance of any equipment using them. Contact potential had been
the villain in many an ill-fated marriage of tube and circuit. However,
a deeper insight into the inter-relation between contact potential
and other tube parameters revealed how it might be possible to exploit
the very feature that generated the apprehension.
of this many-sided situation is depicted in Fig. 2. This is a plot
of plate current vs heater voltage in the 12CR6. By shifting the
screen voltage for each value of control-grid resistor, it is possible
to adjust Ip so that it is always .5 ma. with 12 volts
on the heater. Hence, all the curves intersect at this point. Note
the dramatic difference in slope between the condition of zero grid
resistor and that where 10 megohms is employed. Even using 1000
ohms makes a considerable improvement. These phenomena stem from
the fact that with higher heater voltage (and consequent greater
cathode temperature), more current flows to the No. 1 grid because
of the greater velocity possessed by the emitted electrons. Of course,
more current will go to the plate and other electrodes as well.
If the resistance in the grid-to-cathode path is low, there
can be no change in grid potential as the grid current increases.
If this grid resistance is high, the grid develops a negative voltage
which will increase with increasing cathode temperature. This rising
voltage at the grid will prevent the more energetic electrons from
reaching the plate. Hence, there is a kind of compensation which,
as Fig. 2 shows, tends to hold the plate current more nearly constant.
In the same manner Gm and other tube characteristics
are leveled out as well. Also, this compensation affords a measure
of balance when the effective cathode area or the cathode activity
changes. At Tung-Sol, several new tube designs have been evolved
embodying this principle.
the composition of a hybrid auto set, there appeared to be these
problems the tubes must solve:
1. Driving the power output
transistor: This step required a shift from voltage amplification
to current amplification, calling for some power output from a vacuum
tube. It was estimated this might be as much as 50 milliwatts.
2. Getting gain: Voltage amplification could use either
low-impedance or high-impedance circuitry. The earliest endeavors
took the form of tubes with larger cathodes giving higher Gm
at the price of lower Rp and greater grid-plate capacitance.
After some preliminary experimentation, this approach was temporarily
side-tracked because the conventional tubes with lower Gm
but high Rp and low Cgp appeared to give a
quicker answer. Already designed i.f. transformers could be used,
and some early sets were made using standard tubes selected for
the best low-voltage operation. As a matter of fact, while the design
of most of these tubes has undergone considerable revision, one
of them, the 12AJ6, remains unchanged. It is merely a 12AV6 with
special processing, tested under the low-voltage conditions.
3. The a.g.c. problem: The fact that the input signal at the
first r.f. grid is commensurate with the plate voltage posed some
new and nasty problems.
4. Stability of gain: Since 12-volt
auto sets are expected to function effectively at supply voltage
from 10 to 16 volts inclusive, this apparently heroic requirement
had to be met. Good life is also required under these conditions.
Returning to problem q, the transfer of power, it was felt
50 milliwatts might be needed to drive the transistor. Whether this
will remain so depends, of course, upon the power gain of available
transistors. In some respects, a suitable driver tube is the keystone
of a successful hybrid radio set.
As Fig. 3 shows, conventional tubes, even relatively high-power
examples, are inadequate to do the job. Only the newly developed
12K5 comes anywhere near the power-output objective. The availability
of peak current approaching 40 milliamperes in the vicinity of zero
bias - is achieved in the 12K5 by employing the structure depicted
in cross-section in Fig. 4. The plate family and transfer characteristics
obtained with the special design are shown in Fig. 5.
Fig. 3. Power output in milliwatts of several tube
types. Only the newly developed 12K5 comes close to providing
the drive needed by the power transistor.
the 12K5 the space-charge grid principle is employed. This was first
disclosed by Langmuir in 1913. By providing a No.1 grid adjacent
to the cathode with a positive accelerating potential applied to
it and a control grid disposed between this accelerator and the
plate, Langmuir was able make tetrodes with very high transconductance.
