A E1T Decade Scaler Tube raised from the dead

Ronald Dekker

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An E1T clock
The making of the E1T


The E1T tube you see on the left has been "in the family" for as long as I can remember. Probably my father took it home from work (he worked at the Shell laboratory in Rotterdam) or he bought it in one of the surplus shops in Rotterdam my father and I used to visit every Saturday morning when I was a child. Because it was such a strange and funny looking tube we always used to play with it. Being under the assumption that the tube was broken, I was never particularly careful with it, nor did I make any attempt to bring the tube to life.

From the commonly available datasheet, I knew that this was not just a display device, but that the tube actually counts pulses and additionally displays the sum. This implies that the tube has a memory of the equivalent of 3.5 bits! On top of that, the tube even has a provision to generate a carry for the next digit. How did they do it? I work at the central Philips Research Laboratory in Eindhoven, Holland. Obviously, the Lab keeps a complete archive of all research reports and papers. Browsing through some old volumes of the Philips Technical Review in the library, I happened to stumble across a paper, which described the E1T tube in great detail [1]. It is one word beautiful! The whole paper reads like a poem, it is so elegant and ingenious; undoubtedly one of the masterpieces of vacuum tube engineering.

Unfortunately there does not appear to be a fancy and concise name for the tube. Dieter categorizes the tube as a "beam Deflection Decade Counter Tube" [2]. I would have liked to believe Joris Roehrenbude who categorizes the E1T as a Trochotron [3]. It sounds a hell of a lot more interesting than a "Decade Counter Tube". Unfortunately on further investigation it appears that a Trochotron is really something completely different [4]. Philips itself finally adopted the name "Decade Scaler Tube" [5]:
"The E1T, previously referred to as a "counter tube", is now termed a "Scaler Tube" to preclude confusion with counter tubes such as Geiger-Muller tubes etc."
You will appreciate that I feel an emotional and moral obligation to follow that convention.

Digging in the lab's archives I found two additional "Internal Research Reports" related to the E1T. The first report dates from February 1949 [6]. It is a short 3 page memo with some measurement results on 6 sample tubes, which had been prepared by the "proefafdeling" (sample department?). The tubes worked apparently as expected. It was emphasized that exchanging a tube for another tube did not require any adjustment of the biasing components. Apparently it was feared that the biasing of the tubes would be extremely critical. The second report is from September 1949 [7]. It was an interim report marking the transfer of the tube from research to the "radiobuizenlab" (radio tube laboratory). The nine page report shortly points out the critical construction aspects of the tube and how they relate to the performance. The efficiency of the report is striking. So already early in 1949 working research samples of the E1T were available! The first external publication probably marking the introduction of the E1T on the market I could find is from 1952 [8], it is the Dutch version of [1] published half a year earlier.

Surfing the web I bought a second E1T from Mattijs de Vries [9]. Mattijs really offers excellent and prompt service and I can not recommend him too highly. When it appeared that there was a defect in a socket I bought from him, my money was refunded the same day without questions asked. Having a second tube, I set about trying to bring the tube back to life. It appeared that even the first tube that I had all along was fully functional, and it was very rewarding to see the filament glow-up for the first time in perhaps forty years.

At this point I would like to make an appeal: If you have one or more E1T tubes lying around, and if you do not have any use for them, you can make a clock addict very happy, send me an e-mail. You will find my e-mail address at the bottom of my homepage.

Figure 1. The waking of the sleeping beauty. The filament glowing after forty years of sleep as if nothing happened.

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How it works

In this section you find an edited version of the original description of the E1T counter tube from the "Philips Technical Review" [1]. The full pdf version of the document is unfortunately too heavy for my web-page (3M). On request (see e-mail address homepage), I will be happy to mail it to you. The figure numbers in this section refer to the picture numbers of the scanned images.

