>I was referring to approx. 20 kV plate voltage.
// Thanks. We are talking x-rays.
> With 1/2CV^2, the
>stored energy is large here. Hence the need for good crowbars. They
>protect 30 gauge wire fine when the are working.
>
>As for arcing inside tubes however, there is no fundamental science
>that says smaller tubes with closer spacing and lower voltages won't
>arc and do damage just as easily. Gold balls, parasitics, gas, etc.
>are all possibilities,
// Gold sputtering is not a possibility. It is a reality. When an
R-supp. self-destructs during 75m operation, my guess is that VHF energy
was the cause.
>but I won't toss out everything to speculate
>its only one thing for all small tubes.
// Who said that failures were due to one thing for all small tubes?
>They can outgas if gettering
>is not fully activated, such as when turning on an old tube without
>adequate warmup time. 15-30 minutes is typical for broadcast triodes
>and tetrodes when a new one or old spare is reenergized first time.
>Pushing a tube in plate dissipation may also cause arcing, as
>localized spots are getting hotter than before and outgassing the
>materials. We had a tremendous problem with a particular warped grid
>4CW250,000B 7 years ago, as electron beam was focusing on the anode
>and dislodging and eating into the copper, where the free ions then
>hit the filament. Once something like this occurs, it is a runaway
>condition, the tube never gets better, but only worsens in
>temperment, and number of faults/arcs.
// I have never seen a case of disappearing gas in a ham type. How do
we know that 3500Zs have a getter?
>Higher gain bandwidth tubes do have an advantage in that they have
>much lower L in the leadout to the base or cap, and also many modern
>tubes are well aligned (attention to electron optics), not like the
>hardware cloth grids of the 1920 and 1930 bottles.
// The 8877 has electron optics and low L grid lead-out - yet, about
half of the corpses that I've autopsied have a plethora of gold particles
inside.
>The screen grid
>for tetrodes today may actually shield the control grid from the
>anode. I have a 1kW GL851 at home, which is very long, and has
>absolutely no provision for reduced L or lower Cpg. It was rated by
>RCA for a max of a few MHz.
>
// I have never observed VHF regeneration with an 813.
>Improvements have made tubes easier to use, and pushed the Fmax and
>the self neutralization freq up. But the final responsibility for a
>stable circuit rests on the designer of the amplifier. I may be
>preaching to the choir here.
// There is still an 8877 amplifier manufacturer who uses no parasitic
suppressor.
>
cheers, John
- R. L. Measures, 805.386.3734,AG6K, www.vcnet.com/measures.
end
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