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.
January 1957 Radio & Television News
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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
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.
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 tubes.
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.
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.
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.
One facet 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
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.
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
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.
In 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.
But 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.
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
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
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 special aging.
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 electrons.
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.
Since 10-volt 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.
Thus, 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.
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.
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