p[[[[[[[0000000000000000000000000000000000000000000000000000000000000000000
To: <amps@contesting.com>
000000000000000000000000>>Date: Thu, 11 Dec 1997 09:07:15
>>To: "Ian White, G3SEK" <G3SEK@ifwtech.demon.co.uk>
>>From: Phil Clements <philk5pc@connect.net>
>>Subject: Re: [AMPS] parasitic suppressors
>>
>>At 07:25 AM 12/11/97 +0000, you wrote:
>>
>>>
>>>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?
>>>
>>>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
>>
>You are correct, Sir.
Bias zeners also blow in amplifiers with directly grounded grids -- as
well as in amplifiers where the grid chokes remained intact during the
'Big Bang'. . As I recall, this was the case in the 3-500Z shown in the
photograph on page 15 of the September, 1990 issue of *QST*. The
filament helix has somewhat obviously been bent until it touches the
grid. If a rauchian vanishing-gas anode/grid arc was responsible for the
shorted zener and shorted filament, why is there no arc mark on the
outside of the grid? Before I opened the envelope of this kaput tube,
the vacuum tested normal.
... ...
>Eimac states that an amount on energy in excess of four Joules will
>permanently damage EITHER the cathode OR the fine wire grid structure during
> repetitive arcing in typical circuits used by amateurs. ...
The grid of an 8877 is rated at 25W dissipation. The grid of a 3-500Z is
rated at 20W dissipation. The "in excess of four Joules will permanently
damage" statement seems to be questionable. .
[note: a Joule (J) is 1 watt for 1 second}
- For a tube whose grid is rated at 1W, 4J might possibly cause damage.
For a tube whose grid is rated at 25W, how could 4J damage the grid? I
find it hard to believe that a legitimate power grid tube engineer wrote
such a statement, Phil. Do you know the name of the person who wrote it?
... ... ...
Rich...
R. L. Measures, 805-386-3734, AG6K
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