[Amps] grounding grids

[Joe Subich, W4TV]

> 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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