Joe Subich, W4TV wrote:
>> 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
>>>>>
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