On Thu, 11 Dec 1997 07:25:28 +0000 "Ian White, G3SEK"
<G3SEK@ifwtech.demon.co.uk> writes:
>Rich Measures wrote:
>>>The Big Bang will generate tremendous amounts of RF, at that
>instant, of
>>>unknown frequencies. Zeners and the like do not handle that very
>well.
>>
>>- The current path from the alleged anode/grid vanishing-gas arc is
>not
>>through the bias zener. However, with a parasitic oscillation, the
>fault
>>current path would be through the bias zener.
>
>Let's try to stay with detailed circuit analysis, and not fall back to
>vague arguments based on "lots of RF around".
I would love to if you would be so kind to tell me exactly what
transpires during a Big Bang internal gas arc Ian. After all these
months there have been nothing BUT vague comments.
As far as my comments about RF floating around I feel it deserves serious
consideration and not a flippant put-down.
In the SB-220 the insulated stud and hardware of the bias zener protrudes
right into the RF cavity; I use the word cavity deliberately. Anyone with
VHF/UHF experience should be able to see the potential of RF pickup from
this probe. Is it just a coincidence that the SB-220 has a very high
percentage of zener failure during an arc wheras the TL-922, with the
zener in a different location, often survives?
To me in the service end of things I call this basic fault
analysis...look for a pattern and then the why. Sitting in front of a
SB-220 schematic would not reveal this.
>Rich is obviously correct in saying that the bias zener is not in the
>path of a surge current loop that goes B+ - anode - grid - chassis -
>B-.
>However, what happens if the grid is connected to the chassis by a
>choke
>or resistor which then burns out?
That was the basis of my question last week; what goes first, the choke
or the zener? Thank you for repeating it.
I am the student here and asking questions of the experts. So far I have
very few answers.
>When the grid floats, wouldn't the electron stream then switch to the
>cathode? That would blow the bias zener, unless something else such as
>a
>fuse blows first.
Absolutely, in my limited opinion, but Rich has never brought this up,
even after repeated prodding. Maybe I should have phrased it as a
positive statement but it is often more interesting reading Rich's
answers.
It fits both circuit and fault analysis IMO and has been discussed
privately with several TL-922 owners in recent weeks.
There are many fine minds on this reflector but they are intimidated by
certain constant barrages of repetiveness and do not publicly contribute.
So the question of the week remains: Exactly what happens during an
internal gas arc? Both the DC and RF components, and puuuleeesee lets not
get off on parasitics again.
Assume the amp is in standby and biased at cut-off.
Has anyone a spectrum analyzer photograph of the event?
73 Carl KM1H
>73 from Ian G3SEK Editor, 'The VHF/UHF DX Book'
> 'In Practice' columnist for RadCom (RSGB)
> http://www.ifwtech.demon.co.uk/g3sek
>
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