Why would an electron-emitting heated coil require AC?

[greypilgrim]

Hi.

At school, I once worked with a Teltron tube that looked very much like this (I don't remember the brand):

The electrons are emitted from a coil that must be heated with a voltage around 5 V. But for some reason, it must be AC, it doesn't work with DC (and it took me a long time of troubleshooting until I found this to be the problem). Why would that matter?

 

[sophiecentaur]

When you say it "doesn't work", what do you mean? Does the filament glow or not in both cases? Are you reporting an actual event or just reading a written spec / instruction?
There's a theoretical possibility that there is a transformer somewhere I the filament supply. But not likely as 5V is the sort of voltage that many filaments use.

 

[greypilgrim]

I mean that there was no dot caused by the electrons on the fluorescent screen with DC. If the filament was glowing I don't remember (it's a while back), I'm not even sure it can be seen from the outside.

The instruction says AC, but I didn't have an AC supply around back then and thought it shouldn't matter. Only after a long time of troubleshooting I finally dug up an AC supply and voilà, the dot was there.

 

[artis]

in the diagrams i found on google it seems like the anode is connected electrically to one of the legs of the filament, the filament emits electrons by thermionic emission but if you connect the DC supply such that the leg going to anode is negative then all the electrons will be repelled even though the filament might still shine orange because there is current running through it no matter which way you connect it's legs.In order to form an electron emission beam just a hot filament is not enough you also need an electrode which is at a different (more positive ) potential than the filament itself.
If this were not true every ordinary incandescent bulb would be a vacuum tube.

 

[sophiecentaur]

in the diagrams i found on google it seems like the anode is connected electrically to one of the legs of the filament, the filament emits electrons by thermionic emission but if you connect the DC supply such that the leg going to anode is negative then all the electrons will be repelled even though the filament might still shine orange because there is current running through it no matter which way you connect it's legs.In order to form an electron emission beam just a hot filament is not enough you also need an electrode which is at a different (more positive ) potential than the filament itself.
If this were not true every ordinary incandescent bulb would be a vacuum tube.

From what you write, this thread seems to be based on memory of past events. Are you not in a position to repeat the experiment? If not then we may be dealing with evidence based on imperfect recall.
If the filament were coated with the same sort of rare Earth's then every lamp filament would have a cloud of thermionic electrons round it. I have to assume we all know the basics of operation of an electron gun.
You seem to be suggesting that any electron gun needs an AC filament supply. An AC supply can cause Hum on some audio amplifiers so DC filament feed is very often called for..

In a thermionic valve the filament is not 'connected to' the Anode (Apart from via the EHT supply); it is right next to the Cathode and may or may not be connected, depending on the detailed design. (Normal thermionic valves have two distinct connections to the filament and the Cathode potential is always 'near' that of the filament.) It is Anode - Cathode volts that cause the motion of the electron beam. The Anode is tubular in a Teltron tube and is close to the cathode (looks like parallel plates in the diagrams on Wiki etc.. The accelerating voltage is much higher than the voltage across the filament so the polarity of the supply would not be relevant. Have a look at the diagram in this link for a 3D diagram.

The call for AC volts on the Teltron filament is probably based on convenience and AC is cheaper then DC. The Cathode (=filament) will be near Earth potential.

 

[tech99]

I think it is more likely that the DC supply could not furnish the required current.
We do see effects caused by DC filament action in a triode tube because it depends which side of the supply the grid is connected. But I think the effect will be small for the Teltron diode.