Gridless tube from 1938

Fellow Radiophiles,

Prof. Rudolph has posted an interesting article in the German side of the forum  that was originally published in the German press, about an American tube design from 1938 that was made without any kind of lattice shaped grids. I post here a Google assisted translation of the original article. Prof. Rudolph kindly reviewed the translation for accuracy.

---

Funk, No. 17/1938, Weidmann Publishing House, pp. 471-472

A grid-less tube with power steering control

Tube designers in the USA have recently applied the control principle by current steering and developed a tube in which the actual control grid is missing entirely. The electrodes that serve for power control have no lattice anymore, but they are rod-shaped. The tube has no electrodes formed as a lattice.

The spatial arrangement of the electrodes in the schematic representation of Figure 1 includes the Anode, auxiliary anode and control electrodes which are composed of two symmetrically arranged sub-electrodes electrically connected to each other within the tube. The anodes are formed as semi-circular curved plates with the cathode between them. By itself, the anode may also comprise a single piece of metal tube, but then there would be a large capacitive coupling to the auxiliary anodes. These have a flat shape and are closer to the cathode than the anode. 

The control electrodes, act as a current compressor that distributes the cathode current according to the amount of the negative control voltage for the benefit of the auxiliary anode or anodes. Figure 2 shows the current control.

In Figure 2a, the control voltage is very negative, therefore virtually all of the cathode current goes to the auxiliary anode. In 2b, only a small negative control voltage is present, so that the current flows to the anode and to the auxiliary anode. In the last figure (2c), the control voltage is finally very low negative or even positive, so that the higher anode voltage can draw over the major portion of the cathode current to itself.

Figure 3 shows a circuit diagram for the application of the new tube in an intermediate frequency amplifier stage.

In the IF stage an AGC control takes place. Besides the control electrodes, the auxiliary electrodes also support the AGC. When the AGC voltage is suddenly reduced - due to tuning from one station to another – the voltage of the auxiliary electrodes will also increase due to the coupling by the capacitor C, and this leads to a reduction of the anode current which results in a sensitivity reduction which has a pleasantly noticeable quenching effect on inter station noise.

The constructors of this tube attributed many valuable properties in detailed discussions, so that we miss even more, the presence of curves with precise data on the gain, distortion, internal capacity, etc.. In enumerating the advantages, we can agree that the simpler structure  eliminates the need for the usual number of nested grids. This allows an easier, and therefore cheaper, production method. The simple design and execution of the electrodes also gives a greater consistency in the electrical characteristics for mass production, which is valuable for push-pull and parallel connections. But why this tube is recommended for short waves, is questionable, because the capacitance between the control electrodes on the one hand, and the cathode, or auxiliary anode or anodes on the other hand, appears rather larger than smaller, in comparison to grid-controlled tubes. Furthermore, the control electrodes are subject to serious feedback from anode voltage swings, which have been virtually eliminated in the usual tubes by a screen grid. Also, one can not readily accept the view than no secondary emission effects with their known adverse consequences, could occur in the new tube. It seems quite possible that secondary electrons emerge from the auxiliary anode, which go to the higher anode voltage, or otherwise affect the characteristics.

Finally, attention is drawn to the greater input dynamic range of the tube , including the fact of the AGC control used in Figure 3 – which is at odds with a curved characteristic.

No doubt that the principle of power steering control still opens great possibilities. However, it is to be seen what benefits these structures offer, when compared to the normal control method. One does not need to that end, to move away from using lattice-shaped electrodes, as is the case with gain control in hexodes. By the way, it was recently pointed out by a German tube expert that the normal intensity control arranged around the cathode by the the control grid works like a power steering control. According to this theory, the electron cloud of the cathode undergoes through the grid voltage, a division of electrons into two groups. One group overcomes the inhibitory reaction of the grid voltage and reaches the positively biased electrode, the other group is rejected and goes back to the cathode. With this in mind, normal control is nothing more than power steering control between the positive electrode of the tube on the one hand, and the cathode on the other hand.

H. Boucher; drawings by the author

---

Note some topological similarity to Russian Subminiature Rod Tubes. However, the relative dimensions are quite different. The very thin filamentary cathode which is very near to the control grid rods in the Russian tubes, accounts for extraordinary transconductance efficiency that is unlikely to be duplicated with the inherently much larger indirectly heated cathode shown above.

Prof Rudolph also points out a topological and operational similarity to the rod control used in frequency converting heptodes and hexodes.

The steering action between the main and auxiliary anodes described above is also analogous to the operation of the suppressor grid G3 in dual control pentodes, where G3 steers the cathode current between the Anode and the screen grid G2.

The positioning of the auxiliary anodes with respect to the control rods suggests a tetrode wired for space charge operation, where G1 is tied to a positive voltage and G2 is the input.

Regards,

-Joe

 

To thank the Author because you find the post helpful or well done.

 Fellow radiophiles,

Prof. Rudolph's finding of the name of the inventor of this gridless tube, Henri Francois Dalpayrat, in the original German language post, made it possible for me to find the original patent for this tube design. The text of the patent indicates that ownership was assigned to Radio Corporation of America, a corporation of Delaware.

 

 

Dalpayrat had several other US patents to his name. A quick search at Google-Patents turned up several patents, mostly in the 1930's. The following table summarizes my findings, and includes my description.  Ownership of a number of these patents was assigned to RCA.

Note that patent 2156650 is for an electron gun based tube that shares some of the electron optic control seen in the gridless tetrode shown above.

The full content of these patents can be downloaded for free at Google-Patents, Espacenet.com or from a number of other patent web pages.

Regards,

-Joe

To thank the Author because you find the post helpful or well done.

A guest, Mr Pac Low from South Tirol, sent a link to "Radio Craft 1 - 1937" where on pages 396 - 397 & 428 a paper from Henry F. Dalpayrat on "Gridless vs. Grid Tubes" is published (PDF 6.2 MiB).

Many thanks to Mr Pac Low for this information.

Regards,

Dietmar

To thank the Author because you find the post helpful or well done.

Download PDF

Add a : into link to make it work, or delete the http // on your browser address bar.

 

Beam switching

It also reminds me of the later 7360. I restored an SSB set that used it. Perhaps a Swan 500.

 

Wonderful article.

 

To thank the Author because you find the post helpful or well done.

The same link obviously works in the German Tread. You also may use this link, please.

Regards,

Dietmar

To thank the Author because you find the post helpful or well done.