SOFT VALVES 



according to type. Higher voltages may be had by connecting tubes in 

 series (Figure 7.3). They may be of different types if this helps to get the 

 required running voltage, but design is easier if they are all intended by the 

 makers to work over a similar current range. Having chosen the tube or 

 tubes to use attention must be given to the minimum value F„ can have in 

 order that the tube or tubes shall strike. For a single tube this is merely the 

 quoted striking voltage. For several tubes, proceed as follows. Arrange the 



Figure 7.3 



tubes in order so that those with the greatest difference between their running 

 and striking voltages are at the bottom and those with the least are at the 

 top. Now take a set of high resistances, in number one less than the number 

 of tubes, the lowest of the order of megohms and of values such that each 

 is, say, 5 times the value of its predecessor. 2 megohms and 10 megohms 

 would be suitable for a chain of 3 tubes. Connect them as in Figure 7.3. 

 Now consider what happens when K„ is connected. Let the running vohages 

 of the tubes be Fj, Kg and V^. At first the whole of F„ is applied to the 

 bottom, most refractory tube by way of the high resistances, ensuring that 

 it strikes. Then we have K„ — Fg applied to the upper two tubes, but 

 because of the potential divider now made by the resistances, I of it is 

 applied to V^, ensuring that it strikes. Finally we have K„ — (Kg + V<^ 

 applied to the upper, least refractory tube, and this will strike if F„ — (Kg + 

 Kg) is greater than its striking voltage, or if F„ — (Kj + Kg + Kg) exceeds 

 the voltage differential of this tube. The rule is that the tubes will strike if 

 the difference between the unstabilized input and the stabilized output 

 voltages exceeds the difference between the running and striking voltages 

 of the least refractory tube, provided the tubes are arranged to strike in the 

 right order. 



Having found K,,min, we can look about for some point in the power 

 pack from which to tap off F„. Stabilizers are not perfect and in fact there 

 is some change of voltage across them when the glow current changes, so 

 that as far as increments of voltage and current are concerned they may be 

 allotted an equivalent resistance. Thus if a reference tube voltage changes 

 from 69-9 to 70-1 volts for a glow current change from i to 1| miUiamps, 

 its incremental resistance is 0-2/10"^ = 200 ohms. Note that this is a non- 

 linear device and the incremental resistance is not the same as the absolute 



120 



