Low Plate-Potential Tubes
January 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.
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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 possible.
With the transistor in the output stage, use of these new tubes in auto radios makes possible a vibratorless
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.
Probably 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.
Incidentally, 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 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.
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.
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 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.
In 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.
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.
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.
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
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 special
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
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