>Rich, you say:
>
>>4. High grid current caused high EM force on the hot (1800 degree C)
>>tungsten filament -- which caused the filament to bend and short against
>>the grid.
>
>Now if the EM force is going to bend the filament, the current has to
>be DC grid current, not AC at 130MHz - surely?
Surely - it is rectified DC.
>Well, maybe 'surely', but
>it seems unlikely that the reversing magnetic field could overcome the
>inertia if it was AC.
indeed.
>So the grid current (DC) has to be sufficient to
>burn the choke out.
.......yes
>But even saturated, the best a Thoriated flament
>will do (according to the Reference data for Radio Engineers) is
>100mA/watt so 7 Amps is the max emission, and probably lower.
However, that is a repetitive - not a one-shot event - capability
>How long
>would the parasitic have to be there to burn out a 1 amp rated choke?
Good question, Peter. We know that: without doubt, the grid chokes
occasionally burn out. The pi-sections on the choke typically implode.
Even though the insulation on the wire is generally intact - the copper
wire is burned in two, complete with copper melt-balls.
(Twilight Zone theme)
>And to get 7Amps of grid current, how much emission is left to go the
>plate? If it even had say 3 Amps of grid current, wouldn't it take
>rather longer to go?
A look at the evidence indicates that whatever happened, happened fairly
fast. The accompanying "big bang" suggests a sudden flow of current
somewhere.
>
>It would be interesting to see the grid of the the failed tube:
I have discarded dozens of grid-fil. shorted tubes. Last month, 5 went
in the recycling barrel -- 2 of which appeared to be virtually new. If
you want to see what a grid/fil. short looks like, see the photo on page
15 of the September, 1990 *QST* ('Parasitics Revisited') In our
centrifuge, it takes close to a minute at 10-G, with 5.8v on the
filament, to produce this much displacement in 3-500Z filament helices.
- During a one month horizontal operation (1-G) test with 5v applied to
the filament, the filament displacement was about 15% of that needed to
produce a grid filament short. (based on the observed change in
grid-filament breakdown potential)
>wouldn't be surprised to see a bent filament or grid, ...
The grid seemingly bendeth not -- even though the amount of
electomagnetic force thereon is obviously equal and opposite to that on
the filament. However, the filament is at roughly 1820 degrees K, so it
apparently bends easier during the moment of force.
>but I wonder at
>the cause.
>Puzzled
I was puzzled until, upon the advice of an OT (W7MOI), I decided to start
having a closer look at VHF parasitic suppressor assemblies in amplifiers
in which a tube had grid-filament shorted. I started finding zapped
resistors that showed no outward sign of foul play. Such damage seems
quite unlikely to have been caused by HF energy, or by an alleged DC arc
in an allegedly gassy tube. However, when tested on a high-pot., said
tube subsequently proves to have a good vacuum. . . . IMO, there is no
such thing as intermittent gas, except perhaps at chili-bean tasting
parties.
cheers, Peter
Rich---
R. L. Measures, 805-386-3734, AG6K
--
FAQ on WWW: http://www.contesting.com/ampfaq.html
Submissions: amps@contesting.com
Administrative requests: amps-REQUEST@contesting.com
Problems: owner-amps@contesting.com
Search: http://www.contesting.com/km9p/search.htm
|