> A previous post mentions a phenomenon wherein is written
> "parasitic oscillation" and what I deduce as filament sag.
>
> Is it possible that these two conditions are related?
No.
> It is understood how a strong magnetic field within the
> tube envelope
> can perhaps alter the filament geometry, but can this
> happen as a result
> of a VHF "excursion?"
If you calculate the amount of magnetic force produced by
current, you'll see that force for something the size of a
filament helice is on the order of a few grams from a dozen
amperes of current. It is not only impossible for a
parasitic to cause a problem, it is also impossible for
filament inrush of 50 or 100 amperes to cause significant
bending forces.
It gets worse than this. I mapped the voltage distribution
in an SB 220 (SB 221) tank circuit at VHF and from that we
can see it is also impossible for a VHF oscillation to cause
an arc in the tank.
http://www.w8ji.com/SB221/sb-221.htm
I actually saw cartoons about parasitics on the walls of
Eimac when I visited! You know those little office jokes
that get circulated. One that stuck in my mind was a
something about a contra-wound parasitic suppressor causing
a being connected in push-push and causing a push-pull
oscillation that caused a huge explosion with parts flying
everywhere. It's sad that Hams and ham myths like this are
now the subject of technical jokes. This parasitic thing is
entertaining but unfortunately it has set back group
knowledge of how and why amplifiers actually fail by many
years.
73 Tom
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