Fig. 3a shows a cross-section of the decade counter tube, whilst fig. 3b shows the diagramic representation which will be used in circuit diagrams. The cathode is of the conventional type as used in receiving valves, apart from the fact that it emits electrons from on side only of its rectangular cross-section. A control grid (g1), four rod-shaped focussing electrodes (p1, p2) and an accelerating electrode (g2), together with the cathode, form the electron gun. This has been so designed that the cross-section of the electron bean thus obtained is not circular but ribbon-shaped. A ribbon-shaped electron beam has the following advantages:

  1. It is easy to obtain a fairly strong beam current at a relatively low voltage, for example 1 mA at 300 V. This results in high-speed operation.
  2. The dimensions of the tube can be kept small.
  3. It is necessary to align the electrodes, which are fixed between two mica discs, in one dimension only. This makes manufacturing more easy.
The ribbon-shaped beam thus obtained proceeds between two deflection electrodes (D, D', fig. 3). These have been so positioned that the beam almost touches one of these electrodes in each of it's extreme positions, deflection sensitivity thus being at a maximum.

At given values of deflection, the beam passes through one of the ten vertical slots in the slotted electrode (g4), which is at a positive voltage. It will be shown later how a special circuit ensures that the beam can occupy only one of these ten positions. Some of the electrons passing through a slot impinge on the anode (a2) placed behind the slotted electrode; the remainder pass through an aperture in this anode and impinge on the envelope. The part of the envelope situated behind the anode is lined with fluorescent material, so that a fluorescing mark is produced behind the opening in the anode through which the electrons pass. The number opposite this mark thus indicates the position of the beam.

When the beam passes through slot 9, it has not reached its extreme position, but can be deflected slightly further under the influence of a following pulse. When this occurs, it impinges on the so called reset anode (a1). Before discussing this, it will be shown how the beam is fixed at well defined positions.

Step-wise deflection of the beam

Fig. 4a gives a schematic representation of an imaginary cathode-ray tube, containing an electron gun, a set of deflection electrodes and an anode. When the potential vD of one of the one deflection electrode is kept constant and the potential vD' of the other is varied, the electron beam will move along the anode, but the anode current ia remains constant (line I in fig. 4b). When the deflection electrode D' is connected to the anode and these two electrodes are fed via a common resistor Ra from a battery with voltage Vb (fig. 4c), the following equation, however, applies: va2 = vD' = Vb - ia*Ra. This relation is represented by the line II in Fig. 4b. The point of intersection P of the lines I and II gives the state of equilibrium; the electrode D' then assumes a potential which corresponds to the abscissa of P, and according to this given potential the beam occupies a well-defined position.

Advantage is taken of this principle for fixing the position of the beam in the counter tube. In this case too, the anode a2 and the deflection electrode D' (i.e. the one nearest to the figure 0) are connected to direct voltage source Vb via a common resistor Ra2 (fig. 5); the potential of D and a2 is denoted by vDa2. The straight line I of fig. 4b is now, however, replaced by the undulated curve I of fig. 6; the way this curve is obtained will be shown presently. This curve is intersected 19 times by the resistance line II. Each of these points of intersection corresponds to a condition of equilibrium - although only ten points of intersection numbered from 0 to 9 represent stable positions. From this it may be seen that the position of the beam indicated diagrammatically in fig. 3a is one of its stable positions, i.e., passing partly through a slot and impinging partly on the metal on the left of this slot: in fact, when the beam is deflected further to the left the current initially increases, in accordance with characteristic I near the points of intersection 0...9 (fig. 6).

How the characteristic ia2=f(vD',a2) is obtained

In fig. 7a a slotted electrode and an anode have been drawn as flat planes. It is assumed that the ten slots have the same dimensions and are equidistant and that the thickness of the ribbon-shaped beam exceeds the width of the slots and is less than the width of the spaces between the slots.

When the beam is now made to move from slot 0 to slot 9 by (continuously) lowering the voltage vD',a2 it might be expected at first sight that a characteristic ia=f(vD',a2) as shown by curve I of fig. 7b is obtained: each time the beam is redirected on a slot, ia2 reaches a maximum (which is the value for all slots), and each time the beam points to the centre between the slots, ia2 drops back to zero.

