Understanding the Barkhausen–Kurz tube

[Atomillo]

TL;DR

How is the oscillating voltage in the grid of a Barkhausen–Kurz tube created?

Hi:

Due to the quarantine I have more free time, I've decided to learn more about electron tubes. Currently, I'm trying to understand how the Barkhausen–Kurz electron oscillator works.

Now, I've been able to understand the calculation of electron motion inside the device and why they oscillate around the grid. According to Wikipedia (Wikipedia), the reason the current ouput is so low is because it consists entirely of the electrons hitting the grid and plate. What I don't understand is how the voltage appears on the grid.

Again according to Wikipedia, it is "induced". At first I thought about electromagnetic induction. However, the magnetic field produced by the moving electron is always perpendicular to the grid (in a parallel plane geometry), and thus the magnetic flux is always cero right? What is "inducing" the voltage then?

Thanks for all the help.

PS: I post this in the physics forum instead of the engineering one because I'm interested in the physical mechanist that causes the voltage itself. If it is in the incorrect place tell me please.

 

[Vitina]

Do not think of the grid as a grid, it is the plate of this setup, the plate is a reflector to the electron flow. The analogy would be a slide whistle, where the volume and top opening determines the frequency . The balance between the + grid voltage , - plate and capacitance do this. The plate voltage acts as a spring to oscillate electrons across the grid, the combination distance from the cathode to grid and balance of grid to plate voltage determine the range of tuning (see grid pot on the receiver).
This design was later replaced by cavity resonant , and Klystron tubes. The magnatron in your microwave oven is a cavity resonant tube that uses magnetic "steering" to develop massive output.

 

[Atomillo]

As I said, I understand the electron motion inside the tube. My question is how this electron motion "induces" a voltage between grid and plate.

 

[Vitina]

The "in line" induction is by electric charge, as electrons overshoot the grid (because electrons have mass), the "grid" goes more negative (with respect to its positive bias) and the "plate" goes more positive (with respect to its negative bias), and do not forget that the cathode is also affected by this traveling charge.

 

[Atomillo]

Why does this traveling charge alter the potential in the grid and plate? Is there a name for this phenomenon? How could I calculate this variation (obviously assuming a simple geometry like parallel planes)?

 

[Vitina]

This is induced charge, the same as a charged object inducing an opposite charge on a conductive surface, only at the speed of the electrons movement.

 

[Atomillo]

Two questions:
1.- Because the capacitance is fixed (i.e the electrodes are fixed) this causes a voltage to be created?
2.- Isn't the voltage fixed at the grid and plate due to the external power supplies?
Thanks for all the help, I think I'm getting there

 

[Vitina]

Yes, capacitance's are fixed but the "current" at the plate is AC on the DC bias as well as grid, remember that an inductor or voltage divider is in the circuit of both plate, cathode and grid.

 

[Atomillo]

I'm afraid I did not understand this last thing. Is the voltage created by electrostatic induction or by the flow of the current (caused by the electrons hitting the grid and plate) through the inductances?
I attach the circuit I found in Terman, which I suppose is what you a referring to.

 

[Vitina]

I understand. If you look at the electron movement drawing imagine the charge (voltage) on the opposite plates to vary with the position of the electron. Trying to read the books explanation even confuses me.

 

[Atomillo]

Yeah, the explanation is not really the best. My original question is why and how could I calculate this variation in the voltage. Is there any resource I could use to read about electrostatic induction in situations like this? Wikipedia wasn't really helpful

 

[Vitina]

I am sorry, the times I worked with these circuits date back to the mid 60's and they were outdated then, and had been replaced by beam power tubes. The downfall of these was very limited power and unstable performance due to shifting frequency with temperature. AND AGE

 

[Atomillo]

Okey, think of it another away.
When the electron is being deaccelerated, he's doing work on the system. That energy goes to the capacitor formed by grid and plate right? Because the capacitance is the same, that means that the voltage across the capacitor must change. Is my reasoning correct?

 

[Vitina]

YUP, the biggest confusion I always had, was understanding that the electrons had mass and were constantly being accelerated and decelerated. And to complicate this a bit the cathode capacitance to grid and plate are part of this,, the electron is "bouncing" between cathode and plate.

 

[Atomillo]

What do I make then of the whole electrostatic induction thing then?

 

[Vitina]

As the electron approaches the plate an opposite (+) charge is induced but not enough to counter the bias(-). So this shows up as a shift in grid to plate voltage , the electron is repelled but slightly after (remember the electron has mass). This change happens at near the speed of light (shifted by the electrons momentum).
Negatively charged electrons always 'induce' a positive charge proportional to their distance.
I hate using the term but this type of oscillator creates "packets" of electrons create this effect.