EP0718867A1 - Grid electron tube with improved performance - Google Patents

Abstract

The tube includes electrodes (K, G1) which are mostly cylindrical, mounted coaxially about an axis (XX'). The electrodes, with a cathode (K) surrounded by a grid (G1), partly define an input resonant cavity (34) having an active zone (30). The input cavity is tuned so that the central part (C') of the active zone corresponds to a voltage antinode. The cavity extends on both sides of the active zone. The input cavity is folded back on itself, towards the electrode axis, on one side of the active zone (B'-A'). Pref. the cavity terminates at two ends (A', E') which form part of the tube stem, at different radial distances from the axis. At least one end of the cavity is pref. terminated by a short-circuit tuning stub (36). The two cavity ends may be short circuits for operation at the lower end of the tube frequency range; for operation at the frequency range upper end, one cavity end is an open circuit.

Description

The present invention relates to grid tubes used in particular as television amplifiers, or in industrial heating devices, etc. These grid tubes are in particular of the triode or tetrode type.

FIG. 1a schematically represents a half-tetrode. It comprises cylindrical electrodes mounted coaxially around an axis XX '.

The central electrode is the cathode K emitting electrons when it is heated. Around it is a control grid G1, a screen grid G2 then an anode AN. In a triode, there is no screen grid.

The cathode K and the control grid G1 contribute to forming an input resonant cavity 1. The input resonant cavity 1 has an active zone 10 between the marks B and D and extends on either side of this active zone 10 towards the foot of the tube between the marks D and E and towards its top between the marks A and B. The active area corresponds to the area where the electrons emitted by the cathode pass before crossing the control grid G1. Mark C shows the central part of the active area.

The input cavity 1 comprises means 4 for introducing a signal to be amplified in the amplification application. The screen grid G2 and the anode AN contribute to forming an output resonant cavity 2. It includes means 5 for extracting the amplified signal.

The resonant input 1 and output 2 cavities are generally closed, at the foot of the tube, by a mobile short-circuit piston 3 which makes it possible to adjust their resonant frequency

et électrique $\overrightarrow{\text{E}}$

ont été représentés. Ce type de cavité est généralement accordé en quart d'onde. Cela signifie qu'entre le repère A au sommet du tube sur l'axe XX' et le repère E au niveau du piston de court-circuit 3, la longueur électrique I de la cavité d'entrée est: $$\text{I = (2n + 1)λ/4}$$

   n est un entier ≥ 0 et λ la longueur d'onde de l'onde s'établissant dans la cavité d'entrée.In the input cavity 1, it is generally the TEM mode which is established. Magnetic fields $\overrightarrow{\text{B}}$

and electric $\overrightarrow{\text{E}}$

were represented. This type of cavity is generally tuned in quarter wave. This means that between the mark A at the top of the tube on the axis XX 'and the mark E at the short-circuit piston 3, the electrical length I of the inlet cavity is: $$\text{I = (2n + 1) λ / 4}$$

n is an integer ≥ 0 and λ the wavelength of the wave establishing in the input cavity.

In this TEM mode, the scalar potential V varies between the reference A and the reference E as follows:

The reactive surface currents I on the walls of the cavity are also not constant. The representation which is made in FIG. 1b of the voltage V and of the reactive current I (in absolute value) corresponds to a length l of cavity between the reference A and the reference E equal to λ / 4. We chose an integer n zero to simplify. The voltage V is shown in solid lines and the reactive current I in dotted lines.

Between the reference A and the reference E, the voltage V substantially follows a quarter of a sinusoid, this voltage V being maximum (belly of tension) for l = 0 at the level of reference A and zero (node of tension) for l = λ / 4 at the level of the mark E. At the top of the tube, at the level of the mark A, on the axis XX ', there is an open circuit and at the foot of the tube, at the level of the mark E a short circuit. Between the marks B and D, at the two ends of the active area 10, the voltage V and the current I vary. This means that the cathode K is not homogeneously stressed in its active part 10. We also note in FIG. 1b that the reactive current I is greater at the level of the mark D than at the level of the mark B. This implies that there is a heating of the tube in the part (mark D) of its active area 10 closest to its foot while it is the part of the active area 10 least stressed. Indeed the peak current supplied is more important in B than in D.

In this type of tube, the height of the active zone (BD interval) is limited by the frequency and the power of the tube. The higher the frequency, the greater the variation in voltage V in the active area 10. The power of the tube is limited since a compromise must be observed between the height of the active area 10 and the diameter of the cathode K. For example , the most powerful tubes operating in UHF have a cathode K whose diameter is of the order of 40 mm and whose height of the active zone is approximately 2 cm.