Assuming for the time being that the characteristic had this form, it would be possible to proceed according to fig. 5, and to give VB and Ra2 such values that the line II, which represents eq.(1) graphically, intersects all waves of I (fig. 7b). In practice, however, with a slotted electrode according to fig. 7a, the characteristic ia=f(vD',a2) will not assume the form I of fig.7b, but that of 7c. This is caused by a defocusing of the beam as it is displaced further to the left due to the asymmetry in deflection (vD constant and vD' variable). The disadvantages of the latter form are that the magnitude of Ra2 (slope of line II) must remain within very narrow limits.

In order to obtain the much more favorable characteristic I of fig. 6, the slotted electrode has been provided with additional apertures, which are also scanned by the beam, so that an additional current is passed which increases as the beam proceeds further to the left. The most obvious solution is to make a triangular aperture in the slotted electrode (O in fig. 8). It would nevertheless be very difficult to obtain the desired improvement in this way. This is mainly due to the presence of the suppressor grid (g3) in front of the slotted electrode. For this reason the idea of a narrow triangular aperture was abandoned in favor of rectangular apertures whose positions with respect to the grid wires are less critical.

Mechanism of the displacement of the beam from 0 to 9

The positive-going pulses to be counted are applied to the left deflection electrode D via a blocking capacitor. In order to understand how the beam is shifted to a following position at each pulse, it should be recognized that the characteristic I shown in fig. 6 is applicable to a constant voltage vD at the left deflection electrode, and that the angle of deflection is a function of vD'-vD. An increase of vD by an amount Vi therefore corresponds to the line I being shifted to the right over a distance corresponding to Vi (fig. 13).

As a starting point it is assumed that the beam occupies one of its stable positions, for example position 0. If a positive pulse is now applied to the left deflection electrode, so that vD is temporarily increased, the beam will tend to move to the left (fig. 3a); the number of electrons passing through slot 0 decreases, i.e. the anode current ia2 is reduced. If no stray capacitances were present, the decrease of ia2 would result in a rise of the potential of the anode and of the right deflection electrode connected to it, and this increase of vD',a2 would counteract the deflection to the left, so that the beam would be retained at position 0.

In practice, however, the stray capacitance to earth of the electrodes a2 and D' and their wiring, represented by Ca2 in fig. 5, is shunted across Ra2 and impedes sudden changes of the potential of a2 and D'. Provided the condition is satisfied that the leading edge of the pulse is sufficiently steep, the potential vD',a2 may be considered constant during the rise time of the pulse. This amounts to the line I of fig. 13 being shifted to the right (I') over a distance equal to the amplitude Vi of the pulse, the anode current thus assumes the value A1A. This differs from the original value; the difference is supplied by the capacitance Ca2.

Provided that the second condition is also satisfied, namely that the decay time during which the input pulse decays from Vi to zero, is sufficiently long, the characteristic I' will gradually return to I, and A will be shifted to the adjacent stable point of intersection between I and II, i.e. point 1.

In order to ensure that the beam is shifted just one step to the left, a third condition must be imposed to the beam, namely that the amplitude Vi should be roughly equal to the voltage difference Ve which corresponds to the horizontal difference between two adjacent stable points of intersection and amounts to approximately 14 V; it will be clear that at too small an amplitude the beam will return to its original position, whereas at too large an amplitude it will advance two or more positions.

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A Transistor E1T Tester

Before starting the description of the tester, it should be stressed that the voltages used in this circuit are lethal! Never touch any part of the circuit while the high tension supply is on. Figure xx depicts the circuit diagram of the tester. The circuit consists of three different parts: 1. The E1T valve with biasing circuit, 2. a pulse generator and pulse shaping circuit, 3. the reset circuit.

Figure 2. Circuit diagram of the E1T tester.

The upper part of Fig. 2 forms biasing circuit. It is directly copied from the E1T datasheet. The voltage devider around R14, R15 and R16 provides the biasing voltages for the "input" deflection plate D (156 V) and the control grid g1 (12 V). Any transients on these voltages are removed by decoupling capacitors C9 and C8. The current through the voltage divider is ca. 2 mA. This is enough for the low-current LED D1 to indicate that the high voltage supply is on. The total energy dissipated in the divider is ca. 0.6 W, a bit too high for standard 1/4 resistors. One watt resistors would do, but I only had two watt resistors. Although the tube is designed for a 6.3 V filament voltage, I use 4 V just not stress these old beauties too much. The most crucial resistor in this part of the circuit is R21. This 1M resistor links the anode current and the deflection plate voltage. The datasheet is very fussy about the tolerances of the resistors, especially of the 1M anode resistor which is specified at 1%. If you are not interested in pushing the tube to it's counting speed limits 5% resistors will also work well. In my circuit the maximum counting speed was about 1 kHz.

