You are right Joe. It was the SB200 that used the cutoff bias through the
antenna relay scheme.
73,
Gerald K5GW
In a message dated 7/29/2009 8:46:28 P.M. Central Daylight Time,
lists@subich.com writes:
> Didn't Heath use the ungrounded grids as a way to apply
> cutoff bias during standby?
Not in the SB-220, the grid circuit is a 1 mH RF choke
bypassed with three 200 pF disk ceramic capacitors.
The SB-200 used a single 200 pF capacitor to bypass the
grids of the 572B with a single 33 Ohm resistor per tube
for both operating and cut-off bias. The SB-200 is even
worse that the SB-220 for "floating grid" problems due to
the relatively high bypass impedance. Among other common
problems are damage to the 330 Ohm (1 W) resistors from
being cooked by RF.
73,
... Joe, W4TV
> -----Original Message-----
> From: TexasRF@aol.com [mailto:TexasRF@aol.com]
> Sent: Wednesday, July 29, 2009 6:19 PM
> To: audioguy@charter.net; lists@subich.com;
> alexeban@gmail.com; amps@contesting.com
> Subject: Re: [Amps] grounding grids
>
>
> Didn't Heath use the ungrounded grids as a way to apply
> cutoff bias during standby?
>
> Seems the internal antenna relays were connected between a
> -130vdc supply and the grids. The connection to the grids was
> wired to a rear panel jack where a ground application
> operated the relays and shorted the bias all at once.
>
> The chokes would have been there to keep rf out of the bias supply.
>
> 73,
> Gerald K5GW
>
>
> In a message dated 7/29/2009 5:10:18 P.M. Central Daylight
> Time, audioguy@charter.net writes: 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@subich.com>
>
> Date: Wed, 29 Jul 2009 17:49:20
> To: 'Alex Eban'<alexeban@gmail.com>; 'Ham-amps'<amps@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@gmail.com]
> > Sent: Wednesday, July 29, 2009 1:29 PM
> > To: 'Bill W5WVO'; lists@subich.com; stevengrant98@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@contesting.com
> > [mailto:amps-bounces@contesting.com] On Behalf Of Bill W5WVO
> > Sent: Wednesday, July 29, 2009 7:58 PM
> > To: lists@subich.com; stevengrant98@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@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.
> > >>> _______________________________________________
> > >>> Amps mailing list
> > >>> Amps@contesting.com
> > >>> http://lists.contesting.com/mailman/listinfo/amps
> > >>
> > >> _______________________________________________
> > >> Amps mailing list
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> > >>
> > >>
> > >>
> > >>
> > >> _______________________________________________
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> > >
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> >
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