A very nice explanation, Tom! A nice tutorial (which I am going to save).
Bob, WA2CKY
----- Original Message ----
From: Tom Rauch <w8ji@contesting.com>
To: ditzian@alltel.net; amps@contesting.com
Sent: Wednesday, August 22, 2007 1:35:29 PM
Subject: Re: [Amps] AL-811H maximum Ip
>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
Jan,
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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