Figure 3. Front side view of the E1T tester.

The pulse generator and reset circuit were designed only using npn and pnp transistors. The primary reason was that I thought these would be more "rugged" than ICs in such an high voltage environment. Additionally I wanted to show that for these simple functions there are still simple alternatives for the relatively complex (and boring) integrated standard solutions. The additional advantage is that the circuit works over a wide supply voltage range. The heart of the oscillator is the multivibrator around T2 and T3. At "non-inverting" output, the collector of T3, we find 330 µs pulses (determined by C2 and R6) separated by a much longer variable length pause (determined by C1, R4 and R5). By adjusting R5 the repetition frequency is adjusted between 0.5 and 200 Hz.

As explained in the previous section, the rising edge of the input pulse to the E1T should be steep enough so that the voltage at the node of anode a2 and deflection plate D' can be considered constant. The datasheet specifies a leading edge of at least 20 V/µs. On the other hand the slope of the trailing edge should not exceed 1.2 V/µ second so that the potential vD', a2 can follow the trailing edge. Both conditions are met by using an active pull up by means of transistor T1 and resistor R1 for a passive pull down. The base of T1 is connected to the "inverting" output of the multivibrator. This output is normally high, and is low during the 330 µs pulses. During the low time T1 is switched on and driven hard into saturation. The trailing edge of the pulse is determined by C5 and R1.

Figure 4, a rear view of the tester.

When a new input pulse is given when the tube is already in the ninth position, the electron beam advances to the left again an now impinges on anode a1. This anode is connected to the 300 V supply via resistor R18. The anode current now flowing through R18 will cause a negative trailing edge at a1. After removal of the DC component by blocking capacitor C6, this pulse is used to trigger the one-shot pulse generator formed by T4 and T5. It appeared that also during normal counting from 0 to 9 small pulses where induced at the anode a1. Capacitor C3 is used to filter these pulses so that they do not cause unwanted resets. The one-shot produces on the collector of T5 a positive going pulse of 2.2 ms. This pulse will switch on T6 which in turn will cause a negative pulse of -15 V on the control grid g1. This negative pulse will cut-off the electron current in the E1T. As a result the potential on anode a2 and deflection plate D' will return to 300 V again. When the valve is turned on the beam is in the right position again, in other words being reset to zero. Isn't it beautiful !?

Figure 5. The working of the transistor one-shot multivibrator: A. rest position, B. potentials in rest position,
C. situation during pulse.

Although most people are fairly familiar with the transistor multivibrator circuit, the one-shot circuit is less know. Its working is easy to understand. In Fig. 5A the npn version of the one-shot circuit is depicted. Lets for the moment assume that the one-shot is in its stable rest position. The capacitor will be charged then, so that we may think it removed from the circuit (Fig. 5B). In this situation the base of T2 is pulled high by R2 so T2 is conducting. As a result the collector of T2 will be low so that T1 is not conducting so that it's collector will be high. When in this situation a positive pulse is applied to the base T1, T1 will start to conduct, pulling it's collector low. Since the capacitor was charged to 15V as indicated in fig. 5A, the base of T2 will initially be pulled down to -15V, assuming that the emitter-base junction of T2 does not break down (Fig. 5C). This situation continues until the capacitor is charged so far that T2 starts to conduct again. The one-shot is now back in its rest position.

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Results and Discussion

The tube worked directly without any adjustments. At a filament voltage of 4 V it takes about 30 seconds for the filament to warm up. I did not make any special attempts to push the tube to its speed limits. The rather lengthy reset pulse limits the counting speed to ca. 1 kHz. Although designed for a HT supply of +300V, the circuit of Fig. 2 works fine for voltages as low as 200V.

Figure 6. Anode a2 = deflection plate D', voltage while stepping from 0 to 9.
The scale is 50 V per division, 1- indicates 0 V.

