On Aug 29, 2006, at 1:52 AM, Peter Chadwick wrote:
> There are a number of things I don't understand about the parasitic
> suppression business. According to Terman, a tuned amplifier with a
> plate circuit tuned LF of the grid circuit is unconditionally
> stable because the grid has a positive input resistance. If the
> grid is tuned LF of the plate, it has a negative input resistance.
> On this basis then, a tube with a good short grid cone (8877?) with
> a collet type connection has a good chance of being stable without
> any suppression.
What is the reactance of C-feedback in an 8877 at 500MHz?
> A 2C39 should be pretty good, too....As an aside, why were there
> amplifiers in the 1950s with series inductors in the grid for
> parasitic suppression? That appears, on the face of it, to be
> ridiculous.....
Not everything we did in the 1950s was well thought out, Peter.
> The 'conventional' parasitic approach is, if I understand the
> various arguments, to lower the circuit gain at VHF by reducing the
> plate load impedance. I don't see how a resistance wire suppressor
> does that, since a resistance and inductance in series have an
> increasing impedance, ...
To see it, one converts the R/L suppressor's series R to Rp, its
parallel-equivalent resistance -- which is what the anode sees at its
VHF self-resonance. . This conversion is not a simple task.
http://www.somis.org/YtoZ.GIF
"Finding Impedance by Solving for Admittance"
> So the circuit gain would increase, ...
VHF gain decreases because Rp decreases.
> 73
> Peter G3RZP
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R L MEASURES, AG6K. 805-386-3734
r@somis.org
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