[Amps] grounding grids

[Gary Schafer]

Rich Measures claims that the grid bypass capacitors used in the SB 220 are
series resonant at or near the parasitic frequency of the tubes and actually
provide a lower impedance to ground than a piece of copper strap.

73
Gary  K4FMX

> -----Original Message-----
> From: amps-bounces at contesting.com [mailto:amps-bounces at contesting.com] On
> Behalf Of Joe Subich, W4TV
> Sent: Wednesday, July 29, 2009 11:22 AM
> To: stevengrant98 at yahoo.com; 'Ham-amps'
> Subject: Re: [Amps] grounding grids
> 
> 
> 
> > This was used on newer tubes (3-500Z - ect) to raise the
> > resonance of the grid to control parasitics.
> 
> The grid resonance of the 3-500Z and similar tubes has been
> measured well above 100 MHz.  There is no need to "float" the
> grid.  In fact, floating the grid will lower the grid resonance
> by increasing the amount of inductance in the grid lead.
> 
> The floated grid was misguided attempt by Orr to introduce
> some degeneration (negative feedback) to improve IMD.  Since
> the level of degeneration was based on the Xc of the "bypass"
> capacitors, it varies greatly from band to band and is
> ineffective for that purpose.
> 
> In addition, the bypass capacitors introduce a parallel
> resonance in the grid circuit at UHF which converts the
> nominally stable grounded grid amplifier into a TPTG
> oscillator.  One need only look at tubes like the 8874,
> 8877, etc. with very high VHF amplification factors to
> see how difficult they are to tame if there is any
> impedance in the grid circuit.
> 
> While the 3-500Z and other older glass tubes don't have
> as much gain as the newer external anode tubes, the
> principle is the same.  If there is enough gain and the
> feedback is of the correct phase the tube will oscillate.
> Placing any impedance in the "grounded" grid of a common
> grid circuit increases - not decreases - the chances of
> oscillation.
> 
> 73,
> 
>    ... Joe, W4TV
> 
> 
> 
> > -----Original Message-----
> > From: amps-bounces at contesting.com
> > [mailto:amps-bounces at contesting.com] On Behalf Of Steven Grant, W4IIV
> > Sent: Wednesday, July 29, 2009 10:45 AM
> > To: Herzog; Ham-amps; Bill Fuqua
> > Subject: Re: [Amps] grounding grids
> >
> >
> > This was used on newer tubes (3-500Z - ect) to raise the
> > resonance of the grid to control parasitics. It is not needed
> > for older tubes
> >
> > STEVEN GRANT W4IIV
> >
> > --- On Wed, 7/29/09, Bill Fuqua <wlfuqu00 at uky.edu> wrote:
> >
> >
> > From: Bill Fuqua <wlfuqu00 at uky.edu>
> > Subject: Re: [Amps] grounding grids
> > To: "Herzog" <herzog at frontiernet.net>, "Ham-amps"
> > <amps at contesting.com>
> > Date: Wednesday, July 29, 2009, 9:28 AM
> >
> >
> > Maybe to measure individual grid currents?
> > That would be the only reason I can think of.
> > 73
> > Bill wa4lav
> >
> > At 08:35 PM 7/27/2009 -0400, Herzog wrote:
> > >RE: > >Hi.
> > >  > >This has probable been asked before on this group,but... Is it
> > >good
> > >  > >practice to physically tie the grids to ground via one
> > connection,or via
> > >  > >low value resistors and decoupling capacitors. I was
> > going to go for VHF
> > >  > >practice and go for direct grounding,but would like
> > some comments before
> > >  > >progressing this project.
> > >  > >               73`s Jon
> > >
> > >REPLY:
> > >
> > >I'm curious - what would be the reason for using resistors and
> > >decoupling caps?
> > >
> > >73, Bill W6WRT
> > >===========================
> > >Herzog proffers a reason, to be able to measure the grid current.
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