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A Fig. 6. This decade counler is built around a single glow-transfer tubę. Reset swilch iorces Ki to K« positWe. returning the glow only to cathode Ł.

Fig. 7. Derice with ^ 5 glow-transfer tubes can count to 100.000. Position of glow in each tubę can be read by external digits marked on the panel.

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Fig. 8. The anodę ot a glow-transier tubę is ringed by ten cold cathodes.

CONSTANT CURRENT

AUIOmaIIC Mam KMminG ’ OUTPUT (TARGET)

Fig. 10. Outer magnet rings MBS tubę.

of clements. Let us assumc that this is cathode in Fig. 8. When the first pulse is applied, the G, pins become morę negative than the cathodes. Thus the glow at K» transfers to the nearest G, guide pin, which happens to be the one to the right of it. going eloekwise. When the G, pulse ends, the G, guide pins are the most negative electrodes in the tubę. so the glow darts eloekwise again to the nearest G_s pin. How-ever, the G, pulse passes quickly and the cathodes. which are maintained at the lowest fixed potential applied to the tubo, become the most negative electrodes. Thus ionization glow transfers to the closest cathode, which is K, in this example.

The fixed potential at the cathode. in the absence of input pulses, is kept morę negative than that on other electrodes by the application of a smali positive bias to al) guide pins from the "B+” linę, as shown in Fig. 6. Al-though each cathode has its own resis-lor in Fig. 6. a common resistor may be used for all cathodes except K,. in cer-tain applications. lndividual resistors, however. permit greater flexibility in using the tubę as a ring counler. for an output pulse may be taken from any one of the stages (cathodes) within the tubę for external use. When the tubę is used only to count by tens, the common resistor is suflicient. In fact, some glow-transfer tubes are designed so that all cathodes except K« are tied together internally and brought out to a single base pin. This simplifies Circuit wiring.

A glance at Fig. 6 shows that. when the normally closed re-set switch is opened momentarily. a positive potential is applied to cathodes K, through /v\ inclusive. At this moment, the cathode is the most negative electrode in the tubę, and the glow will occur only at that point. The same end can be achieved by using a negative re-set pulse or voltage and applying it to the independent cathode. Which-ever method is used. a definite starting position is provided from which the counting can begin. Once counting has begun, the application of evcry tenth pulse will return the glow. in its eloekwise progression, to the K- position. The drop that then occurs across the separate resistor of this cathode may then be used as an external output pulse, for application to the next decade in Cascade or for other use.

In some counting applications where the ratio is less than 10 it is neverthe-less desirable to use glow-transfer tubes. The tubę is madę to produce a lower counting ratio by the method known as forced re-setting. Actually, this technique is related to the use of feedback, described in earlier articles, for reducing the ratio of other counting circuits. For such applications, of course, there must be separate ex-ternal connections for the cathodes; a tubę with nine of its ten cathodes tied together internally is limited to decade counting only.

The Beam-Switching Tubę

The last device to be considered here is one that functions like the glow-transfer tubę in many ways. from the practical viewpoint, but is quite unlike it in important respeets. The mag-netron beam-switching (or MBS) tubo has a single, centrally located cathode that is surrounded by ten anodes also known as targets; it thus reverses a relationship already discussed in the glow-transfer tubę. Another important difference arises from the fact that the MBS tubę is a vacuum type. This per-mits higher switching rates than can be achieced with gas-filled devices. with some MBS versions capable of switching at rates in excess of five megacycles.

Important to the operation of this tubę is a cylindrical magnet attached to the glass envelope. The magnetic field produced by this structure inter-acts with the fields existing beween electrodes inside the tubę in such a way that it can influence the direction of electron flow. This beaming or dellection technique will be familiar to anyone who knows that magnetic fields, such as those produced by ex-ternal yokes and focus coils, control the deflection of the electron beam in a TV picture tubę.

In generał terms, operation of the MBS tubę may be described as follows: As a result of interaction of the vari-ous fields involved, electrons flow from the common cathode to one of the anodes. Each time an input pulse is applied. this flow from the cathode is transferred to the next target. The

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59

October, 1959