Hi Ian,
> Tom, please explain in your view, why tube type amplifiers are fitted with
> suppressors. And when amps with 3-500z's (for example) go band, why do
> they go bang?
>
> Ian, ZS6BTE
Tubes flash over because something either exceeds the normal
breakdown voltage, or because something inside the tube lowers
the breakdown voltage.
It's pretty common for 3-500Z's to gas up a little and flash over. The
gas comes either from internal material outgassing or seal leakage.
The arc itself can getter the tube, and remove the gas. As a matter
of fact controlled arcs can be used to getter certain tubes in
manufacturing.
The 3-500Z normally getters itself when the anode reaches
temperatures that show some color. In order to completely getter,
some 3-500 anode designs have to be heated to a red color.
There is nothing sinister about an arc.
What amazes me is how people conclude a tube will arc from
some momentary parasitic that occurs one every year, but the very
same tube WON'T arc from 8 kV of peak RF voltage at the signal
frequency.
What is particularly amazing is the anode impedance and Q of the
circuit is many times lower at VHF than at the operating frequency,
and yet some people actually seem to believe more voltage is
developed at VHF than at the fundamental frequency!
This whole parasitic thing is pretty odd, from a technical standpoint.
73, Tom W8JI
w8ji@contesting.com
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