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Re: [Amps] AL-811H maximum Ip

To: <>, <>
Subject: Re: [Amps] AL-811H maximum Ip
From: "Tom Rauch" <>
Reply-to: Tom Rauch <>
Date: Wed, 22 Aug 2007 14:35:29 -0400
List-post: <>
>I would like to thank those who almost immediately guided 
>me in tuning
> my AL-811H.  I will use 700 ma as the maximum Ip.
> 73,
> Jan, KX2A


I strongly disagree with the advice you have been given. The 
short term current really doesn't matter at all. Setting or 
limiting the current to 700mA won't do a thing to help the 
tubes live unless other things are correct.

Those tubes are thoriated tungsten filaments. They are NOT 
subject to emission wear or any other problem when current 
is too high.

What WILL kill the tubes is dissipation. If the tubes are 
out of resonance and the carrier is held on too long you 
will be in BIG trouble at only 300mA of current! If the tank 
is in resonance and properly loaded even pinning the meter 
won't hurt a thing for short periods of time.

This is why a pulser or CW keyer will do wonders if you are 
not fast on tuning.

It all really comes down to dissipation over time, and 
current has very little to do with being safe.

Assume you have 1600V HV under load. Each tube is rated at 
60-65 watts CCS dissipation, but will handle 120W in ICAS. 
15 seconds is about the maximum thermal lag time of the 811 
anode to go from idle temperature to melting at 120 watts 
dissipation. So for every 15 seconds on at 120W you need an 
accumulated time of 15 seconds off or the anode will start 
to creep over temperature.

With four tubes you can have about 250 watts dissipation for 
15 seconds while tuning if you allow a 15 second cool down.

Out of resonance you would have 15 seconds at 160mA. Not 
quite 700 is it?

You could have 320mA at about 7-8 seconds, and still be OK.

You could have 640mA at 3-4 seconds and be OK.

You could have 1000mA at a few seconds and still be OK.

Now when the anode is in resonance and power is being 
transferred to the load, the dissipation is roughly a little 
less than Pin - Pout. So with 700mA at 1600V you have 1120 
watts in and you could easily have 600 out.  That's under 
520 watts dissipation, or 130W per tube. So you are good for 
any mode where the duty cycle in a 15 second or shorter 
period is 40-50%.

The only tubes that suffer catastrophic damage from current 
and current alone are metal oxide cathode tubes. They can be 
damaged by any operation that attempts to draw more emission 
current than the cathode can supply. In the tubes you have 
it is all about anode heating.

I hope this gives you a better feel for what the problem 
really is. The problem is heat and time, not current by 
itself. The tubes are NOT safe during tuning just by 
limiting current to 700mA.

73 Tom 

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