[AMPS] ARCING MORE
Richard W. Ehrhorn
w4eto@rmii.com
Fri, 17 Apr 1998 10:48:36 -0600
Hi again Jon...
I agree with Carl 100% on this one. Based on experience I'd guess that
there is virtually zero chance that in your case a defective parasitic
suppressor either caused, or resulted from, the cap arc.
As others have already suggested, the tune or load cap is usually the
"weakest point" when rf voltage is excessive. Maybe that's because cap
plates often aren't perfectly aligned and spacing varies; a spec of dust
between plates effectively narrows the gap and triggers the arc; often
plates have sharp rather than rounded edges, and on higher freq bands
(i.e., 21-28 MHz) where the plates many times are nearly "full-out" those
edges "look at" each other, creating excessively high voltage gradients
which increase the tendency to arc; corona is a function of frequency, so
ionization of the air leading to arcing is more likely at the higher f's.
Just some thoughts...my guess is that all these factors contribute to rf
arcs at one time or another.
73, Dick W0ID
-----Original Message-----
From: Jon Ogden [SMTP:jono@webspun.com]
Sent: Thursday, April 16, 1998 9:24 PM
To: km1h @ juno.com; amps@contesting.com
Subject: Re: [AMPS] ARCING MORE
> but how can operator error produce an arc?
>
>
>Wrong antenna is a classic way to a path of destruction.
>
Understood now.
>
>>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 fail to follow your reasoning...it is certainly not logical. In this
>case the capacitor was the weakest link and failed first. Otherwise it
>could have been the bandswitch or even a tube if it was one of those
>fussy ceramic ones.
>
What I am saying is how do you know the capacitor was the weakest link?
Maybe it is, but perhaps it is a secondary cause. I don't know what
happened to the amp and just cause the cap is arcing does not mean that's
the only thing that is wrong. Maybe it is and maybe it isn't. Perhaps
your experience has shown otherwise and my assumption is wrong. Ok,
fine. But if it were me, I would check probably every other component in
the output to make sure no damage was done to them either regardless of
which component failed first. I've pretty much always done this with any
kind of major failure.
>
>>I'd definitely follow AG6K's advice and check the parasitic supressor
>>resistor resistance.
>
>Pure horse s..t The capacitor arcing has limited the RF current and I
>also HIGHLY DOUBT if any other component is damaged. Since the amp is
>still putting out some RF that pretty much covers that issue...a shorted
>tube would be quite evident. The only other possibility is pecker tracks
>on the bandswitch.
Maybe. However, it is a very SIMPLE thing to disconnect one end of the
resistor and measure it's resistance. Maybe it isn't blown but perhaps
its value has changed over time due to heating, age, etc. I am not
suggesting that his problem is a lurking parasitic. However, the
suppressor IS there for a reason. Without it, I would bet that the amp
could oscillate much easier. The supressor is there for a purpose and
not just to look pretty. As long as the amplifier is apart and all, why
not just check to make sure everthing is copasetic.
>
>Jon, I realize you are an EE but have you ever seriously used or built a
>high power HF tube amp before? Or is the 4-1000A project the first? The
>reason I ask is that anyone with some real world experience has done at
>least a few stupid things to his amp and has suffered the consequences
>and is probably saying "yep, been there done that".
No...The 4-1000A IS my first HF tube amp project. That's why I look like
such an idiot as I stumble through it. I have built numerous amplifiers
at 800 to 1500 MHz and have worked on amplifiers at 6 GHz. However,
these were much lower power and pretty much always operated into fairly
well behaved loads (used isolators).
I've had experience using tube amps, but that was in the past about 10
years ago. I was inactive in the hobby for about 7 years and only got
active again a little over a year ago. So I am also trying to re-learn
everything I forgot about HF tube amps.
I am learning and that's why I make stupid comments sometimes.
>I fail to see why this thread is progressing up to La-Laa land and
>parasitics again.
>
>I suggest letting the Centurion owner file or replace the plates and then
>let US know what results...not going off into some endless loop
>perpetuated by magic or lack of information.
I am not trying to digress to parasitics. I am just suggesting a simple
thing be done in order to make sure nothing else bad happens. If that
little RL circuit in the anode is so unimportant than lets take them all
out of our amps and see what happens.....Why not be sure that it is
working right?
Carl, I don't doubt that you are probably correct in your assesment of
the situation. Cetainly, you have a heck of a lot more experience than I
and I know I could learn a lot from you. I appreciate what I have
learned. At the same time, don't discount the theories of others on
things too.
73,
Jon
KE9NA
-------------------------------------
Jon Ogden
KE9NA
http://www.qsl.net/ke9na
"A life lived in fear is a life half lived."
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