[ARC5] Shunt vs. Parallel

[WA5CAB at cs.com]

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> 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 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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