This morning on NPR, heard that scientists have gotten enough data
from a new satellite studying kozmic debris to say that the universe
is 13.7 billion years old, +/- 1%.
Now, I wonder if we hams will ever come to agreement on something
here, which has used plenty of bandwidth here for the years I have
been reading this. I was almost going to post this last week, then
decided "no, I have plenty of work to do and besides, it hasn't
mattered how many times the references have been quoted, or
experience has been expounded here". The same bent filament helix and
parasite theory keeps getting shoved out as the only possible way
that certain triodes have been dying. So one can assume that this
will perpetuate forever here, until the principals fade away into
senility or SK. However, as Ian said, it is important to restate some
facts for newcomers and for the record, as so many have come to
believe only a certain science is possible, based on the repeated
assertions of a few on the web.
Thanks for repeating the outgassing phenominum Ian. I wholeheartedly
agree with it, as it is based on 70+ years of work in the field of
high power tubes. It happens quite often in our work with larger
tubes here and thank goodness it is explainable. Otherwise our
technicians witnessing this would have to be taught that there is
much more magic in radio frequency engineering that cannot be easily
explained. We understand parasites, can measure the presence of them,
and can find the sources in the circuits if we are careful. What we
don't prescribe to is that VHF parasites are so prevalent to bend the
helix of filaments regularly, or sputter gold in a planar triode, or
other topics most suited for nightime AM talk radio. Some or most of
the failures can be explained by various theories, and MOST can be
explained by flash arcs inside tubes.
Below I have quoted paragraphs and Rich's replies to them - which are
quite predictable (he has a vested interest in his parasite
cure-alls for his business as well as his technical integrity - those
oft' quoted 1980s QST articles and the subsequent 'eimac employee
Foote' letter). I don't disagree that nichrome wire makes a good
suppressor, having built-in resistance that gives the desired result.
In the 1980s I used it for some chokes and suppressors in some high
powered amplifiers and oscillators at two companies that I worked
for. One supressor design was patented, for a particular VHF cavity
amplifier using Eimac tetrodes of up to 20 kW plate dissipation
rating.
I have Richs anode suppressors in my SB220 at home, and they seem to
work well, in place of the stock Heathkit suppressors. But I cannot
fall to being a " blind-faith believer" from the continually
insistance that so many of the failed amplifier tubes are from the
obvious VHF parasitic mode. The autopsied carcasses do not
demonstrate this.
We have all been through this fifty times now. It is sad that Bill
Orr is deceased (he used to read here), that he is being dragged in
as having not understood bypassing grids on triodes, etc. Bill Orr is
probably one of the most respected ham/tube engineers of the 20th
century, as I have learned much from his many handbooks and articles,
and chatting with him. I can not write off his designs. I have
learned a lot as a ham from ARRL HB, but the W6SAI Radio HB is
required reading as well, for complete handling of RF amplifiers.
> >But more gas continues to slowly evolve into the "vacuum" space. This is
>>a perfectly normal process, even in a tube with perfect vacuum seals
>>(leakage is a totally separate problem). To maintain the quality of the
>>vacuum throughout the life of the tube, the manufacturer creates a
>>specially activated metal surface inside, called a "getter". The getter
>>will react chemically with any gas atoms that strike it, and will keep
>>them trapped on the surface. It's a kind of passive, maintenance-free
>>vacuum pump.
>
>** Concurrent with a big bang, the circuit breakers typically trip.?
>Does the gettering process take place when the tube is cold? If it
>does, can the gettering process take place in the time it takes to remove
>the tube from the amplifier and test it for gas with a high-pot?
The gas which causes bang is probably not going to show up on the
high pot afterwards. For one thing, if the energy is limited properly
by a good power supply design, it helps to clean up and condition the
tube for further operation. On our big triodes and tetrodes with
built in ion pumps, we see the results of the bang, after the power
supply crowbars, as a brief spike in the ion current meter. Often it
is very minimal, and can only be noted if you happen to be watching
the current. We usually watch this meter as we are raising the power
or duty factor of big tubes as we are working them at higher levels
for the first time.
> >If the arc is extinguished quickly and not too much energy is dumped
>>into the tube, the tube can recover completely. There may not be much
>>visible evidence that the arc ever occurred (depending on the tube
>>construction). If the tube was hot, the getter can collect the gas
>>within a few seconds.
