[ARC5] ARC-5 transmitter frequency drift suggestions

[Kenneth G. Gordon]

On 7 Dec 2008 at 20:17, Neil 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.

Yes. However, the 1626 appears to run with minimal drift if 
the plate voltage is held at 200 VDC.

There was a recent series of articles in Electric Radio 
Magazine concerning the "ARC-5" transmitters: 
measureable drift was unusually low for several 
transmitters tested.

Chirp was non-existent when the rigs were run within 
original specs, and that triple-can capacitor was changed.

> 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.

This is an excellent idea for CW. Doesn't really do much for 
AM phone though. I am going to try simple "cathode" 
modulation with one of mine as soon as I can. 

"The Simplest Modulator" from an old QST article will do 
the job.

> 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.

Try 200 VDC regulated (VR-105 and VR-95 in series). I 
think you will be pleasantly surprised.

> 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.

As previously pointed out by one of our other members, full 
plate-modulated AM would require considerable "over-
drive" from the 1626 which would not be advisable.

As I said above, I intend to try simple cathode modulation. I 
MAY try controlled-carrier screen modulation in one of my 
severely hacked junkers.

Here is a really superb article or series of letters written by 
our own David Stinson on this subject.

http://www.mines.uidaho.edu/~glowbugs/arc5pages.htm

I hope you all find these useful.

Ken Gordon W7EKB