This note was sent with two JPEG files with photos, to 2 and W8JI.
Here is plain copy. I can forward the photos upon request.
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I have inspected the insides of probably a dozen big tubes in my
work, and most - if not all - of them have arc marks inside. Some
were still operating when pulled and opened, as they had excessive
hours and needed rebuilding. We pay a standard fee to the
manufacturer to open each tube and analyze the failure modes and
condition before rebuilding as the rebuilding process is expensive,
but cheaper than a new 7835 grounded grid triode. I usually witness
the opening when a failure mode isn't understood. Autopsy is the best
method to find out what killed it.
Attached are two photographs showing two different tubes at the
manufacturer. In one, for serial number P9, you can see a 1/4 inch
chunk of the grid missing, my pen pointing to the hole. This was
caused by numerous plate to grid arcs. We obviously had problems with
the protective crowbar for this particular amplifier. We also have
vacion guages mounted on each of these big tubes, and can observe the
pressure surges when they outgass. It always occurs on turn on when
the filament is ramped up to full temperature from cold. We call the
first hour of RF power conditioning, as we have to slowly increase
the plate V and the duty factor to keep the vacion reading below a
certain pressure. There are no parasitics which will happen during
this first hour and then subside. It is strictly a materials
function. New tubes are conditioned for over 100 hours to burn off
barnicles and outgas the oxygen free copper and other materials as RF
current heats them.
In the second photo, two anodes are compared, one with cracks in it
due to a manufacturing defect called grain growth. The lower tube has
no such cracks, but instead one can see numerous arc marks around the
large radiused edge of the anode. To see it with you computer, you
should zoom in with your picture viewer program around the upper edge
of the lower tube. There are clearly visible black marks and smudges,
with dings into the copper.
I recommend that hams consider other methodologies which can cause
arcs besides an "all inclusive" parasitic theory; in industry we
don't subscribe to that theory unless other reasons are found to
promote that hypothesis. Even then, the smoking gun is difficult to
find. The Fyler/GE paper on parasitics from the WLW 500 kW
transmitter that I sent to Rich about 4-5 years ago told of many
instances of parasitics which had to be snuffed. A reading of the
complete paper reveals that they were breaking new ground with high
RF voltages and standing waves in large circuit layouts. Flash overs
were common in that rig, which was the first of its kind. Parasitics
were really a problem. One certainly hopes that these are not the
sort of problems which we have today, 80 years later. In our
amplifiers at work it certainly isn't, and the photos prove that
outgassing is the predominant cause of internal arcs in tubes with
handles. Now the reasons for sudden outgassing are many, including
changing RF loading, power supply voltage fluctuations, changing
drive power, changing duty factor, changing VSWR (fast!) or loss of
cooling. I am sure there are other reasons also, but in essence we
are getting a thermal condition which will push the materials to a
new plateau of operation, with subsequent outgassing until they
settle down at the new level.
73
John
K5PRO
2 sez:
>// Of the dozens of kaput tubes I've autopsied, I have yet to find one
>with an internal arc mark.
>
>>If the tube and other components have enough HV breakdown to
>>operate at the desired frequency, they almost certainly have
>>enough for any unwanted oscillation.
>>
>// ... whistling in the dark?
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