Hi Carl,
> I doubt if a "mature" 811A is outgassing Tom....good dance try though.
Why the personal comments implying I am telling a "story"? If you'd
listen as much as you spew insults, you'd learn.
ALL tubes, no matter how "mature", outgas. Outgassing is a normal
part of the life of the tube.
For example, thoriated tungsten filament tubes outgas carbon dioxide
and carbon monoxide from the filament. They do this after
years and years of use. Outgassing occurs from the glass as well as
all the other element of the tube when it is new, and outgassing
continues to occur throughout the life of the tube when elements are
heated.
That's why power grid tubes containing getting agents. Seals also leak,
and allow air ingress.
> >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 .
Why the personal comments? The 811A is included in this. Small tubes
are 6CW4's, although they outgas also.
> >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??
Come on Carl. The tube could easily have a breakdown voltage above
the normal no load voltage on standby, and outgas enough to
pop when the anode heats slightly when quiescent current is drawn.
A normal tube test procedure is to draw some dc current and heat the
anode, and quickly remove the anode voltage and check leakage
current in a reverse direction (the supply is reversed so the
filament emission does not ruin leakage readings).
None of this is that secretive! It's a normal part of testing tubes.
B & K even did this with small receiving tubes in TV and radio tube
testers!
> >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.
During an arc, I have clearly stated the arc can burn through the
filament. If current is not limited to several dozens of amperes,
magnetic flux can apply enough force to an in line element to reach
several ounces.
That's all fine, but the parasitic can NOT start an arc in an
otherwise healthy tube. That's a fact. The tube would need an
equivalent anode circuit Q that was very high (much higher than the
unloaded Q of the HF tank), and feedback driving the tube to very
low conduction angles. While it is possible to generate peak voltages
several times the anode voltage in these conditions, the HF tank
would be much more likely to provide the proper anode impedance to
create high voltages.
That is the problem in the FL-2100. The tubes oscillate at HF on
standby, because the bias voltage is only 15 volts. The input and
output of the PA are unloaded, and the tube is not cut off. The high
feedthrough capacitance of the 572 allows the PA to oscillate as a
tuned cathode tuned plate oscillator.
If you have an unloaded tank, un-neutralized tubes with high feedback
capacitance, and tubes drawing current you could have an
oscillation. But if the tubes are healthy and have proper breakdown
voltage, they are the last components that would arc.
It's a nice theory Carl, it certainly shows more thought than the
magnetic field theory of filament damage, but when the facts are
looked at carefully it requires some very rare and special
conditions.
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
|