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[AMPS] 3-500Z

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Subject: [AMPS] 3-500Z
From: (Tom Rauch)
Date: Fri, 28 Sep 2001 20:44:29 -0400
> Is the 500mA limit one imposed on safe conditions for the anode, or
> for the cathode?

Neither. It is a typical operating parameter based on a certain ratio 
of peak to average current. 
> 600mA @ 2600V = 1560Wdc input; output (measured) = 1100W; apparent
> efficiency 70%, apparent Pd = 460W (realizing some drive power appears
> in the output).  Under these condx the amp is just hitting a small
> amount of compression (<1 dB) but with more drive the output will
> continue to climb; problem is Ig, which gets hard to keep below 150mA
> when you drive it too hard.

As long as IMD is OK, you are OK. Unlike MOX cathode tubes, 
thoriated tungsten tubes are NOT damaged by excessive peak 
emission demands. They simply limit at a certain current. That is 
why you can operate a thoriated tungsten filament at reduced 
heating voltage with no damage, and why the tube can actually be 
operated while the filament is still warming up without damage. 

Now here is a neat thing to think about.

Rich claims parasitics can somehow bend the filament from the 
extreme currents making magnetic fields, many amperes! The 15 
amperes of heating current won't bend it, but the plate current will.

He also claims the tube's emission will start to saturate at 400mA 

Which one is correct at what moment of time?

Factually, the saturated emission current is just over ten amperes 
in a 3-500Z...unless the tube has gas.
73, Tom W8JI 

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