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In a vacuum tube, are the anode and the cathode connected in any way? Answered

In the wikipedia article, they refer to the cathode as a filament, which I thought was a bit odd. The only other filament I'm familiar with is in a light bulb. I was reading a book about vacuum tubes earlier, when I looked them up in wikipedia searching for a diagram: vacuum tube. Basically, a vacuum tube is a capacitor with a vacuum as the dielectric? Does a vacuum tube retain a charge? They seem to have the same principle: anode, cathode, dielectric. In a way, a battery seems to also be similar; the difference being the galvanic process involved. I attached a picture of a Daniel's cell from back in the day.



Best Answer 7 years ago

A vacuum tube is not a capacitor.  And does not hold a charge.

The anode and cathode are not connected in any way.

The filament is hot and can give off electrons.

Those electrons can be controlled or amplified by other parts of the tube, like the gate.

I was looking at the similarities rather then the differences. Most of the images I've seen looked like a one turn inductor with a capacitor plate in close proximity attached to the positive lead of the inductor. The wikipedia image seemed to be a later model, with a cylindrical filament that wasn't a loop of wire. The way that last vacuum tube looks, it seemed to me like there would be some sort of capacitance, however small. I see what you mean. That cylindrical filament really threw me off.

Thermionic Valve.PNG

There is a capacitance between the elements but it is very small and shows up more as freq. goes up and in miniature tubes. The capacitance of the tube is not used like a capacitor. It is more a characteristic of the tube that the designer works around. In most circuits it is totally ignored.

There is also some "inductive" coupling since there is a current flowing thru the filament.  This is usually ignored since it is extremely small.

The " Thermionic emission" from the filament is much larger and overshadows the other two.

Erase everything you think you know about tubes because you are way off. The heating element is the filiament and yes it is like a lightbulb filiament. however, they are not lit up brightly. they usually have 6 or 12 volts applied to them and they light up dimly. Now imagine there are electrons bouncing up and back down from the hot filiament. Kind of like frying grease in a frying pan. Those electrons just frying away bouncing UP and back down to the filiament. That is your cathode. now imagine another piece of metal nearby but not connected to the cathode. It has its own wire attached and it is the ANODE. If you put a POSITIVE charge on the anode... it will attract those electrons bubbling off of the cathode. Therefore current will flow. If the anode is charged NEGATIVE... then no current will flow because LIKE charges repell LIKE charges. If you built that it would work OK. If you want to improve the operation, you might wrap a SPECIAL METAL around the filiament. This special metal emits electrons when it gets HOT from the filiament. So that is what they do. However, you really don't want the FILIAMENT to electricly touch the cathode. so it is insulated electrically. If they touch each other it would be a defective vacuum and it is called a HK (heater to cathode ) short. So far what we have built in our imagination is a simple DIODE. it will only conduct in ONE direction. The voltages being used on the tubes cathode and anode are several HUNDRED volts. Tubes work at higher voltages than transistors. To make a long story short... we really want to build some kind of controllable diode that could be used as an amplifier. They place a kind of metal screening between the cathode and the anode. When a voltage is placed on this SCREEN ... we can controll the current flow from anode to cathode. Some tubes have 2 heaters because they have 2 tubes in one. Some tubes have other plates and screens that perform similar controll function of current flow.
That is the best i can explain it.