On Mon, 12 May 1997 20:31:53 +0000 w8jitom@postoffice.worldnet.att.net
writes:
>
>> >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.
>
>Not really, the elements also heat from the quiescent current. It's
>quite normal to have outgassing that causes and arc when the
>elements are elevated in temperature.
I doubt if a "mature" 811A is outgassing Tom....good dance try though.
>As a matter of fact, that is how large power grid tubes are aged
>at the factory. The filaments are run over temperature, the elements
>are overheated, and a vacuum is pulled while the tube cooks. If they
>miss something, or if there are impurities in the tube, you might
>very well change the breakdown voltage by warming up the anode.
You are seriously Drifting again....I am NOT discussing large power
tubes, graphite plates, etc. Please stick to the 811A .
>With that thought in mind, remember my other comments. If a
>parasitic caused the arc on standby, the tube must have had low
>breakdown voltage anyway.
Nothing happened on standby so you cant use the excuse of gas, poor
breakdown, etc. The BANG only happened when the tube was conducting in
STRICTLY a DC state. No RF drive PERIOD, NADA , NONE,
ZERO.....Comprende??
>> 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.
>
>No, only a fractional amperage increase. The saturated emission
>current of a thoriated tungsten 25 watt filament is three amperes or
>less. That current must be integrated over time to cause heat, plus
>it is the PEAK current. The effective heating would be the time
>integrated value of this RF current. If driven by a sine wave, the
>heating would be .707 times 3 amperes, or 2.1 amperes times the
>resistance. If the conduction waveshape was the same as a typical RF
>PA, the current considered for heating would be less than 1/3 the
>peak current, or under one ampere.
Very wrong again Tom. You are staying completely hung up in "proper" tube
performance and there you are correct.
What you refuse to admit is that an instantaneous discharge is a function
of the voltage and current and has absolutely no bearing on the design
parameters of the tube.
A VHF parasitic can reach many times the breakdown voltage of the tube
when it can not be absorbed by the load and is dependent upon the
UNLOADED Q of the tank circuit. Parasitics can vary in intensity,
depending upon that Q and the amount the load can absorb. Basic stuff
Tom. Even the ARRL Handbook has the formula correct.
Note that I keep insisting on the word "instantaneous" since that is a
key. What you may observe visually during an arc is thousands of times
longer than the actual event. A parasitic at say 100MHz is over and done
with in one cycle or so which is what... .000000001 seconds? How many
cycles does it take to establish the breakdown path.....very few IMO.
That 10, 20 or more KV...yes KV, arc is dumping all of its energy in
that time thru the filament and a few components beyond.
Do you really expect anyone to belive that it is only a few hundred ma?
I may be dense, but not stupid.....
>> 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.
>
>The analysis would prove nothing except the filament got hot. It is
>impossible to determine the cause of the heating after the fact.
Well, golly gee whiz...it got hot. Just what you said would cause a
filament failure ...HEAT. Where do you suppose it came from; not a UFO I
hope. Even a NoCoder can probably calculate the various iterations of
Ohms Law.
>>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.
>
>The chemical composition is altered by gas or distributed high
>temperatures, not by the arc. All the arc does is cause localized
>heating at the point of the arc.
You are correct BUT the arc is PRECEEDED by distributed or localized HIGH
TEMPERATURES causing the fracture of the filament. Thank you Tom for
admiting that altho it was probably not intentional. Just a dig....no
flame..its getting late.
>If you had an entire filament that crumbled, as you stated, the cause
>would have to be exposure to gasses and/or excessive temperatures
>along the filaments entire length.
I'll stick with the temperature angle.
>I'm not surprised a gassy tube that eventually arcs has a weakened
>filament structure.
The only gas around this discussion is pure methane flatulence.
73...Carl KM1H
>73, Tom W8JI
>
--
FAQ on WWW: http://www.contesting.com/ampfaq.html
Submissions: amps@contesting.com
Administrative requests: amps-REQUEST@contesting.com
Problems: owner-amps@contesting.com
|