[Amps] Design VS parasitic

[Bill, W6WRT]

ORIGINAL MESSAGE:

On Mon, 24 Aug 2009 12:44:13 -0400, "Carl" <km1h at 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