There is one.... A thing which doesn't show up here is the transit time
parameter: the time it takes from electrons to travel from the cathode grid
area to the anode. It's a few nanoseconds for common power tubes. At
frequencies where the transit time becomes an appreciable fraction of the
period time, all thing happen, including self sustained internal modes of
oscillation that have nothing to do with external components and everything
to do with internal strays. That also explains, at least in part, why they
are so constant. I call this "magnetron mode". One of the most effective
ways to impede it is to change the load characteristics drastically, like
from capacitive to inductive. That's what the suppressor does: instead of
seeing the input capacitor of the tank, the tube sees the inductance of the
suppressor in series with it.
As for the lossy suppressor: in the old Handbooks it was claimed that the
parasitic energy was developing a voltage across the suppressor and the
resistor's role was to dissipate the associated energy. Actually all it does
is to reduce the Q of the inductor.
So, it's back to what Carl says: cut it the old way, make it of the best
conducting material, de"Q" it with a real resistor and keep the degrees of
freedom the setup provides.
Alex 4Z5KS
-----Original Message-----
From: amps-bounces@contesting.com [mailto:amps-bounces@contesting.com] On
Behalf Of Bill, W6WRT
Sent: Monday, August 24, 2009 8:29 PM
To: amps@contesting.com
Subject: Re: [Amps] Design VS parasitic
ORIGINAL MESSAGE:
On Mon, 24 Aug 2009 12:44:13 -0400, "Carl" <km1h@jeremy.mv.com> wrote:
>And also please explain why the natural parasitic frequency of a tube as
>determined by math and in a test jig doesnt change, except minimally if at
>all, when in an amplifier circuit.
REPLY:
I had never heard of "the natural parasitic frequency" of a tube until you
mentioned it in a previous post. I am assuming you mean the combination of
anode-to-grounded-element capacitance together with the internal lead
inductance?
Of course that characteristic of a tube is a SERIES resonant circuit and
will
not sustain oscillation by itself. You must connect the tube to the
operating
tank circuit and by doing so, you inadvertently create a PARALLEL resonant
circuit which is the actual source of the high VHF impedance necessary to
sustain VHF oscillation.
The length of those connecting leads is highly important in establishing the
VHF
parasitic frequency and shorter is always better. Shorter leads moves the
VHF
resonance higher where tube gain is less and parasitic suppression can be
done
with a smaller inductance, which reduces the tendency to smoke the
suppressor
resistors when operating in the high HF region.
This should not be confused with the natural self-neutralized frequency of a
tube. That frequency applies to the actual operating frequency, the
frequency
being amplified, which is entirely different from the VHF parasitic
frequency.
73, Bill W6WRT
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