>Rich-
>
>Somethings I've always wondered about...
>
>determining the required inductive component of the 'suppressor' for a
>particular amp.
** for optimum effectiveness, the suppressor inductor (L-sup) needs to
have roughly the same # of ohms of reactance (at the anode's vhf-resonant
frequency) as the ohms of the suppressor's resistor. This arrangement
divides the vhf ringing-current semi-equally and optimizes the
stagger-tuning effect that broadbands (decreases the Q) of the vhf
resonance and proportionately decreases the damped-wave ringing potential
when anode current changes.
>
>the problem is positive feedback through coupling between the plate
>circuit and the
>grid circuit. right?
** In a tetrode or pentode, right. In a g-g triode, the problem is
coupling between the anode/plate output and the cathode input.
>
>In order to get enough feedback for an unwanted oscillation, we need a
>resonant circuit, right?
** correct. And there always is a VHF-resonant circuit in a HF
amplifier. The lowest freq I've seen in a HF amp was 43MHz (4cx20,000A).
the highest was 170MHz (8873).
>The inductive component of the suppressor changes the resonance,
>to a frequency where we can not get enough feedback to sustain oscillation,
right?
** Shorted or not shorted, L-sup/R-sup barely shifts the anode circuit's
resonanant frequency.
>If so, this means the
>inductance does not have to be some specific value, so long as it's
>enough to move the
>resonant frequency enough.
>
>So what's the resistor for?
** The resistance lowers Q which lowers peak ringing V, and it provides
a low-L path to establish the stagger-tuning effect. Resistance in the
inductive path also lowers Q.
>
>Isn't this the same thing we do when we neutralize a tube with
>additional capacitance
>in the feedback loop?
** no
>Why isn't this additional capacitance damped?
>Should it be?
>
** The Bruene neutralization bridge needs to be tuned for minimum SWR at
the operating freq. - which cancels grid reactance. When this is done,
the C-divider is then adjusted to null the feedthrough signal at the
anode. Since a uniform 180? phase shift is needed over the entire
operating range in order to cancel feedback at the operating freq.,
adding R would seem be a hindrance. [see Figure 5 on my Web site]
cheers, Bob
- R. L. Measures, a.k.a. Rich..., 805.386.3734, AG6K,
www.vcnet.com/measures.
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