To: <amps@contesting.com>
> Date: Thu, 14 May 1998 01:21:12 -0800
> From: Rich Measures <measures@vc.net>
> Subject: Re: [AMPS] Re: Parasitics
> >Rich claimed a K5 had parasitics in his 922 because "mistuning and
> >misloading can not cause excess tank voltage or arcs. The K5 showed up
> >and correct Rich, and said the nichrome Rich sent him was never
> >installed. It turned out the cause of the arc was a bad gas discharge
> >tube in his lightning arrester.
>
> Is a bad lightning arrestor breaking down the same thing as deliberate
> mistuning?
Yes, so far as the PA is concerned they are identical. That's why I
said "mistuning or misloading".
During a load fault, the PA is no longer properly matched to the
load. If, during the mismatched condition, impedance at the tank end
of the tube rises. With a high Q resonant tank for load and nothing
extracting all the available energy form the tank, anode conduction
angle shortens. Tank voltages soar, just like in any flyback system.
(Old automotive ignition systems used this principle, that's why they
quit working if the capacitor on the points opened up. TV sets and
switching supplies use this principle to step up voltages far beyond
what the turns ratios would provide.)
Even more important, a sudden burst of excess drive power can cause
the same end result. Arced bandswitches and grid damage are common
when certain ICOM rigs that output 300 watt RF pulses are used with
PA's, unless the PA is loaded extra heavy to handle the drive pulses
without saturation.
My amp dumped from grid overload fault and let out a quick "bzzzzt"
when I run an early 775DSP with the amp loaded to normal grid
current. I had to overload the amp and pull the grid current back to
about 25% of normal when using that particular rig. My peak storage
power meter showed a 260 pulse on occasional key closures!
Driving a PA loaded to handle 100 watts with 260 watts is a bad idea,
and will eventually tear something up (even if it's just people in
the neighborhood).
73, Tom W8JI
w8ji.tom@MCIONE.com
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