Vacuum Tube & Thyratron plate capacitance

In summary: Okay, external capacitor. Is that something that you would need to do for every Thyratron?No, not necessarily. I've seen designs that use an external capacitor in conjunction with a feedback loop to achieve oscillation.No, not necessarily. I've seen designs that use an external capacitor in conjunction with a feedback loop to achieve oscillation.
  • #1
EinsteinKreuz
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What factors determine the anode plate-to-grid capacitance? I know that the capacitance of a regular capacitor is determined by the surface area of the plates and is inversely proportional to the distance between them. Does this hold for vacuum tube grids and anode plates? The reason I'm asking is what are the ways to vary the anode-grid capacitance and in particular, how would this factor into designing a Thyratron that has sufficient capacitance to self-oscillate when there is a cathode-grid feedback loop. The idea of this device is to use it as a relaxation oscillator that can oscillate at audio frequencies(so sufficient large capacitance is required).
 
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  • #2
EinsteinKreuz said:
The idea of this device is to use it as a relaxation oscillator that can oscillate at audio frequencies
Here's something that may interest you from www.junkbox.com/electronics/sheets/GE_Glow_Tubes_ETI-176.pdf

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  • #3
Don't know if this is on topic, please excuse if not

in high school(early 1960's) we used 2D21 thyratron for an audio relaxation oscillator
but it required external capacitance

to my considerable surprise somebody still makes thyratrons, you might contact them
http://pulsepowersolutions.com/switches/thyratron/

here's a couple links Google found for me just searching on '2D21 thyratron'
http://forum.allaboutcircuits.com/threads/yes-useful-thyratron-relaxation-oscillators-exist.9095/
http://www.electricstuff.co.uk/rca2d21.html
http://www.ebay.com/itm/2D21-RCA-Tu...485?pt=LH_DefaultDomain_0&hash=item259db2c425
 
  • #4
jim hardy said:
Don't know if this is on topic, please excuse if not

in high school(early 1960's) we used 2D21 thyratron for an audio relaxation oscillator
but it required external capacitance

to my considerable surprise somebody still makes thyratrons, you might contact them
http://pulsepowersolutions.com/switches/thyratron/

here's a couple links Google found for me just searching on '2D21 thyratron'
http://forum.allaboutcircuits.com/threads/yes-useful-thyratron-relaxation-oscillators-exist.9095/
http://www.electricstuff.co.uk/rca2d21.html
http://www.ebay.com/itm/2D21-RCA-Tu...485?pt=LH_DefaultDomain_0&hash=item259db2c425

Very Eenteresting! But it doesn't say whether or not the Metasonix thyratron sawtooth oscillator has external capacitors or is self-oscillating. Other types of tubes can oscillate without external capacitors, like the Tetrode for example.
 
  • #5
EinsteinKreuz said:
Other types of tubes can oscillate without external capacitors, like the Tetrode for example.

Oh ? In my experience that indicated something bad wrong in the circuit.

Anyhow i hope you find what you're after.

Good luck

old jim
 
  • #6
jim hardy said:
Oh ? In my experience that indicated something bad wrong in the circuit.
Yep. My old tube mentor taught me how to tap tubes to see if they were "microphonic".

Microphonics or microphony describes the phenomenon wherein certain components in electronic devices transform mechanical vibrations into an undesired electrical signal (noise).
 
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  • #7
jim hardy said:
Oh ? In my experience that indicated something bad wrong in the circuit.

Anyhow i hope you find what you're after.

Good luck

old jim
Well old jim, I might have to actually do the hard work of designing my own kind of Vacuum tube. Tetrode self-oscillation, often called parasitic oscillation is considered undesirable in amplifiers but for an oscillator circuit that's another story. There is an assortment of electron tubes that will oscillate when a DC voltage is applied to them but those are mainly radio frequency and rely on electron bunching rather than capacitance, like the Barkhausen-Kurz tube, the cavity Magnetron, and the feedback Klystron.

In the case of the Thyratron I'm aiming for, both the anode plate and the grid would have a sufficiently large surface area and be positioned close enough together to act as an internal capacitor. There is a feedback lead from the cathode to the grid plate. A negative DC voltage is applied to the cathode and a positive DC voltage is applied to the anode. The inert gas acts a dielectric between the 2 plates. When the current is turned on, the positive anode potential draws electrons from the cathode lead into the grid. The capacitance must be high enough(along with a heating element to generate sufficient heat)so that when the grid plate is fully charged to capacity, it will reach ignition voltage. When the tube conducts, electron from both the cathode will move through the ionized gas to the anode in an avalanche cascade causing the cathode-grid voltage to drop and then the tube cuts off.
 
  • #8
Interesting apparati, thyratrons...

Okay, I'm aware of the need to neutralize in RF amplifiers. In my high school days we'd place a short piece of wire vertically adjacent tube for a capacitor plate. But that was picofarads.
http://www.w8ji.com/neutralizing__amplifier.htmI just never thought about oscillation from Miller effect at audio frequency.o_O

old jim
 
  • #9
EinsteinKreuz said:
The idea of this device is to use it as a relaxation oscillator that can oscillate at audio frequencies(so sufficient large capacitance is required).
So why use a specially designed and built thyratron when you could use an external capacitance?
 
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1. What is the significance of plate capacitance in vacuum tubes and thyratrons?

Plate capacitance is a crucial factor in the operation of vacuum tubes and thyratrons. It refers to the capacitance between the plate (or anode) and the surrounding electrodes. This capacitance affects the performance and stability of the tube, as it can influence the resonant frequency and bandwidth of the circuit.

2. How is plate capacitance calculated?

The plate capacitance of a vacuum tube or thyratron can be calculated by taking the geometric dimensions of the electrodes and the dielectric constant of the surrounding material into account. It can also be measured experimentally using specialized equipment.

3. What factors can affect plate capacitance?

There are several factors that can affect plate capacitance, such as the distance between the electrodes, the material and thickness of the dielectric, and the shape of the electrodes. Any changes in these factors can alter the value of the plate capacitance and impact the performance of the tube.

4. How does plate capacitance impact the frequency response of a vacuum tube or thyratron?

The plate capacitance can act as a low-pass filter, limiting the high-frequency response of the tube. This is because the capacitance will create a reactance that decreases with increasing frequency, thus reducing the gain at higher frequencies.

5. What techniques are used to minimize plate capacitance in vacuum tubes and thyratrons?

There are several techniques used to minimize plate capacitance in vacuum tubes and thyratrons. These include using larger electrode spacing, using low-dielectric materials, and using specialized electrode shapes. Additionally, tuning and matching circuits can be used to compensate for the effects of plate capacitance on the overall performance of the tube.

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