TWT-Traveling Wave Tube

Fellow Radiophiles,

Prof Rudolf has posted yet another very interesting article; this time about Traveling Wave Tubes.

This article is one of a series where Prof Rudolf presents various tubes that deal with the time that it takes for electrons to travel in the vacuum space inside the tube. This time is often called the Transit Time of a tube.

The article is in German, but it translated pretty nicely into English, with Google-Translate.

The article addresses transit time from control grid to plate in a conventional tube as the ultimate speed or bandwidth limitation for a tube. This degradation is noticeable if the transit time is longer than 1/10 of the signal frequency.

The traveling wave tube turns transit time limitation into an a great advantage to be exploited. Whereas a conventional tube must have less than 1/10 of a wavelenght delay from control grid to anode, the TWT has several wavelenghs of delay from input to output.

This is a good example that a physical phenomenon can be a hindrance in one technique and a great advantage in another.

Note that "wavelength" relates to the lenght of the wave along the electron path and is a function of the electron speed, not the speed of light.

Regards,

-Joe

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Dear Joe,

the article you picked from German section is an example of the limitation of Google translator. I am not able to fully enjoy the article, but there is even a link to another fascinating topic on distributed deflection CRTs. Of course I have had severe difficulties to read the articles, since the output of the translator is full of mistakes and refers to pictures not visible. And, even if I have something to add, I will never be able to approach the discussions in German language.

I am extremely interested in velocity modulated (VM) tubes to which TWTs belong. VM tubes were developed for microwave applications to overcome the transit time limitation of traditional tubes. They include many families, Heil tubes, Klystrons, Magnetrons, TWTs and their derivatives, such as BWOs and VTMs. As far as I know, klystron, magnetron and TWT families included both amplifiers and oscillators. Operation could be CW or pulsed and a certain tuning by voltage variation was possible, depending upon the Q of the associated resonators. Heil tubes found just niche applications by British STC as low-power oscillators. The research work on these tubes has its origin in the ‘920s, but practical devices were available starting from the mid ‘930s.

>Heil tubes: In 1935 Oskar Heil and his wife, Agnessa Arsenjeva, coming from the Department of Physics at the Leningrad Polytechnic Institute, published the article describing their idea on how electrons could be bunched by velocity modulation. O. Heil started the first practical development of a VM tube with the British STC that for years produced a line of oscillators. Heil tubes are loaded in Radiomuseum, but as ‘MICROWAVE AND RADAR, general’ and is impossible to say how many items are coded. For some examples, look at V233A/1K, V241C/1K or V246/1K.

>Klystrons: The klystron was patented in 1937 by Russell and Sigurd Varian, two brothers that worked for a while at the Stanford Physics Department with Professor Bill Hansen. Hansen had developed the resonant cavity, called ‘rumbatron’, and the related theory, essential for the klystron structure. Sperry Gyroscope Company was attracted by the klystron for a blind-landing system and financed its initial development. Klystrons have been made for CW or pulsed operations, for powers up in the megawatt range and for frequencies up to 100GHz and over. Reflex klystron tubes operate as oscillators, while linear types can operate as amplifiers or as frequency multipliers. Radiomuseum lists 326 reflex and 57 linear types.

>Magnetrons: Magnetron has a longer life starting in 1921 as odd laboratory device. The first powerful multicavity prototype, assembled by J.T. Randall and H.A.H. Boot at Birmingham University, operated only on 21st February 1940. Shortly later, in September, the Tizard mission brought a GEC pre-production sample to United States, unveiling its construction details to a qualified group of people from MIT Radiation Lab., Bell and other firms. From that date a lot of types were developed. Magnetron tubes were made for frequencies from about 700 Hz (yes, GE Z-2061 was characterized from 700 cps to 1 GHz) to over than 30 GHz, for pulsed or CW operation, with output power from few milliwatts to peaks of several megawatts. Some variants were also developed. Amplitron or Raytheon Platinotron were designed to operate as power microwave amplifiers. Voltage Tunable Magnetron, VTM, could be tuned over a wide frequency range just varying the supply voltage as in the BWO: here the bunching effect is made possible due to a virtual cathode; give a look at ZM-6086 and at ZM-6222. For these devices I found a little confusion, since there are 218 magnetrons (not clear what types included here), 209 pulsed magnetrons and 37 CW types.

