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Re: [Amps] Direct-grounding SB-220 grids

To: "Joe Subich, W4TV" <>
Subject: Re: [Amps] Direct-grounding SB-220 grids
From: R L Measures <>
Date: Thu, 8 Jun 2006 09:03:45 -0700
List-post: <>
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

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