Forty years ago, it was proposed to obtain a tube with better efficiency, power and frequency more important to place a belly of tension V in the central part of the active zone 10.

Figure 2a schematically illustrates such a tube. This figure is comparable to Figure 1a and the elements which correspond have the same references.

$$\text{n entier > 0.}$$

Now the inlet cavity 1 'has been arranged between the marks A' and E '. The reference C 'which corresponds to the central part of the active area 10 is located on a voltage belly V and therefore a reactive current node l. The input cavity 1 'is tuned in half-wave. This means that between the mark A '(top of the cavity) and the mark E' (base of the tube), the electrical length is worth it: $$\text{l '= nλ / 2}$$

$$\text{n integer> 0.}$$

In the example of FIGS. 2a, 2b n is chosen equal to 1 for simplicity. Now the voltage V evolves substantially like a half-sinusoid.

It is the top of the tube which has been modified with respect to the tube of FIG. 1a. In A 'and E' there are pistons 3 in agreement and the input cavity ends in short-circuits.

Compared to FIG. 1a, the part of the tube situated between the marks A and C has been removed and it has been replaced by a part identical to that situated between the marks C and E. The new inlet cavity 1 ′ shown on FIG. 2a, has an active area 10 'between the marks B' and D 'and therefore extends on either side of the active area 10' towards the foot between the marks D'and E 'and towards the top between marks A 'and B'. This inlet cavity 1 'is now symmetrical with respect to a plane normal to the axis XX' passing through mark C '. The inlet cavity 1 'shown in Figure 2a is equivalent to two inlet cavities 1 in Figure 1a mounted head to tail. The outlet cavity 2 ′ is also equivalent to two outlet cavities of the tube of FIG. 1a mounted head to tail.

This configuration gives good results from the electrical point of view. In the active zone 10 ′, the voltage V varies very little and the current I is not too large. On the other hand, this structure is difficult to achieve from the mechanical point of view because it is very difficult to maintain good coaxiality between the cathode K and the control grid G1 since they are now joined by their two ends to the foot and to the top of the tube. The distance between the control gate G1 and the cathode K is a few tenths of a millimeter in the active area while the height of the active area is worth a few tens of millimeters.

The present invention aims to remedy these drawbacks. It offers a grid tube which has electrical performance comparable to that of the tube in FIG. 2a but which is much easier to manufacture, simpler to install for the user, and therefore has a lower price.

The electronic tube according to the invention comprises generally cylindrical electrodes mounted coaxially around an axis, including a cathode surrounded by a grid. The cathode and the grid contribute to delimiting an input resonant cavity comprising an active zone, this cavity extends on either side of the active zone. The resonant entry cavity is folded back on itself, towards the axis, on one side of the active zone. The central part of the active area is subjected to a tension belly.

Preferably, the inlet cavity ends in two ends which contribute to forming the base of the tube.

At least one end of the inlet cavity can be closed by a short-circuit piston so as to tune the frequency of the tube. To operate in a lower part of the frequency range of the tube, the two ends of the cavity can be closed by a short-circuit piston.

To operate in a high part of the frequency range of the tube, at least one end of the tube can form an open circuit. To function in an intermediate part of the frequency range of the tube, at least one end of the tube can have a capacitance connected between the grid and the cathode.

The capacity may include a dielectric element pressed against one of the electrodes using a conductive clamping device. In contact with the other electrode.

Preferably the clamping device is removable and the value of the capacity can be adjusted as required.

The clamping device may include a conductive plug screwed into a collar secured to the other electrode. A spring can be used to improve the contact between the collar and the plug.

The dielectric element will advantageously have the form of a ring. It can be made of mica.

• les figures 1a et 1b, déjà décrites, respectivement une représentation schématique d'une tétrode de type connu et un graphique de la tension et du courant présents dans la cavité d'entrée;
• les figures 2a et 2b, déjà décrites, respectivement, une représentation schématique d'un tube connu formé de deux tétrodes tête-bêche et un graphique de la tension et du courant présents dans la cavité d'entrée;
• la figure 3 une coupe partielle d'un tube selon l'invention ;
• les figures 4a et 4b respectivement une représentation schématique du tube de la figure 3 et un graphique de la tension et du courant présents dans la cavité d'entrée;
• la figure 5 une coupe partielle d'une variante d'un tube selon l'invention ;
• les figures 6a et 6b respectivement une représentation schématique du tube de la figure 5 et un graphique de la tension et du courant présents dans la cavité d'entrée;
• la figure 7 une coupe partielle d'une autre variante d'un tube selon l'invention ;
• les figures 8a et 8b respectivement une représentation schématique du tube de la figure 7 et un graphique de la tension et du courant présents dans la cavité d'entrée.

