Question about electron gun design

[Rinehart]

I've come across a curious statement in Tube: The Invention of Television by D.E. & M.J. Fisher. Describing Zworykin's visit to the Belin Laboratory in Paris, they mention that he was shown a new CRT using a high vacuum and electrostatic deflection. (They use the word "steering," but since they make a distinction between that and focusing, this is presumably what they meant.) It had not yet been made practical, but Zworykin saw how it might be: "[by] relocating the electric steering field, placing it closer to the electron source than they had it. This would allow him to steer the beam before it was accelerated by high voltage to gain brightness, and this would give him much better control [over the direction of the beam.] He would accelerate the beam after it had already been steered in the proper direction; then the acceleration would not affect the steering, so he wouldn't have to worry about losing focus." (p.169)
This suggests that he would place the deflection plates in between the first and second anodes, but I have never seen a diagram or a photograph of an electron gun so constructed, and even the diagram of a kinescope tube on p. 185 doesn't look to be constructed that way. Does anyone know what this might mean?

[jr_tech]

Sounds to me as if they had the basic concept of a "Post Deflection Acceleration" (PDA) CRT in mind. Checkout this patent, for example:

http://www.patents.com/us-4767964.html

jr

[Rinehart]

I'm afraid that I still don't understand. The diagram I've attached is almost exactly the same as the one in the book. If I have understood correctly, in order to deflect the beam before the second acceleration, the vertical and horizontal deflection plates would have to be mounted in the neck between the first and second anodes, which they clearly aren't. I have the feeling that I've misunderstood something pretty basic here. Albert Abramson, in his biography of Zworykin, describes exactly the same thing as do Fisher and Fisher, and it is inconceivable that he's wrong.

[bandersen]

There's really a 3rd anode after the deflection plates. It's the internal conductive surface tied to the external anode connection.

[miniman82]

As an aside, I never really understood why 'static tubes don't get ion burns. Is it because the deflection field is strong enough to move both the electrons and ions, sweeping them both across the screen? Would it be a good idea to aluminize scope tubes?

[Rinehart]

So, is the third anode at the same potential difference relative to the cathode as the second anode is, or is there a further voltage drop?

[bandersen]

It would be a much higher potential to accelerate the electrons.
I just remembered I have a 12FP7 radar CRT like this. Here's a picture of it next to a 7JP4 for scale. It has external 3rd anode connection, but it's not visible in this photo.
Yes, that huge 12" CRT uses electrostatic deflection.

[jr_tech]

Nice Pix! Some that I have seen use 3-5 "wall bands" that are operated at ever increasing potential as the screen is approached. The idea here is to deflect electrons as they are moving slowly (for high deflection sensitivity) then accelerate the electrons as they approach the screen (for high brightness). The trick is to do this without loosing too much of the deflection sensitivity or producing distortions as the beam is accelerated. The "scan expansion mesh" was one of the better solutions.
I have attached a Pix of another early scheme that worked fairly well...a resistive spiral is deposited on the inside of the bottle. At the end near the deflection plates the spiral is connected to a low voltage (similar to that on the deflection plates)... the other end, to 10kV or so, to produce a nice bright image.

jr

[ppppenguin]

Don't forget the fundamental properties of electrostatic CRTs: deflection sensitivity is inversely proportional to EHT voltage. In magnetic CRTs there's a square root law so the deflection power doesn't increase as fast with higher EHT.

PDA overcomes this inherent problem with electrostatic CRTs. I suspect it only really works with narrow deflection angles, possibly due to geometry problems and the usual trouble of the beam hitting the deflector plates.