[ARC5] Shunt vs. Parallel

[Tim]

And of course the Navy always flies higher than the Army ...so there's
that....  hihi

GO NAVY!
Tim
N6CC

On Sat, Jul 15, 2017 at 9:59 PM, Scott Robinson <spr at earthlink.net> wrote:

> Hi Robert,
>
> I'm with you; I bet the altitude is key. Most or all of WWII warbirds were
> not pressurized, and arc-over is easier at low pressures.
>
> /scott
>
> On 7/15/17 9:33 PM, WA5CAB--- via ARC5 wrote:
>
>> It is, at least theoretically.  In practice not so much.  Linearity with
>> screen or suppressor grid modulation is poor at high modulation
>> percentages.  But the tuning capacitor in the series fed circuit has DC
>> supply voltage plus peak RF voltage across it to ground.  In the shunt fed
>> circuit, the coupling capacitor passes the peak to peak RF voltage but not
>> the DC.  So the voltage across the tuning capacitor to ground is +/- peak
>> RF without the additive DC.  This is true in both CW and AM.  But remember
>> that the transmitter output power is dropped when operating AM by dropping
>> the PA screen voltage to a regulated 150 VDC by the 0D3 being turned on.
>> It would take considerable work to determine whether the peak RF plus DC
>> was any higher in an AN/ARC-5 transmitter running AM than it was in an ATA
>> or SCR-274-N.  But shifting to the shunt fed circuit reduced the peak
>> voltage across the tuning capacitor by about 575 Volts in CW and 540 in AM.
>>
>> However, the primary reason for the change may have been that aircraft
>> service ceilings went up significantly during the War.  The ATC and
>> AN/ART-12, which already had shunt feed, switches to low power at around
>> 21,000 feet MSL by dropping the high DC plate supply voltage from around
>> 1200 to around 750 VDC.
>>
>> But this is all just guessing.  I went through the manuals again and
>> there are no hints.
>>
>> Robert Downs - Houston
>> wa5cab dot com (Web Store)
>> MVPA 9480
>>
>> In a message dated 07/15/2017 20:52:40 PM Central Daylight Time,
>> neilb0627 at gmail.com writes:
>>
>>> I thought that the peak plate voltage was the same for both screen-grid
>>> modulationand plate modulation. Maybe I'm wrong. The ARRL Handbooks don't
>>> mention it.
>>>
>>> 73 de Neil ZL1ANM
>>>
>>> On Sun, Jul 16, 2017 at 12:11 PM, <WA5CAB at cs.com <mailto:WA5CAB at cs.com>>
>>> wrote:
>>>
>>>> I have nothing to substantiat this guess with but I have always assumed
>>>> that when they went to plate modulation they discovered, or were afraid
>>>> that they would have, arc-over problems with the tuning and padder
>>>> capacitors.
>>>>
>>>> Robert Downs - Houston
>>>> wa5cab dot com (Web Store)
>>>> MVPA 9480
>>>>
>>>> In a message dated 07/15/2017 18:19:14 PM Central Daylight Time,
>>>> neilb0627 at gmail.com <mailto:neilb0627 at gmail.com> writes:
>>>>
>>>>>
>>>>> Why was there a change in the AN/ARC-5 to shunt feed to the 1625
>>>>> plates?It meant one extra component, the choke. I've never in 50 years seen
>>>>> an
>>>>> explanation for it.
>>>>>
>>>>>
>>>>> 73 de Neil ZL1ANM
>>>>>
>>>>
>>>>
>>
>>
>>
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