[ARC5] ARC-5 transmitter frequency drift suggestion

[Dennis Monticelli]

In order for a PA to be driven to 100% AM, the drive to the input must
be sufficient to support 4X peak power.  In other words, it must be
overdriven.  I suppose if the PA is grossly over-sized for the power
desired, then AB1 would represent sufficient drive.  Otherwise, Class
C running full rated grid current is warranted.

Dennis AE6C

On Sat, Dec 6, 2008 at 11:17 PM, Neil <neilb at ihug.co.nz> wrote:
> Frequency drift in the ARC-5 and SCR-274N transmitters can be mostly
> attributed to the 1626 oscillator being run at high power level in order to
> supply adequate drive to the 1625 grids.
>
> It occurred to me some time ago that if the 1625's were to be run in Class
> AB1 instead of Class C, the drive power required would fall to virtually
> zero. This could be easily done by lifting the grounded end of the 1625
> grid resistor (15K) and feeding -35V bias to it.
>
> The 1626 would now be required to supply a maximum of 35 volts of drive
> (at virtually no power), versus the previous 100 volts or so at about 7mA
> (half a watt). Its plate voltage could be scaled back accordingly, to
> perhaps
> 150 or even 105 VDC regulated.
>
> I'm sure this would work very well in the case of CW, but I can't recall
> anything in the literature describing AM plate modulation of a Class AB1
> power amplifier. Has anyone tried this or can point to literature on the
> subject?
>
> 73 de Neil ZL1ANM
>
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