I went back to the 1941 paper by Heyboer of Philips, titled "A Discharge
Phenomenon in Large Transmitter Valves".
The abstract (part of it):
"In transmitter valves a flashover sometimes occurs between the electrodes
under high voltages in spite of a good vacuum (Rocky Point Effect). In
large valves this phenomenon may result in considerable damage, especially
to the filaments, if no precautions are taken. The damage can be explained
qualitatively, and to some extent also quantitatively by the ponderomotive
forces which occur between different branches of the filamnent as a result
of the current which flows through these branches when such a flashover
occurs. ..."
Here are some snips that seem to answer the questions that are being asked
over and over here. Sorry I don't have the entire document scanned, I did
have it, and mentioned that a few months ago. I did not use OCR software on
it, as the thing didn't photocopy well, and has formulas. So it was a big
file, too big to email to most. Sorry but I deleted it since then, and
havent' found time to scan it again. So i will only snip what is below. If
you want to read it all, I can mail it to you. Now he is talking about
tungsten filaments, not thoriated tungsten. But the concept may help
understand what some are groping over on this mailing list.
"In practice indeed such a defomation of the filament has several times
been observed after a flashover, and the deviation of the wires was
sometimes such that mutual contact or contact with the control grid
occured. In one case even one of the wires was torn free at the terminal.
At the same time traces of the discharge were observed on the filaments,
sometimes in the form of many-branched figures, usually as irregular
cavities in the wires."
He went on to discuss the calculation of the discharge current using AC
theory. Using a damping R for the discharge, he described a damped
oscillatory current (sinsusoidal). It's a classical power supply resonance,
excited by the arc discharge. An example of a real Rocky Point damaged tube:
12 KV plate voltage
L=1950 uH (in the power supply lead to plate)
C = 32 uF (in power supply)
filament at 2500 deg K, 10.63 cm long wires, 0.0892 cm diameter wire. tungsten
Specific resistance of W (tungsten) at that temp is 74 x 10^-6 Ohm-cm,
making overall R = 0.07 Ohms for two parallel wires.
Peak current calculated to be 1540 Amperes, and the oscillatory damped
current is shown, lasting for over 0.3 seconds, for 20 cycles to the half
current value. Spice analysis should be able to replicate this even today.
He then calculated the mechanical forces for such currents. Using the
current divided by the spacing r, one gets the magnetic field H. Each wire
has i/2 in it. Then the force per unit length is H x i/2
or i^2/(200 x r) dynes/cm force. The current being in the same direction in
two parallel wires, means that they will be pulled together. Putting the
current into the force calculation, he said that the force has a vibratory
frequency of 1280 cycles (1.28 KHz) and a force of 9620 dynes or 10 grams
per cm length of filament.
He continued the report with a calculation of the bending of the wires,
assuming they are like rods supported at both ends. With the elastic
properties of the wire, he showed that the there are mechanical resonances
at 121, 1085, 3020 Hz, etc. Therefore he concluded that it is not mechancal
resonance in the filaments. He assumed that the damping could be neglected
for the first two full cycles of flexure for the sake of calculation. The
deflection is 1.83 mm in the center of the wire. The tensile stress is
calculated to be 2000 kg/cm^2. Since the yield of the hot tungsten is 470
kg/cm^2, it will break or undergo permanent deformation.
Finally, Heyboer actually tested the theory on a filament in a glass bulb,
with a charged capacitor allowed to discharge through two parallel filament
wires. It did what he predicted. Amazing science.
His caveat (which gives the parasite theory room to still survive):
"In these experiments, where the mechanism which was observed was assumed
as an explanatory one is as it were isolated, and other phenomena which
might play a part in the complex structure of a transmitter valve are
excluded, the typical deformation of the Rocky Point effect actually do
occur."
Heyboer went on to install varous series limiting resistances, like 40
Ohms, in the plate connection. He is able to show that the bending is
minimal, and that it is important to limit the power supply current as
such, and do it with a noninductive R. He also includes a more exact
derivation of the bending force in the appendix.
I hope this will satisfy some of you, and also give you a great feeling
that yes, arcs can bend filaments, and yes, cause marks in a vacuum tube,
and yes, gas can be a cause, and yes, it is all measurable, calculatable,
and physical. An oscillation of the current can be at low frequency from
the L, C, R in the power supply. Now, it doesn't say that parasites
couldn't do something similar, such as helping to start a breakdown in a
vacuum tube. However, sudden burst of gas from the electrodes, in high
power tubes, called Rocky Point effect in the old days, is a certain way to
bend filaments in a poorly current-limited tube circuit.
OK, now that I finally spent the time to type this, I will back away,
hoping that I don't see 50 more emails overnight, debating whether tube
arcs can happen in the vacuum and bend filaments. The science was proven in
1941, at least published then. I gotta go, time to get a life.
73
K5PRO
John
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