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[AMPS] emission

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Subject: [AMPS] emission
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Date: Wed, 14 May 1997 10:33:32 +0000
> >The specific peak emission is what is applicable to the parasitic 
> >"theories". That is the absolute maximum emission current available 
> >under any bias voltage condition. 
> No Tom, that is only the peak "permissable" current and has absolutely no
> bearing on fault condition parameters. You keep dancing all around the
> subject but refuse to stay on too Rich. 

Specific peak emission is the saturated emission of the 
filament.  It is impossible to obtain values much larger than this 
emission, the tube would arc first. Let me give an example, a 3-500Z 
(a 75 watt filament) has a specific peak emission of about 9 amperes. 
It takes a few thousand volts of control grid to cathode voltage, 
with 4000 volts anode voltage, to reach 12 amperes. That's an 
impossible situation in a grounded grid PA because of a simple 

Metal oxide cathode tubes have high ratios of peak to RATED emission, 
but thoriated tungsten filaments do not.   

>From "Principles of Electron Tubes"....

"The maximum current of which a cathode is capable at the operating 
temperature is known as the saturation current."

"The absolute maximum current of which a cathode is capable of 
providing is the specific peak emission" 

The CW value is the typical maximum operating current. 

On page 15 and 16 of Varian's Care and Feeding of Power Grid Tubes 
you can find a repeat of the values I gave. CW filament emission of a 
thoriated tungsten tube is about 10 mA per watt of filament power. 
Specific peak emission is listed as about 100 mA per watt.

The metal oxide cathode is NOT listed as a specific peak emission 
because it does not follow simple rules of thumb, but it does list a 
CW current of 300 mA per watt of filament power.

Emission pretty sharply limited in a thoriated tungsten 
tube, but is poorly defined in metal oxide cathode tubes. In metal 
oxide cathode tubes it is given as the saturation current, that 
current is defined as the point where emission increase with voltage 
increase eventually fails to follow a three-halves power of the 
voltage change relationship. In a metal oxide cathode the curve rolls 
over at saturation.

In a thoriated tungsten filament, the specific peak emission is a 
sharp limit. 

> In a catastrophic failure mode the current can be whatever it takes to
> cause the damage. A VHF parasitic can approach 20-30KV or more, even in a
> simple 811A circuit.   It  depends largely upon the unloaded Q of the
> tank circuit since the load IS NOT absorbing ANY parasitic energy. 

It depends on the conduction angle of the tube and load Q. If the Q 
was so high the voltage could reach 20-30 kV, the PA would oscillate 
ALL the time. It would not oscillate just once in a blue moon 
with "certain high gain tubes" if things were all that sinister. 

There appear to be too many contradictions to justify an oscillation 
as the cause of a common failure.

> >Other tube design and engineering books agree with this, the highest 
> >number I've found is 125 mA per watt.
> >
> >The CW emission capability of a metal oxide cathode tube is typically 
> >200-300 mA per watt of heater power, about 30 times the CW emission 
> >of a thoriated tungsten tube.

Exactly from the Varian book.  All my other Vacuum Tube design and 
Physics books agree closely with these numbers.

All tubes outgas Carl, no matter what the age. Let's share knowledge, 
instead of insults. I'm on this forum to learn and help, not to 
argue and insult. I'm sure we both have enough to do without arguing 
or implying someone else has a character flaw. 

73, Tom W8JI 

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