I am amazed at the lack of thought (I mean science) behind
statements which frequently are repeated here. In the past years,
this has been discussed and validated on this forum, and in
professional RF power practice its not even a question to
regurgitate, and yet, here it is again.
As a professional involved in RF amplifier designs, I completely
support and agree with Ian White's reply and references listed at the
bottom of this note, and have also presented physical evidence in the
way of photographs of the insides of tubes which have arced (2 weeks
ago here). They were not due to parasitics. Energy is energy. Whether
we have 3kV with 80 uF or 30 kV with 80 uF of capacitance. I pointed
out that in the latter case, one MUST use an active shunt crowbar
device in addition to series resistance. In ham sized tubes, series
resistance alone may work. But the cause of an unexplained discharge
within the envelope of a vacuum tube is NOT always because of an
undesirable oscillation. Gas pressures inside a tube may change from
effects from high voltage conditioning, changing RF level, DC
current, load VSWR. We should ask other EE's and RF practicioners who
design amplifiers to speak here; unfortunately, most will shun the
unprofessional approach of the 'scientific' method as practiced on
many internet discussions and won't bother.
I have attempted to give my own experiences to this forum. I
certainly haven't built as many amateur amplifiers as many here. Or
troubleshooted them. It is extremely frustrating to read people
preaching stuff which they believe in like religious zealots. We have
enough of them in the news in the USA right now.
On 8/30, Richard Measures <2> responded this way:
JL first stated:
"I recommend that hams consider other methodologies which can cause
arcs besides an 'all inclusive' parasitic theory....
<2> replied to this:
"// The all inclusive parasitic theory originated with ....."
"in industry we don't subscribe to that theory."
"//nor do I"
OK, we finally hear that there are other possibilities in mind?
To which <2> wrote yesterday:
>// The purpose of the glitch-R is to protect the electron tube from
>failure during an intermittent oscillation condition.
> A suitable glitch resistor in series with the positive HV bus will happily
>do this job - but only for those who have more brains than money.
Ian White wrote:
> >>Make that "...from failure due to a current surge occurring for *any*
<2> (Richard Measures) wrote:
> >// Please name and explain some other causes of potentially fatal
>>current surges, Mr. White.
Ian White replied:
>You appear to have forgotten that this very subject was discussed at
>some length back in January/February.
>Here are a few quotes from the literature.
>"Although a very high degree of insulation between two electrodes can be
>obtained... by modern high-vacuum technique, it is nevertheless possible
>for this insulation to break down spontaneously and completely. This
>phenomenon is known by various names, such as... the "Rocky Point
>effect", after the American wireless station, which is one of several at
>which it has been observed. ... Time lags [after application of
>voltage] can vary between a few seconds or minutes to tens, hundreds or
>even thousands of hours."
>Gossling (British GEC), 'The Flash-Arc in High Power Valves', 1932.
>"Most power tubes are subject at some time to a phenomenon known as the
>Rock Point effect, which derives its name from experiences with power
>tubes in communications transmitters at Rocky Point, Long Island.
>This phenomenon manifests itself as an internal flash-arc developing
>with little warning on power tubes which apparently are of good design
>and operated in a conservative manner... The cause of this phenomenon is
>not thoroughly understood..."
>Parker and Hoover (RCA), 'Gas Tubes Protect High-Power Transmitters',
>"An arc is a self-sustaining discharge of electricity between electrodes
>in a vacuum environment... Since any high voltage vacuum device may arc
>at one time or another..."
>Eimac, 'Fault Protection', Application Bulletin #17, 1987.
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