[Amps] [Spam] Re: grounding grids

Joe Subich, W4TV lists at subich.com
Wed Jul 29 22:12:25 PDT 2009 

> Perhaps resistors or inductors are sacrificial components to 
> protect the control grid in case the tube arcs over.

That is not the reason Heath gave for the design. Simply 
opening the grid lead does not and will not protect the 
grid as a HV arc can and will damage (puncture) the grid 
and continue on to the cathode.  Such events are well 
documented. 

> I seem to recall placing EIMAC suggest placing a resistor 
> in series with the HV to the RF choke of an amplifier. 

Eimac (and other tube manufacturers) have been consistent in 
their recommendation for current limiting "glitch resistors"  
- *NOT* fusible or sacrificial devices - to limit any fault 
current to approximately five times normal operating values. 
In circuits with higher voltages (and thus more stored energy) 
the tube manufacturers also recommend "crowbar" circuits along 
with fast acting primary power interrupters to prevent damage 
from arcs.  In no case should the secondary of the power supply 
become open circuit or unloaded in the event of an arc. 

73, 

   ... Joe, W4TV 
 


> -----Original Message-----
> From: Fuqua, Bill L [mailto:wlfuqu00 at uky.edu] 
> Sent: Thursday, July 30, 2009 12:42 AM
> To: lists at subich.com; audioguy at charter.net; 'Alex Eban'; 'Ham-amps'
> Subject: RE: [Spam] Re: [Amps] grounding grids
> 
> 
> Perhaps resistors or inductors are sacrificial components to 
> protect the control grid in case the tube arcs over. I seem 
> to recall placing EIMAC suggest placing a resistor in series 
> with the HV to the RF choke of an amplifier. 
> These would do the same thing. Reduce the arc current thru 
> the grid to ground.
> 
> 73
> Bill wa4lav
> 
> ________________________________________
> From: amps-bounces at contesting.com 
> [amps-bounces at contesting.com] On Behalf Of Joe Subich, W4TV 
> [lists at subich.com]
> Sent: Wednesday, July 29, 2009 9:35 PM
> To: audioguy at charter.net; 'Alex Eban'; 'Ham-amps'
> Subject: [Spam] Re: [Amps] grounding grids
> 
> >From Page 83 of the manual ...
> 
>   Pin 2, 3 and 4 of each tube are connected together internally.
>   Each of the three grid pins is bypassed to ground.  This
>   combination of RF chokes and capacitors provides a predetermined
>   level of negative feedback at the tube grids to further reduce
>   intermodulation distortion.
> 
> Unfortunately, as shown by others this is not accurate and 
> since the negative feedback varies significantly from band to 
> band the floating/bypassed grids simply can't work as Heath claimed.
> 
> 73,
> 
>    ... Joe, W4TV
> 
> 
> > -----Original Message-----
> > From: audioguy at charter.net [mailto:audioguy at charter.net]
> > Sent: Wednesday, July 29, 2009 6:11 PM
> > To: lists at subich.com; 'Alex Eban'; 'Ham-amps'
> > Subject: Re: [Amps] grounding grids
> >
> >
> > So why did Heath do it then? (Chokes et al)
> > Inquiring minds want to know
> > 73, --jim
> >
> >
> > Sent from my BlackBerry® wireless device from U.S. Cellular
> >
> > -----Original Message-----
> > From: "Joe Subich, W4TV" <lists at subich.com>
> >
> > Date: Wed, 29 Jul 2009 17:49:20
> > To: 'Alex Eban'<alexeban at gmail.com>; 'Ham-amps'<amps at contesting.com>
> > Subject: Re: [Amps] grounding grids
> >
> >
> >
> >
> > > From an RF point of view, the grid is grounded and well,
> >
> > The Grid is far from "well grounded."  With 220 pF in series, the 
> > grids are above ground by 25 Ohms (Xc) at 28 MHz and 206 
> Ohms (Xc) at 
> > 3.5 MHz (the design operating range of the SB-220) ignoring 
> the Xl of 
> > the long and thin leads on the capacitors and the effects 
> (including 
> > distributed capacitance and internal
> > resonances)) of the RF chokes.
> >
> > 25 to 200 Ohms is far from "well grounded" and an opportunity for 
> > oscillation at some frequency.  For a more rigorous analysis
> > see: http://www.w8ji.com/vhf_stability.htm
> >
> > Again, placing ANY impedance in the grid portion of a "common grid" 
> > circuit is an invitation to instability and the impedance 
> of all grid 
> > connections should be minimized.  Eimac's air system sockets even 
> > include slots to allow the shortest possible strap connections from 
> > the grids to chassis.
> >
> > 73,
> >
> >    ... Joe, W4TV
> >
> >
> >
> >
> >
> > > -----Original Message-----
> > > From: Alex Eban [mailto:alexeban at gmail.com]
> > > Sent: Wednesday, July 29, 2009 1:29 PM
> > > To: 'Bill W5WVO'; lists at subich.com; stevengrant98 at yahoo.com; 
> > > 'Ham-amps'
> > > Subject: RE: [Amps] grounding grids
> > >
> > >
> > > ...the key to the stability is the capacitors! From an RF 
> point of 
> > > view, the grid is grounded and well, since capacitors 
> work well at 6 
> > > meters.
> > > Alex        4Z5KS
> > >
> > > -----Original Message-----
> > > From: amps-bounces at contesting.com 
> > > [mailto:amps-bounces at contesting.com] On Behalf Of Bill W5WVO
> > > Sent: Wednesday, July 29, 2009 7:58 PM
> > > To: lists at subich.com; stevengrant98 at yahoo.com; 'Ham-amps'
> > > Subject: Re: [Amps] grounding grids
> > >
> > > Joe Subich, W4TV wrote:
> > >
> > > > 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.
> > >
> > > How would you characterize, then, the design of the SB-220, where 
> > > the 3-500Z
> > >
> > > grids are grounded through RFCs and bypassed by dsk ceramic caps? 
> > > Particularly since this particular amplifier has been 
> converted to 
> > > 6m use, but the grid
> > > components are still originals from the HF version.
> > >
> > > Would this amplifier be more stable with the grid pins directly 
> > > grounded to chassis?
> > >
> > > Bill W5WVO
> > >
> > > >
> > > > 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.
> > > >>> _______________________________________________
> > > >>> Amps mailing list
> > > >>> Amps at contesting.com 
> > > >>> http://lists.contesting.com/mailman/listinfo/amps
> > > >>
> > > >> _______________________________________________
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> > > >>
> > > >>
> > > >>
> > > >>
> > > >> _______________________________________________
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