On Jun 7, 2006, at 1:53 PM, Joe Subich, W4TV wrote:
>
> Measures wrote:
>>
>> The shortest possible Cu strap has c. 15nH of L. At the 3-500Z's
>> grid resonace (c. 88MHz) this represents roughly pos. 9-ohms of X.
>> The original 200pF cap used to ground the grid has c. neg. 8-ohms of
>> X. Will the resonant frequency by increased by the use the cap or
>> the strap?
>
>
> Don't forget the original circuit has THREE 200 pF capacitors
> and a 1 mH RF choke on each tube. What are the internal resonances
> in the choke, what are the lead impedances (the capacitors certainly
> have significant lead inductance given their long thin nature)?
> What is the EFFECTIVE impedance between the grid of the 3-500
> and ground.
At what frequency? Between 87 & 88 MHz, the 3-500's grid looks
pretty much like an open circuit.
>
> Properly installed, the inductance of the CU strap on each grid
> pin is in parallel resulting in less X than you postulate.
Fred E. Terman was wrong? [ref p.47 - 64]
> There
> is no way to know what the real X is with the L/C networks in the
> SB220 ... there is no way to account for the various installation
> techniques, coupling of the RFC to the chassis, lead lengths. etc.
> but I suspect that much of the variability in performance and
> stability of the SB-220 family of amplifiers is related to the
> unnecessary L/C networks.
In my experiences, the grid resonance does not change much whether
the grid pins are strap-grounded or cap-grounded.
>
> 73,
>
> ... Joe, W4TV
>
>
>
R L MEASURES, AG6K. 805-386-3734
r@somis.org
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