>
>** reference please
See below, also experience with over one hundred tubes in my job.
> >So it's all a matter of time-scales. An arc can happen faster than the
>>getter can handle the gas release - but the getter can do its job faster
>>than anyone can possibly pull the tube out of the amplifier to test it.
>>
>** please explain the reason for a bent filament helix after a big bang.
Here is what I was planning to post a week ago, and then decided to
let it rest. Here are some references, for the record, the archives
and the newcomers here. Please do not request reprints of these from
me, as I have not scanned them, and am not being paid to be another
RF consultant for amps@contesting.com. One should be able to find
them at a resonable technical library in a corporation or university.
Maybe when I retire someday I will start a website reprinting old RF
and tube papers. I have put a few of them up on the AM Window
tutorial pages, those related to AM transmitter designs.
I had looked these up on the subject of flash arcs and their damage
in big tubes. I encounter occasion burps in large tubes in my work,
usually they trigger a crowbar and no lasting damage occurs. Most if
not all of them are what I would term outgassing related or a
breakdown of materials inside the tube as the average power is raised
above previous operating points. Most high power transmitter and
industrial RF users are accustomed to a sometimes lengthy
conditioning phase of 'breaking in' a new tube in the rig.
Rich (AG6K) often refers to Flyer's IRE paper from the early 1930s on
the WLW 500 kW RCA rig, in which much early work on suppressing
parasitics was published. This paper does much to demonstrate ways to
eliminate parasitics by balancing amplifiers, shortening leads,
deQing stray resonances, using good bypassing, decoupling power
supplies and so on. Fyler discusses trigger parasites, which pop up
only during part of the cycle of an RF or AF amplifier. Also negative
resistance effects of grid current, making dynatron oscillation.
Choke coil parasites, lack of neutralization parasities, relaxation
oscillator parasites, TGTP modes which can cause parasites and
others. The importance of this paper is that they succeeded in
building a 500 kW MW transmitter at that time-1935. They had many
parasites to conquer.?
Fyler also discusses a surge phenomena in transmitter tubes called
'gas flash' or 'Rocky Point effect'. It is stated as another type of
surge which can stimulate trigger parasites (which cause high
voltages to pop up at unlikely places in the circuitry). To quote
"The random occurance of the flash arc in vacuum tubes, frequently
not concurrent with modulation peak potentials, indicate spasmodic
gas-pressure variations. The intensity of the visual flash arc
varies from a barely noticeable incipient breakdown in the tube
producing a momentary audible ping, to an intense power arc resulting
in a virtual short circuit between tube anode and cathode or grid
accompanied by a loud metallic sound from the anode. It is possible
to burn out the filament and grid wires with a sustained arc.
Two of the original studies on flash arcs - which were originally
called Rocky Point effect - were published in the 1930s.
"The Flash-Arc in High power Valves" by Gossling, GEC Ltd, Wembley,
in Jour. IEE, V. 71, page 460-487, 1932. Its a long treatise on the
subject, and parasitics are discussed as well as other causes. Most
of the problems were found in certain valves made in a batch for one
particular transmitter. A lot of this paper discusses evidence of
arcs or bursts inside tubes. The main points are that paralleling a
lot of tubes is dangerous (a common practice then to get higher
power) as the energy deposition can be quite high in one valve if it
arcs internally. Also that the tubes seem to clean up from the flash
arcs if the energy is limited.
"A Discharge Phenomenon in Large Transmitter Valves" by Heyboer,
Philips Technical Review, V. 6, #7, page 208-214, July 1941.
This one does discuss the forces which can bend the primative
filament wires oftransmitting valves. Heyboer calculates the
mechanical force on a filament wire. It is one of the first
references that recommends a series resistor in the plate supply lead
to prevent damage. The actual oscillatory events are recorded and
predicted in experiments with a filament in an evacuated bottle. They
are damped oscillations from the inductance and capacitance in the
power supply and interconnections, not VHF parasitics. On the order
of a few KHz, and only for 20 cycles or so. The evidence is
thoroughly presented in this paper and the fix is obvious: limit the
deposited energy.
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