Other characteristics and advantages of the invention will emerge from the following description, given by way of nonlimiting example and illustrated by the appended figures which represent:

• Figures 1a and 1b, already described, respectively a schematic representation of a tetrode of known type and a graph of the voltage and current present in the input cavity;
• Figures 2a and 2b, already described, respectively, a schematic representation of a known tube formed of two tetrodes head to tail and a graph of the voltage and current present in the input cavity;
• Figure 3 a partial section of a tube according to the invention;
• Figures 4a and 4b respectively a schematic representation of the tube of Figure 3 and a graph of the voltage and current present in the input cavity;
• Figure 5 a partial section of a variant of a tube according to the invention;
• Figures 6a and 6b respectively a schematic representation of the tube of Figure 5 and a graph of the voltage and current present in the input cavity;
• Figure 7 a partial section of another variant of a tube according to the invention;
• Figures 8a and 8b respectively a schematic representation of the tube of Figure 7 and a graph of the voltage and current present in the input cavity.

In these figures the scales are not respected for the sake of clarity.

Figure 3 shows schematically and partially a tube according to the invention.

This tube conventionally comprises generally cylindrical electrodes mounted coaxially around an axis XX '. We therefore finds a cathode K which emits electrons when it is heated. This cathode K is surrounded by a control grid G1, itself surrounded by a screen grid G2, itself surrounded by an anode AN. The emitted electrons are accelerated towards the anode AN and pass through the gates G1, G2. The cathode K and the control grid G1 contribute to delimiting an input resonant cavity 34 into which a signal can be injected. The inlet cavity 34 has an active area 30 between the marks B 'and D'. It extends on either side of the active zone 30 between the marks A 'and B' and between the marks D 'and E'. Conventionally, at the foot of the tube 31 the electrodes are spaced from each other by means of insulating spacers 33. These spacers 33 serve to electrically isolate the electrodes from one another, to hold them mechanically in position and to guarantee a vacuum seal at the active part 30 of the electrodes.

The screen grid G2 and the anode A help to define an output resonant cavity 35 from which the amplified signal can be extracted. The resonant outlet cavity 35 is only partially shown to avoid overloading the figure. It is the part of this outlet cavity 35 located at the top 32 of the tube which has been omitted. In this example, the outlet cavity 35 is terminated by a tuning piston 37.

The inlet cavity 34 is tuned so that the central part (mark C ') of the active area 30 corresponds to a tension belly as shown in Figures 4a and 4b.

Instead of having symmetry with respect to a plane normal to the axis XX 'passing through the mark C', the resonant inlet cavity 34 is folded back on itself towards the axis XX 'on one side of the active area 30. The folded part goes from the mark B 'to the mark A'. The inlet cavity 34 then has two ends, one radially outer (mark E '), the other radially inner (mark A') and these two ends contribute to forming the base 31 of the tube. Coaxiality problems no longer arise. The two ends of the cathode K and of the control grid G1 conventionally end in collars respectively 38.39. Only one connector will be used to receive the tube. A tube according to the invention can then be manufactured and easily installed at low cost. Compared to the tube of the Figure 2a, the tube according to the invention will be more compact which is a significant advantage.

The space inside the cathode is not used in conventional tubes. Here we house the folded part of the resonant inlet cavity 34.

Frequency matching of the input cavity 34 can be achieved by at least one short-circuit piston 36. In FIG. 3, the two ends of the input cavity 34 are closed by such a piston. The inlet cavity 34 is then tuned in half-wave. FIG. 4a very schematically represents the input cavity 34 and FIG. 4b illustrates the variation of the voltage V and of the reactive current I (in absolute value) as a function of the electrical length I of the cavity. The two short circuit pistons are located at the marks A 'and E'.

The same variation in current and voltage is obtained as in FIG. 2b. Between the reference A 'and the reference C' the electrical length is λ / 4. Compared to the tube of FIG. 1a, the height of the active zone 30 is greater, which makes it possible to reach higher frequencies and powers.

The configuration with the two short-circuit pistons 36 generally makes it possible to obtain low frequencies in the UHF range, for example between 450 and 550 MHz.

To be able to work in the upper part of the UHF range, for example between 750 and 860 MHz, it is possible to envisage that at least one of the ends of the input cavity 34 forms an open circuit. This is illustrated in FIG. 5. It shows an inlet cavity 34 between the marks A 'and E'. One of its ends ends in an open circuit at the mark A '. This is the inner end. A short-circuit piston 36 closes the outer end of the inlet cavity 34 at the level of the reference E '.