TWTs and BWOs: TWTs appeared in 1946, thanks to the work of Rudolf Kompfner at the Clarendon Laboratory, Oxford. It can be used as electronically tunable amplifier or oscillator. Tuning is accomplished simply varying the beam accelerating voltage, to synchronize the signal propagating along the helix-shaped transmission line with electrons moving from the gun to the catcher. In oscillators signal moves oppositely to the direction of electrons. For this reason they are referred to as BWO, Backward Wave Oscillator, sometimes called carcinotron as in Radiomuseum (but the latter name should only be referred to a ‘crabwise’ device). 11 carcinotrons are listed in addition to 77 TWTs.

>Distributed Deflection CRTs: Even if the operating principle is quite different, helical transmission lines are used inside distributed deflection CRTs described by Prof. Dietmar in the link said before. Here the successive interactions among electrons and the small plates driven by taps along the helix result in a progressive deflection of electrons and their bunching has no interest in the operation of the tube.

In the field of high-speed oscillography Tektronix soon reached a leading position with its high writing speed 517, its 585, the first oscilloscope with plug-in units in the 100MHz range, and its marvelous 519, the first direct oscilloscope with 1GHz bandwidth and 200 picoseconds risetime. In the 519 the distributed deflection CRT is the complete vertical system, the input signal entering directly in the input coaxial connector of the CRT itself. The sensitivity is in the order of 10V/cm. The article of Prof. Dietmar talks of the Tektronix deflection systems, but references to specific models should be added. In Radiomuseum there are the three CRT tubes covered by the drawings in the paragraph.

T5741-31-2 is the CRT of the 547, with three pairs of vertical plates.
T581 is the CRT of the Tektronix 585. It has six pairs of distributed vertical plates.
T5191-11 is the CRT of the Tek 519. It has 27 vertical distributed plates against a common counterplate.

By the way, Tek 519 was a wonderful piece of craftsmanship, full of ingenious solutions, such as the trigger takeoff head, the countdown tunnel oscillator, the 2 ns/div time base generator or the calibration generator, with 0.1 ns rise time.

Unforunately I cannot share these information with German language people.

Best regards, Emilio
 

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Dear Emilio,

thank you very much for the reference to the TEK CRT's. At least the T581 is very similar to the CRT I had once. Probably this one is identical to that one I got it in 1964 when a TEK scope was refurbished. This scope had  distributed amplifiers with artificial lines, and it was one of the finest scopes then available. So indeed, this probably was a TEK 585 scope.

I've included a link to the T581 CRT in the post <Tektronix 'Distributed Deflecction'>

Best regards,

Dietmar

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Dear Prof. Dietmar,

I have just uploaded some better photos of the Y deflection plates for the Tek CRTs T5741-31-2 and Tek T581. Actually the first CRT has three pairs of vertical plates connected by metal ribbon segments, about 1 mm wide, soldered to the pins protruding from the glass support rods. It is not a transmission line but probably the small inductance of the conductors has the effect of fractionating the capacitance of the plates at high frequency.

I am not able to take detailed pictures of the deflection system of the third CRT, T5191-11, because it is hidden by the mu-metal shield. The shield cannot be removed because of the two factory mounted Gen-Rad coaxial connectors on the tube neck. However the last drawing in your article is just referred to the system first installed in this tube, as described in the 519 oscilloscope manual. Few years later the same vertical system was standardized in the CRTs of the series 7000 mainframes.

Best regards, Emilio
 

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Dear Emilio,

thanks again for the good photos of the delay helix in the CRT's.

My recent posts had the general topic "Helix". 

• Helix as a helical resonator (#5)
• Helix as a delay line

Therefore I would appreciate very much if you could write a text on all the velocity modulated valves you mentioned in post #2.

Best regards,

Dietmar

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