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[AMPS] close ratings

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Subject: [AMPS] close ratings
From: (2)
Date: Wed, 18 Jul 2001 02:39:47 -0700
>Hi John,
>> The plate swing probably will be slightly less than the DC voltage, 
>> for a reasonable efficiency amplifier. Otherwise it will be even less and
>> the amp will be underloaded. So figure that the voltage goes from 0.2 to
>> 1.8 x the DC or something like that, at the plate. The experts here
>> probably know the correct figure. Then on the other side of the blocker,
>> you only have the RF swing without the DC component. So the cap passes the
>> RF through, and blocks the DC, right?
>That is true only for a properly loaded amplifier. In a misloaded or 
>mistuned PA,. or a greatly over-driven PA, the peak RF voltage can 
>greatly exceed the supply voltage on each half of the RF cycle!
/\  During the Grate Parasitics Debate, Tom Rauch made the same claim in 
an attempt to put the blame for bandswitch arcing on operator error.  As 
a result, someone measured the worst-case tank mistuning peak V. in a 
SB-220.  The measurement was 3600V-pk - which is c. 24% higher than the 
supply V.  Nacherly, Mr. Rauch switched to ignore-mode.  [note:  the 
SB-220 bandswitch has a 5000V. breakdown.]

cheers, Tom  

>I can demonstrate, with an amplifier biased to the low edge of class 
>AB, that peak voltages many times the supply rail can be 
>produced. Switching supplies work on this principle.
>If there is not a very fast load-fault circuit, the PA should have a 
>spark gap of some form on the anode side of the tank. 
/\  Most do.  It's the Tune-C.

>In any amplifier biased much less than class A, and unless I had a 
>load-fault trip that would work faster than heat would damage the 
>component that arced, I would include an intentional spark gap of 
>some form.
/\  Fairly good idea - providing the spark-gap has a 50 - 200 ohm low-L 
resistor in series with it. 

>73, Tom W8JI

-  R. L. Measures, 805.386.3734,AG6K,  

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