Excellent explanation Alex. I was trying to think of a word to explain my
natural internal frequency yesterday and magnetron is perfect.
If you look at an engineering drawing of all the internal paths and
interactions, and complete with semicircle arrows, it resembles the maggie
drawings. Some may also see a resemblance to a klystron.
In any event the enclosed and self sustaining oscillation mode has been
clearly established by the tube engineers long ago.
Carl
KM1H
----- Original Message -----
From: "Alex Eban" <alexeban@gmail.com>
To: <dezrat1242@yahoo.com>; <amps@contesting.com>
Sent: Tuesday, August 25, 2009 4:56 AM
Subject: Re: [Amps] Design VS parasitic
> 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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