Figure 6 gives v(a2,D') of the tube while counting. The voltage varies from 245 V for zero, corresponding to 55 µA, to 110 V corresponding to 0.2 mA. The increase in current for every step is caused by the additional appertures in electrode g4 as explained in one of the previous sections. The result is a staircase pattern with 13.5 V steps. The 13.5 V corresponds exactly with the minimum voltage for the pulse generator resulting in accurate counting. Below 13.5 V the counting stops abruptly. By increasing the pulse generator voltage to ca. 20 V, the tube starts counting in steps by two as predicted. When for a pulse generator voltage the HT supply voltage is decreased the tube continues to work fine for voltages down to 200V. Below 200V the tube start to count in steps of 2 again. This can be compensated for by reducing the pulse voltage. The lowest HT voltage I could make the tube work on was 145 V with a pulse voltage of 11 V. For a filament voltage of 6.3 V I could go down to 105 V with a pulse voltage of 6 V. Almost TTL compatible!

Figure 7. Defocussing of the beam when it is deflected further to the left.

For the lower numbers the stripe is most pronounced and sharp. When the beam is further deflected to the left, in other words the higher numbers, also the stripes around the number selected light up slightly (Fig. 7). In [1] it is explained that this is caused by:
"the asymmetry of deflection (vD constant, vD' variable). It is in fact due to this asymmetry that the focusing of the beam on the slotted electrode deteriorates and that the beam thus becomes wider as it is displaced further to the left."

All in all the tube was quite easy to handle. I would love to make a clock out of them, but unfortunately I have "only" two tubes. For a clock I would not make use of the carry anode a1. I think it would be simpler to drive the pulse and reset inputs of every individual tube directly by a micro-controller. This would only require an additional +15V supply and a few simple driver circuits. For the input pulse driver the simple circuit consisting of T1,T2,R1,R2,R3 and C5 would do. For the reset pulse driver T6,R12,R13 and C7 would do. Driving each tube individually would enable some nice visual effects, e.g. a "slot machine" kind of spinning of every digit for every new minute. I personally would not leave a clock with such rare tubes on all the time. I would use one of these simple and low-cost motion detectors that you may also find in garden lights etc.. to switch the clock on only when somebody is in the room for a longer period of time. After reading this page, Vincent Crabtree remarked that the most stressful time for the heater is the moment when the coil is cold and power is applied. He suggested to implement a soft start heater PSU or a constant current source to gently heat the fillament. Thank you Vincent !

Figure 8. The E1T Tester as built by Dusan from Germany.

Figure 9. The E1T Tester built by Roger Leifert from Germany.

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References and Web links

[1] A.J.W.M. van Overbeek, J.L.H. Jonker and K. Rodenhuis, "A Decade counter for high counting rates," Philips Technical Review, vol. 14, no. 11, May 1953, pp. 313-344
[2] http://www.tube-tester.com/sites/nixie/different/e1t-tubes/e1t.htm
[3] http://www.jogis-roehrenbude.de/Roehren-Geschichtliches/Nixie/E1T.htm
[4] http://www.tubecollector.org/vs10g.htm
[5] E.J. van Barneveld, "Fast Counter Circuits with Decade Scaler Tubes," Philips Technical Review, vol. 16, no. 12, 1955, pp. 360-370
[6] J.D. de Hartog and A.J.W.M. van Overbeek, "Gebruiksaanwijzing bij decimale Telbuizen Proef 38 E J B," NatLab Report 4/49, Eindhoven 11 February 1949
[7] J.D. de Hartog and A.J.W.M. van Overbeek, "Enige gegevens voor de ontwikkeling van de decimale telbuis" NatLab Report 71/49, Eindhoven 23 September 1949
[8] A.J.W.M. van Overbeek, J.L.H. Jonker and K. Rodenhuis, "Een Decimale Telbuis voor Grote Telsnelheden," Philips Technisch Tijdschrift, vol. 14, no. 12, December 1952, pp. 349-388
[9] http://www.machmat.com/index.htm

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"Oh, Sorry", Radio Bulletin, no. 7, 1955