>
>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.
>
// I agree that these anomalies in the 7835s are arc marks, John. How
much voltage are we talking about?. The tubes I autopsy - 8877s, 8874s,
3-500Zs, et cetera - are smallish compared to the 7835.
>-----------
>
>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.
// Indeed. I learned about autopsying tubes from Bill Foote at Eimac in
February of 1986. I have two kaput 8877s awaiting autopsy in my tube
morgue/garage. However, Mr. Tom Rauch, W8JI says that nothing can be
learned from autopsying an electron tube. The autopsied 8877 in Figure
24 on my Web site suggests that this may not be the case.
>
>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;
// The all inclusive parasitic theory originated with Mr. Rauch. About
half of the 8877s that I have autopsied died from gold
meltballs/sputtering generated by what an Eimac specifications engineer
(Bill Foote) called an "oscillation condition" (letter on my Web site).
In "Parasitics Revisited" (September and October, 1990 QST) one of the
two kaput 8874s - one of the two 8877s and one out of one 3-500Z that I
autopsied for the article seemingly died from parasites. Three out of
five tubes is hardly "all inclusive". Curiously, Eimac customer rep. Mr.
Reid Brandon, W6MTF, told Paul Pagel (QST Technical Topics Editor) that
Mr. Foote was ''not authorized'' to tell me about such information.
>in industry we don't subscribe to that theory
// nor do I
>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.
// Higher gain, higher frequency capability tubes seem unlikely to have
an ability for self-vaccination against parasite predation.
>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.
>
// Interesting words and photos.
- cheers, John
>
>
>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?
- R. L. Measures, 805.386.3734,AG6K, www.vcnet.com/measures.
end
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