> 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@contesting.com
> [mailto:amps-bounces@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@uky.edu> wrote:
>
>
> From: Bill Fuqua <wlfuqu00@uky.edu>
> Subject: Re: [Amps] grounding grids
> To: "Herzog" <herzog@frontiernet.net>, "Ham-amps"
> <amps@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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>
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