Thermionic Cooling?

[ak_47_boy]

The filament on my 304TL draws 10 amps at 10 volts warming up to glowing. After that the current draw of the filament drops to about 5 amps. The data sheet says it draws 10 amps.
Is this normal? Under maximum plate dissipation would the filament(cathode) be cooled by thermionic emission thus causing it to use more current?

[ulibub]

Hello!

This effect exists indeed, but I doubt that it is strong enough to cause a visible cooling of the cathodes in tubes. To leave the atomic lattice of a metal, each electron has to do a certain amount of "work function" to get free into the vacuum. That's the reason why we must heat the cathode - just to provide this work function for the electrons. And since this is a material constant (measured in electron volts), metals are used with a low work function - such as Barium (provided by thermolysis of Barium Oxyde on oxyde cathodes) or Thorium. The energy needed by the electrons to provide the work function is taken from the thermic power of the heating - and the more emission, the more work function power is needed, thus resulting in a certain drop of cathode temperature on high emission conditions indeed.

But I don't know of any case with tubes, that cathode heating power (voltage, current) had to be reduced during low-emission conditions (to prevent overheating). But I know it from the cathodes of mass spectrometers (tiny hard metal filaments, strongly heated) that these sometimes overheated and melted when no emission was needed (acceleration voltage switched off).

If your filament heating current drops to the half when plate voltage is switched on, you really should see the cathode getting much darker - it's a reduction of the heating power by 50%! And indeed, if a strong thermionic cooling would occur, you should see an increase (!!) of the heating current - a cooler filament draws more current than a hot one! And I think if there was a tube where heating power would have to be reduced during low emission, this would be marked in the data sheets...

I'm quite sure that your results have another cause... Maybe some current limiting effects in your power supply? Or simply a malfunction of your meter?

Uli

[Gold_xyz]

Hi

Datasheet Eimac says :

Mounting : vertical.
Cooling : convection and radiation.

http://frank.pocnet.net/sheets/090/3/304TL.pdf

With 300W anode + 100W heater = 400W dissipation
I think that this is near to limit of natural convection cooling.

If the filament current start from 10A and down to 5A
with a costant 10V supply perhaps it can be the exhausted filament.

Cooling with electrons emission is a very small amount.
(compared to heat of filament)
http://ieeexplore.ieee.org/xpl/freea...isnumber=15953

[astouffer]

Maybe the test leads in your meter are adding some resistance?

[nhuwar]

Possible lose of cross sectional area of the filament causing a high heated resistance. How old is this tube?

[ak_47_boy]

Well i think its post ww2. It is NOS. I was just using a 10 amp dmm meter in series with the filament. It gets bright, just like a light bulb.

I am concerned since running a thoriated filament a little high or a little low can cause damage.

[nhuwar]

The damage can be fixed usally by a procedure made by eimac you can find in the care and feeding of power grid tube manuals found on there website.
The damage can mainly be done by overload of emission.

Power on thorated W can also be ran at a slight lower voltage if you follow eimacs procedure for finding this operating point also found in the care and feeding manuals.

Heres a link to the manuals on eimacs website http://www.cpii.com/library.cfm/9


Nick

[kevinkr]

What are you using to power this tube's filament? Perhaps you should also measure what is happening to the filament voltage across the tube.. You might find it drops over time, that's the only explanation I can come up with for a reduction in filament current - from there you will have to determine the cause.

[ak_47_boy]

A lead acid battery. I checked for voltage drop and their was none.

[d2134]

I think your DMM is fault. Please check it.
It is normal for a cold filament to draw more curent then a hot one and the ratio is normal, I suspect your DMM.