When screen-grid became commercially available after 1926, sporadic
efforts were made to this principle in equipment. However, most
of the tetrodes and pentodes available to the equipment designer
did not operate well under space-charge conditions. Furthermore,
it was not good engineering economics to provide the power required
by the space-charge grid, as the gain in transconductance did not
generally warrant the cost entailed in exploiting it.
in a car radio there is a different kind of logic. Since the voltage
is low, it is easily provided by the car-carried battery. The 50
or 100 ma. required for the space-charge grid is minuscule in the
overall current needs of the modern automobile. Accordingly, this
ancient stratagem, long discarded, was revived for the tube that
became the 12K5. The geometry of this type provides a fine-pitch
No. 1 grid having 150 turns-per-inch spaced .018" from the cathode,
which is fairly large, with an area of 1.8 square centimeters. The
grid-to-cathode spacing is generous and poses no manufacturing difficulties.
The No. 2 or control grid, having 80 turns-per-inch, is a bit closer
to both No. 1 grid and plate. Here the spacing is approximately
.012" each way. Holding this configuration is a bit tricky in production.
This disposition of elements provides the desired features.
It is believed that the fine No.1 grid to which the 10- to 16-volt
car-battery potential is applied accelerates the electrons and groups
them into thin sheaths. It is well known that the factor that limits
current in thermionic devices is the repelling effect of the electrons
upon one another when they are crowded together. The provision of
a multitude of layers where the space-charge density is low probably
helps to achieve high space currents with low applied potentials.
In addition to its power-output performance, the 12K5 is an excellent
low-voltage relay control tube. It is possible to achieve 8 or 10
ma. of plate current with only 2 volts of plate potential.
As mentioned earlier, providing adequate automatic gain control
was a real problem. The usual a.g.c. potential has a magnitude comparable
to the anode potential of the i.f. amplifier. This is insufficient
to protect the No.·1 grid of the first stage against positive excursions
on very strong signals. Accordingly, there is bound to be spill-over
through the first stage, and this can be sufficient to overload
the converter tube. However, designers hit upon the expedient of
applying additional a.g.c. voltage to the suppressor grid of the
r.f. tube in order to limit the amount of signal getting to the
following stage. To do this successfully, certain compromises have
been forced on the tube designer.
With high impedance i.f. transformers, it is necessary that the
amplifier tube have a correspondingly high dynamic plate resistance
(Rp). This has been difficult to achieve in tubes operating
at low anode potentials. The beam-forming plates and suppressor
grids, which are effective in high potential devices, do not give
the same favorable results with operating potentials in the region
of 10 to 15 volts. In fact, the suppressor grids employed in these
hybrid r.f. tubes are probably more useful in reducing the control-grid-to-plate
capacity than they are in providing effective suppression.
Fig. 4. Location of special space-charge or accelerating
grid in 12K5 tetrode.
Fig 5. Characteristics of the 12K5 obtained with construction
shown in Fig. 4.
Fig. 6. Effect of aging on tube characteristics,
using type 12AD6 as an example. Characteristics of partially
aged tube is shown in (A), of fully aged tube in (B)
The Rp of a tetrode is considerably influenced by
secondary emission from the anode. These secondary electrons leave
the plate and are collected by the screen grid. Since the magnitude
of this effect is sensitive to the applied potentials, a condition
exists whereby small increments of anode potential can cause either
a drop in plate current or too rapid a rise in plate current. In
the latter case, Rp will be too low whereas, in the former
case, it will be negative. Either of these effects can be harmful.
On many tubes it was found that a nice balance between negative
and positive Rp along the plate volt-ampere curve resulted
in Rp values which were astonishingly high (several megohms
where, without this effect, the Rp would have been a
few hundred-thousand ohms).
This dip in the characteristic
is shown in the curves for the 12AD6 in Fig. 6A. Those readers who
are old enough to remember the early screen-grid tetrodes (type
24A), will recognize this shape as the dynatron kink. Pentodes were
introduced to cope with this characteristic. A coarse-pitch No.
3 grid placed between the screen grid and the plate served to thwart
the return of electrons from plate to screen. Because of the geometry
involved, this grid did not adversely affect the flow of primary
electrons to the plate to any great extent. As stated earlier, suppressor-grid
techniques do not function as well with tubes at low voltages. While
suppressor grids are still used in the hybrid-set tube designs,
it is mainly to help reduce the grid-plate capacity and because
this grid is sometimes needed to secure adequate a.g.c.
