>Subject: [AMPS] How do filaments break?
>Sent: 5/8/97 6:51 AM
>Received: 5/9/97 7:09 AM
>From: Lee Buller, k0wa@southwind.net
>To: amps@contesting.com
>
>This is extremely interesting discussion.... but.... Could somebody
>explain to me the physics of how a parasitic oscillation can affect the
>filament in a tube enough to break it?
Electromagnetic force occurs at a right angle to the flow of electric
current. This is why, when arc welding with high currents, the cables
tend to twich when the arc is struck.
>Is there so much current that the filament wire separates or burns in two
>pieces?
The filament does not burn in two during an intermittent vhf parasitic
oscillation. The filament shatters in 811As and 572Bs. In 3-500Zs the
filament is pushed sideways at the unsupported center of the filament
helices. This decreases the clearance between the filament and the grid.
[9/90 QST magazine, "Parasitics Revisited".]
Today I tested 3, 3-500Zs that were removed from an LK-550 that had
arc-damaged tune capacitor plates.. The grid/filament breakdown
potentials were 2400v, 2800v and 5100v. However, a normal 3-500Z has a
grid/filament breakdown of 8kV to 9kV. How about the vacuum? Two of the
tubes exhibited under 3uA of anode/grid leakage @8800v. The third tube
was 5uA. In other words, the vacuums were good.
>If that is the case, where does this current come from?
The pulse of current comes from the cloud of electrons that surround the
cathode. In a high Mu triode, oscillation with no load results in high
grid current.
>How much
>more is this current that what is flowing normally through the tube?
In 3-500Z amplifiers, it is not uncommon to see 1A grid to gnd. chokes
open during a push-push vhf parasitic. As I recall, the fusing current
rating of #27 gauge wire it is at least 15a. That is about 100 times
normal grid current.
...snip...
>Is there some physical oscillations that the internal structure of the
>tube
>cannot hold and the filament breaks?
IMO, there is a single pulse of EMF.
...snip...
>
>... the power supply
>would
>hum very loudly, the tubes glowed dull red, the meters would show current
>flow, but nothing ever flashed. This went on for minutes while I
>determined
>what was going on.
You were apparently dealing with a push-pull parasitic oscillation. The
anodes get hot, but nothing arcs. In an SB-220, placing the parasitic
suppressors side by side will produce a push-pull oscillation at about
55mHz.
>And the minutes added into at least an hour or more.
>Never damaged the tubes. ......
That's the difference between the push-pull and the push-push variety.
Rich---
R. L. Measures, 805-386-3734, AG6K
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