[AMRadio] Re Eico 730 and Knight T-150

JAMES HANLON knjhanlon at msn.com
Tue Nov 12 15:14:24 EST 2013

Check in any of the older ARRL Handbooks or Editors and Engineers Radio Handbooks that cover AM, and they will explain that the modulation transformer impedance should be set as follows.  For the T-150, you probably have a final plate voltage of about 700 volts and a plate current of about 200 ma.  The amplifier will then present an load resistance to the modulator of 700 divided by 0.200 or 3500 ohms.  You should select the closest tap on the Eico 7300 output to 3500 ohms.
However, the Eico 730 is rated at only 50 watts output, so it will not be able to 100% modulate the 140 watts input of the Knight T-150.  If you wanted to reduce the input of the T-150 to 100 watts which the Eico would be able to 100% modulate, then the Knight plate current would be 100 divided by 700 or 142.8 ma.  Let's say you decided to run 150 ma.  Then your modulator load resistance would be 700 divided by 150 or 4667 ohms.  
Bottom line, choose the modulator output impedance somewhere between 3500 ohms and 4667 ohms, tune things up, and if possible look with an oscilloscope to see how your modulation looks.  Also get a critical report from folks on the air.
By the way, the vfo in my T-150 is not drifty.
Jim Hanlon, W8KGI

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