I was referring to approx. 20 kV plate voltage. 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, but I won't toss out everything to speculate
its only 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.
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 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.
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.
73
John
K5PRO
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