Hi Jon & all..
How can operator error cause an arc in an unconditionally stable amp?
Easily! If the load impedance presented to the anode(s) is much too high,
VERY high rf plate voltage results due to the high tank circuit "Q". Very
high rf voltage also can occur across the load cap as a result of high load
VSWR.
Unless the amp has protective circuitry to prevent it, an arc WILL occur -
usually across the tune or load cap plates or in the bandswitch. Unless the
arc is interrupted within a few tens of milliseconds, the capacitor or
switch WILL be damaged, almost always in such a way as to decrease its
breakdown voltage so that future arcs are more likely - or almost
inevitable. In some cases aluminum capacitor plates will melt enough to
weld together.
How can excessively-high plate load impedance occur? Most likely (a)
because the operator sets up loading much too lightly (e.g., trying to run
500W efficiently by greatly de-loading a 1.5kW amp); (b) by trying to tune
up on a new band at full drive without presetting the load cap; (c) by
having the amp & antenna on different bands, resulting in very high VSWR;
or (d) due to any other occurrence that results in very high VSWR (e.g.,
damaged antenna, open or shorted feedline or connector, etc.)
So far as I know, only the current ALPHA amps have virtually 100%-effective
arc-suppression circuitry. There may be others, but the vast majority of
amps - even otherwise very good ones - can be damaged by rf arcs if load
VSWR is great enough at an unfortunate phase angle, or if loading is much
too light. It's very common.
73, Dick W0ID
-----Original Message-----
From: Jon Ogden [SMTP:jono@webspun.com]
Sent: Thursday, April 16, 1998 9:50 AM
To: Mike Willis; amps@contesting.com
Subject: Re: [AMPS] ARCING MORE
>It may be. Once the capacitor arced and the plates became deformed
>the arc persisted. The original cause of the arc was operator error,
>much of that cause will be treated by the extraction of the cost of
>the repairs from the users bank balance and lost free time during
>a major opening etc.
Excuse me if I am dense, but how can operator error produce an arc? If
the amp is completely stable and bullet proof to begin with and no
components are damaged, how did the arc begin? Sure operator error could
have damaged another component and that failure caused the arc. That
component would need to be replaced as well as the cap.
I'd definitely follow AG6K's advice and check the parasitic supressor
resistor resistance.
--------------------------------------------------------------------------
Jon Ogden
jono@webspun.com
www.qsl.net/ke9na
"A life lived in fear is a life half lived."
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