[ARC5] Shunt vs. Parallel

[Scott Robinson]

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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