It was found necessary to rely on special materials and processing
in order to iron out the plate characteristic and secure the needed
control over the Rp The efficacy of these measures is
greatly facilitated by the important fact that these tubes are used
at very low voltages, so that there is very little plate or screen
dissipation. The heat developed by the electrodes in more conventional
operation has the effect of disturbing or destroying any surface
film left on the electrodes by special processing. In contrast,
it is possible to obtain stability and long life at low voltages
with special surfaces too fragile to long endure under the conditions
of high voltage operation. Fig. 6B shows the change in the plate
characteristic of the same tube depicted in Fig. 6A after it received
To gain a partial understanding of these
phenomena, reference is made to an article by Matheson and Nergaard
in the RCA Review, June 1951, entitled "High-Speed Ten-Volt Effect."
This is a study of the behavior of anode currents in thermionic
diodes with oxide-coated cathodes, operating with plate voltages
in the region of 10 volts. In an ideal space-charge limited diode
the current is proportional to the 3/2 power of the applied voltage.
It was noted, however, that in the neighborhood of 10 volts, there
was a small deviation from this 3/2 power. This was found only in
diodes with an oxide-coated cathode. Experiments performed to discover
the cause in the coating itself gave negative results, so a study
was made of the anode.
An experiment was performed wherein
an anode which had not previously been exposed to the cathode was
rotated in place of one that had been in position when the cathode
was broken down and activated. This new anode did not exhibit the
10-volt effect until after some time. The effect was then explained
by the fact that a layer of barium from the hot cathode had condensed
on the anode, thus giving rise to secondary emission. This phenomenon
is, of course, applicable to triodes and tetrodes - or pentodes.
The effect is manifested by the erratic change of Rp
as the critical plate voltage is approached.
From the foregoing,
one might think that this reflection of electrons - if such it is
- might be eliminated by providing the anode with an especially
clean naked surface. This has not proved feasible because it is
hard to long maintain such surfaces in operating vacuum tubes. It
has so far not been possible to make good tubes with bright nickel
plates. If, however, carbonized nickel is used and the device is
processed in such a way that there is probably a layer of cathode
constituents on top of this carbon coating, a composite surface
is achieved which seems to be quite effective in curtailing reflected
This situation is analogous to that obtained
when an optical lens or prism is coated with a thin film in order
to increase light transmission by reducing reflection from the surface.
To comprehend this parallel, you must recall that deBroglie in 1924
advanced the theory that electrons had wave properties as well as
the particle characteristics assigned to them. This was verified
experimentally by Davisson and Germer in 1927. According to this
theory, electrons have an associated wavelength which varies inversely
as the speed with which they are traveling.
electrons proceed rather slowly, their wavelength is rather long
as such things go. This wavelength is on the order of 3 or 4 angstrom
units. This is of the same relative magnitude as the film thickness
discussed in connection with the plate. With this state of affairs,
it is credible that there is a relation between the deBroglie wavelength
of the slow-moving electrons and the refraction characteristics
of the various media these electrons move through when they impinge
on the plate. The writer believes some such machinery operates to
curtail reflection which, in turn, makes possible better control
of Rp at these low operating voltages.
by using processing techniques not generally suited to vacuum tubes
intended for conventional higher-voltage service, and by altering
the electrode geometry to take advantage of contact-potential bias,
Tung-Sol's engineers have designed a set of tubes that effectively
perform with all electrodes energized by the 12-volt car battery.
With a suitable power transistor, this makes possible an automobile
radio without a vibrator power supply.
These tubes are not
final in any sense. The writer believes many more and better types
will be developed by the tube industry for this class of service
before the day of the all-transistor car radio arrives.
In the development of these tubes, many engineers made worthwhile
contributions. The program owes a special debt to Mr. Fred Crawford
of the Tung-Sol design department for many important contributions,
especially in the field of tube processing.
Posted August 20, 2013