[AMPS] Filament breakage
km1h@juno.com
km1h@juno.com
Mon, 12 May 1997 14:54:47 EDT
On Mon, 12 May 1997 11:44:58 +0000 w8jitom@postoffice.worldnet.att.net
writes:
>> Instead of wandering all around the subject how about sticking to
>one
>> very basic tube like the 811A. Now explain the cause of sudden
>arcing,
>> bangs, flashes, etc with the end result being a broken filament and
>even
>> other circuit components.
>
>There certainly are multiple causes Carl, but it is quite obvious
>from the emission limitations of the tube and the actual mechanical
>force available it can not be the direct result of an oscillation.
>
>It's my opinion that whenever someone does not understand a problem,
>he falls back on something he can understand. This is the point
>where science departs from rumor and wive's tales.
>
>> I'll even give you a hint...from my perspective; magnetic mumbo
>jumbo has
>> nothing to do with it.
>
>The magnetic issue is indeed mumbo jumbo. It can be clearly
>demonstrated the amount of current required to mechanically
>stress the tube are unavailable from either inrush or oscillations.
As long as we are talking about the 811A only, I will agree...somewhat.
The 3-500Z is a different animal.
>Look at a real world example of how trivial the magnetic force
>really is. A small induction type meter, typically used for a car
>ammeter, has to use very strong magnets, light weight pointers, and
>steel magnetic paths to cause a meter pointer to deflect with
>dozens of amperes of current flowing through a wire a fraction of an
>inch away.
That is irrelevant Tom, you are drifting again.
>It is the uneven HEATING of the filament that causes mechanical
>stresses, not magnetic flux.
Now you're getting warm.
>Two destructive mechanisms are at play in healthy vacuum
>tubes Carl. I*I R losses producing heat, and kinetic energy from
>electrons heating the element they strike. Any heat problem is a
>function of BOTH power dissipation and time.
>
>Kinetic heating is closely defined by the accelerating voltage
>times the current. We do it every day. The anode dissipation is
>simply the cathode to anode voltage times the anode current, at any
>given instant of time. This is true under ANY operating condition.
>
>Peak voltage is determined by the Q of the anode load and conduction
>angle of the tube.
We have covered this ground many times before.
>In normal operation a tube that comes up and idles fine for hours and
>hours on end can arc and fail when RF drive is applied. The reason is
>the anode voltage easily exceeds the quiescent voltage.
You have missed my point for the 3rd time in a row now Tom. I have
repeatedly said that the tube(s) would run fine when in standby but would
blow the filaments when the TX relay was energized and NO DRIVE applied.
Repeat NO DRIVE, NO DRIVE.
Just a bit of idling current and within 10 seconds or so BANG. To my
stubborn Germanic brain that spells Parasitic...at the TOP of the list.
>Outgassing also is a common occurrence. If the elements are heated,
>gas molecules can be brought to the surface and released. Graphite
>anode tubes are very susceptible to this problem, because the
>porous graphite traps gas. It takes longer to pump a graphite anode
>tube down, because the thicker porous anode material often traps more
>gas.
We are into mainly operator error here, mistuning, overdrive, wrong
antenna, etc. The end results are melted glass (remember, the 811A uses
low temp receiving tube soft glass), arced switches and blown parasitic
suppressors; the latter particularly if the amp was on 10/12M. So we now
have 3 different fried components from "cockpit error".
- tube...from overheating out of resonance,etc
- switch from Tesla coil effect
- suppressors from the combined effect of the above plus a parasitic
IMO, it is entirely possible for more than one destructive influence to
occur at virtually the same instant. Which comes first....damn if I know.
>So if you fire up a tube, even if that tube behaved normally at lower
>voltages, and that tube suddenly arcs, it could be from several
>causes. The anode voltage could have reached a higher peak value
>and exceeded the breakdown voltage due to a load fault or exciter
>transient, the tube could have outgassed from additional heat
>(any of the elements inside can release gas, not just the anode), or
>a poor weld or weak area could fail from the thermal stress.
>
>Any of these things can cause a tube to suddenly arc.
>
>There is no single answer to WHY a tube arcs, as much as people
>who sell bottled cures would like there to be. The likelihood it is
>a parasitic is far down the list (actually very near the bottom),
>since system Q and drive power (and hence anode voltage) is MUCH
>higher at the operating frequency.
>
>People are free to believe what they like, no one can prove
>UFO's don't land and abduct people for sinister sexual experiments.
>Just like with these UFO landings, the proponents of parasitics point
>to other abductees or enlist UFO believers as evidence the exist.
Tom, the prior 2 paragraphs do not belong in this reply...keep your war
outta my space!
>When asked to show the math proving the alleged magnetic destruction
>occurs, they fall back on more "people stories".
DITTO
>You asked why the filaments failed. I have no idea, except it must
>have been a mechanical or thermal failure. The anode current is
>limited by a minimum total ESR of about ten ohms in that PA,
In what PA? The AL-811A , SB-200 or what....I think you drifted off with
the UFO or maybe we both did! I started off with Svetlana 572B's in the
SB-200 and asked you to stick to 811A's so I apologize for the drift
here.
and that
>limits discharge current to 170 amperes or less. A 170 ampere arc
>could certainly burn through a filament, but in order to arc the tube
>would have to have been gassy or defective anyway.
>
>A good 811 tube will hi-pot above 8 kV, there is no likelihood at
>all the anode voltage could exceed 8 kV, since other parallel
>components are rated at much less breakdown voltage.
>
>In any event, virtually all 811 tube failures are from holes
>burned in the anodes, not filament death. Filament failures are far
>below glass failures in frequency, while anode failures (accompanied
>by holes melted in the anode) are relatively common.
Since the 811A and the 572B use identical filament structures, I will
include them both in my own "theory".
I believe that both you and Rich are both right in many respects.
Just hear me out here.
An 811A filament has a certain effective cross sectional area at 60Hz;
pretty much 100% of the area. At RF the skin effect reduces that
effective area way down and the RF resistance of the Tungsten increases.
I don't know the amounts...I'll leave that up to the tube engineers. In
GG service that filament is subjected to the driving RF which further
increases the heat dissipated by the filament; the higher the frequency,
the lower the available cross sectional area plus the higher the RF
resistance. Which equals even more heat. When that parasitic occurs at
somewhere in the low VHF region its additional effect is added to the
filament stress. I believe that you, Rich and others already agree that
the instantaneous discharge can add many amps to the equation.
What I am proposing here is that the combined HEAT dissipated by the
filament (AC + Driving RF + Parasitic RF) causes a single or multiple
hot spots that finally cant take any more. It would take a full chemical
analysis of the broken filament to prove or disprove this. My own
experience has been that an accidentally broken good tube (with plenty of
in-use hours) has a filament structure that can take an amount of
handling with out breaking. However an arc destroyed filament pretty much
crumbles in your fingers. To my thinking that tells me the chemical
composition has been altered.
Any other theories or comments out there?
73....Carl KM1H
>
>73, Tom W8JI
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