>Rich Measures wrote:
>>If you lived a bit closer, Ian, I have three, intact, filament/grid
>>shorted 3-500Zs that we could disassemble and inspect. My best guess is
>>that we would find bent filament helices in three of them.
>
>To further things come to mind.
>
>One was the message from Reid Brandon at Eimac, posted a few weeks ago
>by W0UN. Here's the currently relevant part:
>
>>Tungsten wire undergoes a physical change when raised to its
>>operating temperature and returned back to room temperature.
>>In tubes with thoriated tungsten filaments, the end result is that
>>over many on/off cycles the shape of the filament begins to become
>>distorted, ever so slightly but depending on how many cycles, the
>>spacing between the filament and grid (which must be very close to
>>achieve good electrical characteristics) may decrease, and eventually
>>a short may occur.... Reducing the filament inrush current
>>is no solution, tungsten must still go through a large excursion in
>>temperature.
Eimac's engineering department presumably decided that it was best to
limit the 3-500Z's filament inrush current to double the normal 14.5A.
As of December, 1996, Reid Brandon did not work in in the engineering
department. He worked in the marketing department.
- I have seen more shorts in relatively new tubes that I have seen in
tubes that were 10 to 30 years old. Typically, grid/fil. shorts have
been associated with a big-bang and arcing. If a filament gradually
bowed and touched the grounded grid, there should be no fireworks.
>
>That's the first point. The second one is mentioned in RB's message and
>then taken up by someone else:
>
>>Reducing filament inrush current is important for the reduction in
>>the magnetic field between filament wires, and allows the spring
>>tension that exists across the filament structure to gently keep
>>the filament aligned (special springs are used for this purpose
>>in many transmitting tubes.)
>
3-500Zs do not have filament tensioning springs. 4CX10,000Ds and 811As
do.
>Me again: If we combine those two observations, there seems a distinct
>possibility that the filament could gradually migrate towards the grid
>over the course of time (especially if the filament tension wasn't quite
>correct, which wouldn't show up in the normal factory acceptance test).
>We already know that 3-500Z filaments can't tolerate much sideways
>force, which is why they're only allowed to be operated vertically.
The normal grid/filament withstanding V in a 3-500Z is 8 to 9 kV. I once
tested a tube that I believe sustained an intermittent VHF parasitic
oscillation. The tube would short after a few minutes of operation. The
grid/filament withstanding ability was 3500V. In a darkened room, I
could see where the filament was arcing to the grid. I marked the
azimuth of the arc. I operated the tube horizontally with the filament
bulge up, using 5.0v on the filament. After a month of horizontal
operation there was a measurable improvement in the grid/filament V
withstanding ability. I calculated that it would take at least another 9
months of horizontal operation at 5.0v to make the filament helices
straight. However, in a centrifuge we were able to do the job in 30 to
40 seconds.
>When the filament eventually moved so far that it touched the grid, it
>would connect the grid to a voltage tap somewhere along the heater
>circuit, which could indeed produce the burnt-out grid choke and the
>rest of the fireworks.
>
A short from grid to filament shorts out the drive, so there is no grid
current to burn out the choke. There were no fireworks when I conducted
such a test.
Rich---
R. L. Measures, 805-386-3734, AG6K
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