Concretely, a conductive bottom 50 closes the inner end of the control grid G1 and the inner end of the cathode K conventionally ends with a collar 38. The mark A 'is located near the axis XX'. At this point, the voltage V is maximum. The input cavity 34 is then tuned in three-quarter wave, as shown in Figure 6b in conjunction with Figure 6a. Between the reference A 'and the reference C' the electrical length is λ / 2.

To be able to work at intermediate frequencies, for example between 550 and 750 MHz, it is possible to equip at least one end of the input cavity 34 with a capacitor connected between the cathode K and the control gate G1. This is illustrated in Figure 7 which shows only the base of the tube. A capacity 70 is mounted at the internal end of the cavity. It comprises a dielectric element 71, in mica for example, which is pressed against the collar 38 of the cathode K using a clamping device 75 conductor. The clamping device is preferably removable. The dielectric element 71 has the shape of a ring. Its thickness measured along the axis XX 'and its surface in contact with the collar 38 determine the value of the capacity. The clamping device 75 is electrically and mechanically connected to the control grid G1. The clamping device 75 comprises a conductive plug 72 which comes into contact with the dielectric ring 71 and clamping means 73 such as a screw. The screw is screwed into a bore 74 carried by the collar 39 of the control grid G1. The bore 74 is centered on the axis XX '. The collar 38 of the cathode K and the plug 72 form the reinforcements of the capacitor.

An annular spring 76 can be provided to ensure good contact between the plug 72 and the periphery of the collar 39 of the control grid G1. The plug 72 fits around the periphery of the collar 39 of the control grid G1.

The evolution of the voltage and the reactive current (in absolute value) is illustrated in Figure 8b, Figure 8a schematically showing the input cavity with its marks.

The capacity 70 is at the level of the reference A '. At this level, there is a tension belly and a current node. The electrical length between the reference A 'and the reference C' is between λ / 4 and λ / 2.

The realization of the capacity is simple and does not pose a problem because the dielectric ring 71 and the clamping device 75 are not subjected to vacuum. A dielectric spacer 33 has been provided between the collar 38 of cathode K and that 39 of the control grid G1 to guarantee vacuum tightness.

The value of the capacitor 70 depends on the thickness and the area of the dielectric ring. The greater the thickness, the lower the capacity. The clamping device 75 can be removable and a change of capacity is easy. The description which has just been made of the capacity is only an example and other variants of the clamping device in particular can be envisaged without departing from the scope of the invention.

Claims (14)

Electronic tube comprising globally cylindrical electrodes (K, G1) mounted coaxially around an axis (XX ') including a cathode (K) surrounded by a grid (G1), the cathode (K) and the grid (G1) helping to define a resonant entry cavity (34) comprising an active area (30) and extending on either side of the active area (30), the central part (C ') of the active area (30) corresponding to a tension belly, characterized in that the resonant inlet cavity (34) is folded back on itself, towards the axis (XX '), on one side of the active area (30). Electronic tube according to claim 1, characterized in that the inlet cavity (34) ends in two ends (A ', E'), the ends (A ', E') contributing to form the foot (31) of the tube. Electronic tube according to claim 2, characterized in that at least one of the ends (A ') of the inlet cavity (34) is provided with a short-circuit piston (36). Electronic tube according to one of claims 2 or 3, characterized in that at least one of the ends (A ') of the inlet cavity is an open circuit. Electronic tube according to claim 4, characterized in that the open circuit is located at the axis XX '. Electronic tube according to one of claims 2 or 3, characterized in that at least one of the ends (A ') of the inlet cavity ends in a capacitor (70) connected between the control grid (G1) and the cathode (K). Electronic tube according to claim 6, characterized in that the capacitor (70) comprises a dielectric element (71) pressed against one of the electrodes (K) of the tube by means of a conductive clamping device (75) and in contact with the other electrode (G1). Electronic tube according to claim 7 characterized in that the clamping device (75) is removable. Electronic tube according to one of claims 7 or 8, characterized in that the clamping device (75) comprises a conductive plug (72) screwed into a collar (39) integral with the other electrode (G1). Electronic tube according to claim 9, characterized in that a spring (76) improves the contact between the plug and the collar (39). Electronic tube according to one of claims 7 to 10 characterized in that the clamping device (75) and the dielectric element (71) are in the air. Electronic tube according to one of claims 6 to 11, characterized in that the dielectric element (71) is in the form of a ring. Electronic tube according to one of claims 6 to 12, characterized in that the dielectric element (71) is made of mica. Electronic tube according to one of claims 6 to 13 characterized in that the value of the capacity (70) is adjustable.

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